Britbike forum

Another boring question.

Posted By: Tracey Spear

Another boring question. - 10/29/18 12:55 am

Well actually two questions.

I got busy on the cylinders with WD40 and scotchbright hoping to clear the rust and get by with honing. No such luck. So next step is a .20 bore. I reviewed several older threads. A few cautioned about going to automotive machine shops for the machine work. Why is that?

Shouldn't any competent machinist be able to bore the cylinders straight, round, parallel, and to spec?
**********
While at it I also reviewed old discussions on pistons and came away more confused than enlightened.


I did read that I should order the pistons first so that the machinist has them available for when boring. And that the clearance should be 0.004. So recommendations will now be accepted for quality pistons/rings.
Posted By: Jeff K.

Re: Another boring question. - 10/29/18 1:02 am

Ask around and see who the bike shops are using. The critical part is getting the proper clearance. Do not let them tell you what it should be unless they have a lot of experience. Use the piston manufacturer's recommendation. I am lucky and the local auto machine shop is run by a old and very experienced biker/ flat tracker/ machinist.
Jeff
Posted By: Allan Gill

Re: Another boring question. - 10/29/18 12:38 pm

And the bike is? If it’s an A65, bore it to .004-.0045 thou. The later manuals gave this figure, even at .004 it will want to nip up if. It gets too warm before running in. At .0045 I haven’t had this problem. The value is also given for the bottom of the skirt. Could be because most piston rock occurs at the bottom but check the top of skirt clearance also (If one is mentioned) I’ve had auto shops do mine, but they work on other old vehicles too so have a better mechanical sympathy for the old iron.

And yes, get the pistons bought first and bore to suit. Check the clearance before purchasing pistons with the old barrel. Then look at the wear, damage etc. A good shop would give you a good idea before you order anything. As your in the states, Give Ed V at E&V engineering in Howard City, Michigan. He has the best pistons for the job!
Posted By: kommando

Re: Another boring question. - 10/29/18 12:47 pm

A modern reborer will not be used to cast iron rings and will hone too finely, so the rings do not bed in, as long as they have experience of old bikes and cast iron rings they should be ok. But to be 100% safe then use Ed V or similar.
Posted By: BSA_WM20

Re: Another boring question. - 10/29/18 12:54 pm

The first problem is most machine shops index the bore off the head face which is fairly std practice with cars.
However your barrel must be indexed off the underside of the barrel flange
Next most machine shops will ignore what you tell them and bore the cylinder way too close for an old air cooled engine.
Then there is the need for compression plates , some will argue about this
Finally there is the hone, our bikes ilke a really coarse hone 100 to 130 grit where as most shops doing cars with hone it down to 180 grit which will take thousands of miles for the rings to bed in.
Now a machine shop can spend several hours making up plates & jigs to suit a one off job and down here get $ 40 for their troubles or they can set up a 4 or 6 cylinder block in less time and end up with $ 160 to $ 240, so if you had to pay wages , machine leasing and put your kids through school, which one would you choose to do ?
Just about all of the places where I take work to or send others to are run by motorcycle owners who do this work becuse they have a passion for motorcycling
Posted By: Tracey Spear

Re: Another boring question. - 12/10/18 11:31 pm

OK, for A65 rebuild I'm putting Emgo+.20 pistons and Hastings +.20 rings. Everything's ready to go to the shop for bore. I need to request .004-.045 piston clearance, and I need to request coarse hone 100-130. Any particulars I'm missing?
Posted By: Mark Z

Re: Another boring question. - 12/11/18 2:07 am

So far so good. Also, you will need to check the end gaps on the rings and possibly file the rings, unless your machinist does that for you. The last set of Hastings rings I bought had end gaps of .006". According to "the book" they should be .008-.013. There were a couple of long threads on ring gaps a while back. In a nutshell, the consensus of opinion was that the top ring gap should be smaller than the others - also, that too big is definitely better than too small. I filed mine to .010" for the top two rings, and .015" for the oil ring. (I was shooting for .012", but the oil ring files faster than the solid rings.)
Posted By: htown

Re: Another boring question. - 12/11/18 2:49 am

I beg to differ on the honing. Hastings recommends 220 grit for their cast iron rings. Anything between 180 and 220 will work. I've got several engines that seated the rings during their first start up with Emgo pistons, Hastings rings and 200 grit hone. I would shoot for the upper tolerance on the ring gap. Most of these engines won't see high miles and a little bit extra clearance won't hurt. I use four and half thou clearance on the skirts. Give the new pistons to the machinist so he can measure them and hone to the correct clearance. Be sure and scrub the bores with hot soapy water to remove the stone debris, solvent won't work. If you can put some oil on a white paper towel, rub it around and it comes out white they are clean enough. Otherwise keep at it. That light coat of oil is all they need. Don't drench the pistons and rings in oil. Just a spot of oil on each thrust face of the skirt. Use a dino motorcycle oil that's rated SG for break-in. Don't use anything rated SJ through SN. I use Castrol Go 4T 20w/50. I know you will probably get a lot of contradictory advice, just reporting what worked for me.
Posted By: Mark Z

Re: Another boring question. - 12/11/18 3:17 am

+1 on the 180 grit (sorry, I missed that before).
Posted By: bodine031

Re: Another boring question. - 12/11/18 3:14 pm

What Htown says, + assembly lube on the pin to the small end bush. Lucas 20/50 breakin oil. at 1st start turn throttle open, closed not one speed till head warm. closed high vacuum to load rings with compression to force rings out to bite the cyl. wall and bed in rings. complete cool down retorque and valve adjust. take it for a spin constantly loading the engine not lugging it. run it hard for a good 10-15 miles
Posted By: htown

Re: Another boring question. - 12/11/18 3:36 pm

Yeah, on first start up don't let the engine idle, keep opening and closing the throttle 2000-3000 rpm for about 5 minutes or so until nicely warmed up. This is to keep the cam lubed. Then let it cool, retorque and valve adjust. Next start up check timing and adjust carbs and
then ride.
Posted By: triton thrasher

Re: Another boring question. - 12/11/18 4:23 pm

Originally Posted by bodine031
closed high vacuum to load rings with compression to force rings out to bite the cyl. wall and bed in rings.


Throttle open, making the engine pull is what forces the rings out against the bore.

Start the engine and immediately take off up the road.
Posted By: Allan Gill

Re: Another boring question. - 12/11/18 6:31 pm

Originally Posted by triton thrasher
Originally Posted by bodine031
closed high vacuum to load rings with compression to force rings out to bite the cyl. wall and bed in rings.


Throttle open, making the engine pull is what forces the rings out against the bore.

Start the engine and immediately take off up the road.


As TT says. However once a kin to the belief that thrashing it around will help it bed in... it won’t always with cast iron rings, ride the bike normally, not labour the engine and don’t ride it like you stole it. Build the distance up steadily and let it cool fully between runs, 50 miles is about max for a first proper run out, although I ended up running mine in on a fresh cyclinder going to an international, rode it like I normally would Do and it was fine
Posted By: BSA_WM20

Re: Another boring question. - 12/12/18 10:59 am

Originally Posted by Allan Gill
Originally Posted by triton thrasher
Originally Posted by bodine031
closed high vacuum to load rings with compression to force rings out to bite the cyl. wall and bed in rings.


Throttle open, making the engine pull is what forces the rings out against the bore.

Start the engine and immediately take off up the road.


As TT says. However once a kin to the belief that thrashing it around will help it bed in... it won’t always with cast iron rings, ride the bike normally, not labour the engine and don’t ride it like you stole it. Build the distance up steadily and let it cool fully between runs, 50 miles is about max for a first proper run out, although I ended up running mine in on a fresh cyclinder going to an international, rode it like I normally would Do and it was fine



The advice I was given was toss on your riding gear start the bike and off down the road.
As TT said, lots of up & down through the box & wide open throttle to seat or bed in the rings.
500 yards should be enough and in my case it was about 1.5 miles once around the block.
Then back home let it cool down & retorque the head.
Next start, faff around with the carb if you must .
No good playing with tuning till the rings have sealed cause you will have to keep on doing it till the rings have made a proper seal.
Remember this is just for getting the rings to seal against the bore & piston.
It is not running in, a process where brand new bushes & bearings need to run against each other and take up the initial wear.
This is where the gentle riding , o oading the engine, no high revs unloaded or lugging comes in .
Posted By: Tracey Spear

Re: Another boring question. - 12/12/18 5:41 pm

Ok....so, have patience with me, I'm a little confused.

Are you really suggesting that I, who have neither rebuilt an engine, nor completely disassembled/reassembled a motorcycle before, fire it up the first time and gently ride down the road running up and down through the gears to full throttle? And on top of that, it's been near twenty years since I last rode my Norton.

I'm having a little trouble getting my head around that. Sounds like asking for a transfer to the RIP forum. Now I'm obsessing about every detail, and will make sure that all systems are go, to the best of my ability. But the image TT painted so blew my mind, I thought he must be joking.

So what is the proper procedure for seating the rings/running in? I would call it break in, that A) Does it proper so it works well and last, and B) gives this old goat a chance to relearn some skills, and not get killed.
Posted By: Allan Gill

Re: Another boring question. - 12/12/18 6:29 pm

There’s a difference of opinion but I had a set of razor sharped burred up rings when I tried to run mine in hard. It would also blow more fuel past the rings and into the oil which is never a good thing either

A decent break in oil will help wonders, ride the bike normally and don’t labour it in too high a gear.
Posted By: triton thrasher

Re: Another boring question. - 12/12/18 6:57 pm

Half throttle in bursts of a few seconds is probably enough, if that makes you feel better.

I make sure the oil pump feed side is pumping oil to the crankshaft, but my engine is Triumph. I’m not sure how you do that on an A65. Does it have an oil pressure light?

Posted By: htown

Re: Another boring question. - 12/12/18 7:08 pm

Here's what works for me.
Initial start up on the stand, run about 5 minutes or so to get the engine up to operating temps. No idling, keep blipping the throttle between 2000 and 3000 rpm. For breakin, I use Castrol Go 4t 20w/50 which is SG rated. Always use SG rated oil.
Let cool, retighten cylinder base nuts, retorque head bolts, reset valve adjustment
Have timing light ready, start engine up, get it up to operating temps, check timing, rough adjust carbs, don't take time to fine tube carbs.
Now go on first ride. Hopefully a place with little traffic. Run up and down the gears, don't lug it. I never go above 3500 rpm. Don't ride at a steady speed. Close the throttle every so often, this draw oil up into the cylinders.
At 100 miles, change the oil and filter, recheck the base nuts, retorque the head, readjust the valves.
Ride as before, gradually increase the top rpm up to 5000.
At 500 miles, change the oil and filter, recheck the base nuts, retorque the head, readjust the valves. Use whatever oil you plan to run in it.
She should be well broken in at this point.
Posted By: Mark Z

Re: Another boring question. - 12/13/18 12:56 am

I didn't notice this until I indexed my throttle, but I found it's normal to bring the throttle to half during even moderate acceleration, and when climbing hills. OTOH, sustained half throttle at speed will bring my A65 over 70 mph.

I think I live on the perfect break-in road - rural and hilly, a safe speed for most of it is around 50 mph, a little slower on the curves and a little faster on the straights. That is, all I have to do is ride that road as I normally would.

Posted By: BSA_WM20

Re: Another boring question. - 12/14/18 11:37 am

Originally Posted by Tracey Spear
Ok....so, have patience with me, I'm a little confused.

Are you really suggesting that I, who have neither rebuilt an engine, nor completely disassembled/reassembled a motorcycle before, fire it up the first time and gently ride down the road running up and down through the gears to full throttle? And on top of that, it's been near twenty years since I last rode my Norton.

I'm having a little trouble getting my head around that. Sounds like asking for a transfer to the RIP forum. Now I'm obsessing about every detail, and will make sure that all systems are go, to the best of my ability. But the image TT painted so blew my mind, I thought he must be joking.

So what is the proper procedure for seating the rings/running in? I would call it break in, that A) Does it proper so it works well and last, and B) gives this old goat a chance to relearn some skills, and not get killed.


Go to the piston ring makers web site and read the bumpf about how rings actually work and it will all make sense to you.
You need to get high pressure behind the rings forcing them onto the bore very quickly to bed the rings into the bore.
Light throttle leads to light pressure and light pressure glazes the bore.
Just because you open the throttle wide open, does not mean the engine will be doing 7000 rpm.
So you open the throttle wide thus allowing the piston to gulp in a full charge of fuel, by prefference going up a hill and by the time the engine has got to about 4000 rpm, shut the throttle down.
Slaming the throttle closed pressureizes the rings from the other end.
I like to roll the throttle on fully and off in 3rd gear going up a slight rise.
So you open the throttle wide in first then close it down well before the engine gets much past 1/2 full revs.
A couple of times in 1s then the same in 2nd & 3rd.
If the roads allows do the same in top.
I am not tanking about valve bouncing nor doing 100mph, it is all about working the cylinders hard with a full charge .

What confuses people is the instructions in the owners manuals which tell you to run under light loads for the first 200 to 500 miles.
These instructions are for running in a brand new motorcycle because when brand new the rings had already been bedded in before the motorcycle left the factory and you are running in the bearings & bushes.
Posted By: Allan Gill

Re: Another boring question. - 12/14/18 1:52 pm

Happy Birthday Trev!

I agree with your statements, but I once told someone to "let the engine rev a bit" as a dumbed down explanation of getting the rings bedded in, not realising that they would hold it at full throttle in each gear until it eventually got a 4 corner siezure and locked the back wheel.
Posted By: BSA_WM20

Re: Another boring question. - 12/15/18 4:01 am

Originally Posted by Allan Gill
Happy Birthday Trev!

I agree with your statements, but I once told someone to "let the engine rev a bit" as a dumbed down explanation of getting the rings bedded in, not realising that they would hold it at full throttle in each gear until it eventually got a 4 corner siezure and locked the back wheel.


You been reading the mail ?
Birthdays have not existed for this little black duck from the time he left home.
Posted By: Tracey Spear

Re: Another boring question. - 04/14/19 1:58 am

Update:

My cylinders have come back, bored and painted. I specifically ask for 0.04 to 0.045 piston clearance. I haven't had a chance to measure them yet, but I was told the clearance is 0.035. I was told that the machinist called Emgo, and that was their recommendation. A65, Emgo pistons, Hasting rings.

Is that gonna be too tight?
Posted By: Mike Baker

Re: Another boring question. - 04/14/19 12:03 pm

Rob Hall will run his motors at that or smaller clearances with those pistons. But he likes to see how far he can push whatever boundary is in front of him. For most of us, .045 is safer.
Get everything measured by someone that knows what they're looking at.
I call BS the statement that someone at emgo said to bore at .035 clearance. Going from memory, I thought their instructions called for .04 - .05?
Posted By: Shane in Oz

Re: Another boring question. - 04/14/19 10:47 pm

Are we missing a 0 after the decimal point, gents?
Posted By: koan58

Re: Another boring question. - 04/15/19 12:20 am

If they are 3.5 thou you have a heck of a careful break in to do, lots of short runs.
Definitely get them measured, then you know.
The boring fellow should be happy to adjust with a little honing if needed, considering he didn't follow your instructions.
Alternatively, he should accept responsibility for a seizure during normal break in.
Posted By: Tracey Spear

Re: Another boring question. - 04/15/19 2:30 pm

Yes, not that it detracts from the conversation. I'm more concerned about 0.0035 piston clearance and whether to build as is or send back to the machinist than a missing 0 in a forum.
Posted By: Shane in Oz

Re: Another boring question. - 04/15/19 8:39 pm

As per the others, .0035" seems too tight. I'd send it back for another run-through with the hone to take another half a thou out.
Posted By: Ignoramus

Re: Another boring question. - 04/15/19 11:01 pm

Originally Posted by Shane in Oz
As per the others, .0035" seems too tight. I'd send it back for another run-through with the hone to take another half a thou out.


3.5 is too tight for certain
Posted By: Richrd

Re: Another boring question. - 04/17/19 12:31 pm

What concerns me is the machinist not listening to what the customer wants.

I know one shop that had a few tighten up at .004 when they started using emgos so had to go to .0045.
.035 would be fine in a 250, maybe.
Posted By: Tracey Spear

Re: Another boring question. - 12/03/19 6:19 am

My apologies everyone, life kept me away from the project and forum for a few months.

Rather than speculate, I took the cylinders and pistons to a different machinist. He measured the Emgo + .020 pistons:

1 = 2.967, 2 = 2.966

The Bores measured:
DS = 2.9745, TS = 2.974

If Clearance = Bore - Piston. Match Piston 1 to DS and Piston 2 to TS
Then DS Clearance = 0.0075 and TS Clearance = 0.008

He thought that was way too loose, but he doesn't work on old brit bikes.

The A65 General Data for Piston, Clearance, top of Skirt is 0.0094 - .00109.

If I'm reading it right, and if machinist measured right, that leads me to think the holes are a little tight.
But General Data spec is based on '69 pistons.
The original machinist said he contacted Emgo and bored to their clearance specs because modern pistons don't expand as much.

I'm confused. I have an automotive machinist who thinks the pistons flop around, and spec's telling me the holes are too small.

Or I'm totally misunderstanding piston clearance.
Posted By: triton thrasher

Re: Another boring question. - 12/03/19 7:20 am

Measure clearance near the bottom of the skirt.
The commonly available modern pistons such as Emgo do expand as much as the originals.
Posted By: NickL

Re: Another boring question. - 12/03/19 7:22 am

1.5 thou per inch of bore is typical, and always a safe bet. (measure at bottom of skirt this allows for taper on the piston if there is any)
Twice that for iron rings is about right too.

That is for cast pistons, it can be up to double that for forged ones. In either case the manufacturer will advise you.
Posted By: Allan Gill

Re: Another boring question. - 12/03/19 8:21 am

Not sure which manual you have that from. If you look at the early A65 WSM it states around 2-3 thou which is tight. Later manuals say more.
You want at least 4-4.5 thou.

Or as nick says 1.5 thou per inch of bore
Posted By: Mark Parker

Re: Another boring question. - 12/03/19 11:43 am

The manual I have handy says .0039"- .0054". However, I've seen oversize pistons that gave instructions on the box to bore to 'X' size, Hepolite I think, if you measured the pistons and compared that to 'X' they would have much more clearance, like .006-.007"+ probably why they didn't tell you the actual clearance on the box.

Early A65 pistons are different, not as heavy with less valve cutaway depth. If any pistons used had split skirts they would run much tighter. Also stock A65 pistons are thinner than oversize pistons. Over size are basically the same casting machined less so +.060" pistons can be pretty heavy. Which may effect the clearance needed.
An A65 +.040" piston and pin is 425g, a T140 340g, a cast JP 79mm B44 422g, 79.5mm Omega cast 396g, an 80mm JE forged 372g. Ed V's A65/A70 pistons are like the B44 versions, some of the very best, forged by JE and designed to run small clearances, .0035-.004" safely in an iron cyl or .002-.0025" in nicasil alloy cyl. Rings seal better when the piston is not rattling around.

.0075" or .008" is pretty big if the pistons were measured near the bottom of the skirt. JE pistons are measured up a bit from the bottom of the skirt.
Posted By: Tracey Spear

Re: Another boring question. - 12/03/19 5:36 pm

Allan: My reference is the BSA Workshop for 69-70 A65, A50.

Everyone else attempting to help a baby engine builder.
I don't have a 3" mic, but with Vernier Calipers I can get within one digit of the machinist measurement.
he measured the pistons at 2.966 and 2.967. I got 2.967 and 2.968, bottom of skirt, perpendicular to pin. Close enough I think to confirm his values.

If I then accept his bore measurement that still leaves me with Clearance DS = 0.0075 and TS = 0.0080

The manual states Clearance, Bottom of Skirt, on Major Axis is 0.0039 - 0.0054.

Which confirms the machinist_2's opinion that the bore is excessive and the piston too loose.

Which means the original machinist_1 totally f'd up the job.
Which means I have to now order + 0.040 pistons, and re-bore.

Correct?
Posted By: triton thrasher

Re: Another boring question. - 12/03/19 5:58 pm

8 thou clearance might work.
Posted By: kommando

Re: Another boring question. - 12/03/19 6:05 pm

The 4 to 5 thou is the starting point, you are allowed another 5 thou of wear, so 8 thou still leaves you 2 thou to use. That assumes the pistons expand like the originals, f they expand more then 8 may be a good starting point and you have 5 thou to go on wear.
Posted By: triton thrasher

Re: Another boring question. - 12/03/19 7:10 pm

Impress your friends with your “racing clearances.”
Posted By: Allan Gill

Re: Another boring question. - 12/03/19 8:05 pm

Originally Posted by Tracey Spear
Allan: My reference is the BSA Workshop for 69-70 A65, A50.

Everyone else attempting to help a baby engine builder.


Sorry if I touched a nerve, this wasn’t intentional.

However you don’t need a 3” calliper. (Most vernier callipers aren’t accurate enough and the same measurement twice with cheap digital ones can give different readings each time. The biggest micrometers I have are 2” so not big enough for a piston, but if you have feeler gauges or pin gauges in small enough sizes then you will get more use and accurate enough readings from this. I use feeler gauges about an inch or so up the skirt and at the side of the piston/skirt. This will be more than enough and give you a better result than measuring both barrel and
Piston and calculating the gap.
Posted By: Tracey Spear

Re: Another boring question. - 12/03/19 10:44 pm

My apologies Allan. Read back like that, it comes off as quite rude. That wasn't my intention, your reputation is one of always trying to help. All I intended to do was let you know what my source was since you had mentioned an earlier reference. That came off really bad.

Back to my confusion. I think there is something fundamental I'm missing here. Keep in mind, I've never done this before.

The manual states Bottom of Skirt clearance range is 0.0039 - 0.0054.

The manual also states Top of Skirt clearance range is 0.0094 - 0.0109.

Given that, by the math, I have Bottom of Skirt clearance of 0.0075 and 0.0080.

How do I translate Kommando's statement from above " The 4 to 5 thou is the starting point, you are allowed another 5 thou of wear, so 8 thou still leaves you 2 thou to use."

Is "The 4 to 5 thou is the starting point" the Bottom of Skirt clearance?

"you are allowed another 5 thou of wear" is that wear the difference between top of skirt and bottom of skirt 0.0054 to 0.0109?

"so 8 thou still leaves you 2 thou to use." Would you build an engine like that? Seems to me that would be like installing a brand new, worn out piston.
Posted By: Mark Parker

Re: Another boring question. - 12/03/19 11:00 pm

Don't worry about the top of skirt clearance that is machined when the piston is made. The top of the piston gets hotter and expands more in use. The bottom of skirt clearance is what you have the bore finished in relation to, and that's front to back measurement, pistons are oval on the skirts narrower on the sides. You can use 7-8thou clearance. Check the ring end gap width and see what they are at the top middle and bottom of the bore, they will show if it's the same all the way through.
Posted By: Tracey Spear

Re: Another boring question. - 12/03/19 11:32 pm

Back to my problem. I want to verify whether Machinist_1 who delivered a good +020 bore. I took the completed cylinder and pistons to Machinist_2 who balanced the crank for me. His measurements seem to indicate that this brand new bore is at the far range of worn out. What's a neophyte engine builder to do. Slap it together, knowing that seizure during break in will be very unlikely, pistons may rattle around a bit, and another top end job will be in the future.

Or

Suck it up, order + .040 pistons and effectively start over.

What do you more experienced builders recommend?
Posted By: kommando

Re: Another boring question. - 12/03/19 11:34 pm

Quote
How do I translate Kommando's statement from above " The 4 to 5 thou is the starting point, you are allowed another 5 thou of wear, so 8 thou still leaves you 2 thou to use."

Is "The 4 to 5 thou is the starting point" the Bottom of Skirt clearance?

"you are allowed another 5 thou of wear" is that wear the difference between top of skirt and bottom of skirt 0.0054 to 0.0109?

"so 8 thou still leaves you 2 thou to use." Would you build an engine like that? Seems to me that would be like installing a brand new, worn out piston.


If the figures you quote are correct then your 20 thou over rebore is 1/2 worn out already, but go to 40 thou over and you get no running at 20 thou at all and waste a good set of 20 thou over pistons instead of running them for 1/2 their life and pay for 2 rebores. What is your annual mileage going to be, not a lot going by most.
Posted By: Allan Gill

Re: Another boring question. - 12/03/19 11:40 pm

No worries at all Tracey, I think I may have miss read your previous post when I put my reply in and not correctly noted that you referred to the top of the bore clearance.

I could be wrong here with this next statement so someone please pull me up if I am.

There is more piston rock at the bottom of the stroke than at the top, it’s greater again with a long stroke motor with a short rod and lessens with a long rod motor with a short stroke. By measuring the wear at the bottom of the bore you have a better idea of actual wear at the most worn point. On a used motor I would compare the gap at side Of the piston to that front or rear of the piston skirt.

Kommandos Point of having a further .005” on top so a total of 0.010” gap is how far you can go if you measure the bore of a Used motor and determined if it needed the next oversize or not.

For the amount of use most of our bikes get now, you would most likely be fine at a larger gap. And it last 10 years....

Personally I’d get the feeler gauges out and check the bore that way before doing any more.

I was looking on U tube to find a decent example, instead in typical u tube fashion I got distracted but found these videos interesting none the less, the first one with his plastic engine cut away gives you a good idea of what is happening inside the motor, the second one is interesting in other ways but but seemed to have me chuckling anyway.

Video 1

Video 2
Posted By: Mark Parker

Re: Another boring question. - 12/04/19 1:08 am

If you have rings put a compression ring in the cyl and see what the bore size is at different points. Measuring its end gap with feeler gauges. It's not uncommon for someone honing, to finish the bore, honing more from the top, this will show up pushing the ring down to different spots and measuring the gap with feeler gauges. Without a bore mic do not expect to measure the bore accurately nor the piston without a micrometer.

So try what Allen suggests and measure using a feeler gauge between piston skirt, front or back by seeing what size feeler gauge fits. Put the gauge mostly in the bottom of the cyl and slide the piston in over it, you will see when the bottom of the skirt no longer has play and there is a little resistance, then see what size feeler gauge does that. Start with .004". Its only the bottom of the skirt you are measuring and you are not trying to lock it up just till it eliminates the play and has a little resistance.
Posted By: triton thrasher

Re: Another boring question. - 12/04/19 9:30 am

Bores don’t wear fast with the oil they sell nowadays. Having less wear left on your bores because of the bigger clearance really doesn’t matter.

You may get more piston slap noise than others, you may not, but that doesn’t matter much either.

You may as well stop quoting us the stuff about the clearance at the top of the pistons. It’s the clearance low down the skirt that matters here.
Posted By: Hillbilly bike

Re: Another boring question. - 12/04/19 12:26 pm

I wouldn't spend the money to have cylinders bored if the pistons won't be the correct clearance afterwards....Do it right unless it's just a beater.....
Posted By: Richrd

Re: Another boring question. - 12/04/19 3:44 pm

Tracy, don't worry so much. there are things we've learned from experience and things that have changed from when these machines were new and the books were written. All the above guys are saying basically the same thing but to you it sounds confusing.

IF mach 2 measured the bores correctly, and I might question that, the clearance will be a bit big, but you may never wear out the life you do have. put it together and run it.

Next time, talk to local motorcycle shops for recommendations on who to use.

YOU TELL THEM WHAT CLEARANCE YOU WANT AT ONE HALF INCH UP FROM BOTTOM OF SKIRT. not the other way around. 3.5 is too tight. the proper way to measure clearance is with a bore guage which measures the difference between the piston and the bore. there is a chance mach 2 introduced error to his measurements by measuring the bores as a separate step.
Posted By: Fullminator

Re: Another boring question. - 12/04/19 11:09 pm

I would say the minimum clearance bore to piston would be .0045 Anything less will risk a seizure. The seizure will happen after the bike has been sitting unridden for a while and the weather is on the cold side, and within the first 5-10 miles or so. At the same time .008 is too loose. It will work, but when cold the piston will slap about and you will think you have a threshing machine instead of a motorcycle. This can be minimized by using straight 40 or 50 oil, but that is another subject.
The bore clearance at the top of the piston is immaterial. Measurements should be taken around the bottom of the skirt and at the thrust face, not on the wrist pin side. The only reason to measure the bore all around and up and down is to ensure the bore is straight. This is something many machine shops are incapable of achieving, since the majority of them are most adept at boring 4 cylinder Toyotas and Fords.
If your plans for the bike are like most of us-limited use- then the .008 will work and you may not be bothered with it. The other point is why do you have two completely different measurements of the same cylinder and pistons? Someone is having trouble.
From your posts, I suspect your machinist #1 did a crappy job. Believe me, I have been down this road, just as you are now. I am forever amazed at the sub quality work and excuse filled apologies from those that are in the business and professed to be professionals.
Fullminator
Posted By: Nick H

Re: Another boring question. - 12/07/19 3:15 pm

Allan, the second video link you provided doesn't work.
Posted By: Allan Gill

Re: Another boring question. - 12/07/19 3:20 pm

Originally Posted by Nick H
Allan, the second video link you provided doesn't work.


Your right, it doesn’t. Sadly as I stumbled across the video and have little chance of finding it again.

Ive checked the post and the URL is complete so it could be that the video no longer exists
Posted By: Hugh Jorgen

Re: Another boring question. - 12/07/19 3:43 pm

Posted By: JakeH

Re: Another boring question. - 12/09/19 12:05 pm

Tracy, if your measurement is really .0075-.008 thats pretty big.. to save the pistons and Cylinders i would look at coating the pistons. Look up Line 2 Line coatings, He can coat up to .010 total. In your case i would have him coat .0035 total, and put in together once your sure the bore is honed to 150-180.. Now first thing that will happen is ppl are going to say the coating wont work, ect. We have HAD many pistons coated, and taken several apart to check, and u cant see the coating has even touched. Several times a year someone brings something really obscure in to hone that he cant easily get pistons for, hone them, he coats pistons and back running!! it works really well!! On BMWs the running spec is .001-.0025, wear spec .005 . you can hone them to .004-.005 whatever cleans up, and coat the pistons to have a very quiet engine. Good luck, if you do try the coating, let us know your results... Good or bad, it will help someone in the future!!
Posted By: Allan Gill

Re: Another boring question. - 12/09/19 12:34 pm

Many new pistons are ceramic coated, either whole skirt or in portions. Often the crowns are too. It’s great for thermal insulation and tougher than old boots. I doubt the area between the ring lands would get done though, and you might try looking for some +0.010 oversize rings to what you have already to try and eliminate having big ring gaps
Posted By: triton thrasher

Re: Another boring question. - 12/09/19 12:57 pm

Originally Posted by Allan Gill
Many new pistons are ceramic coated, either whole skirt or in portions. Often the crowns are too. It’s great for thermal insulation and tougher than old boots. I doubt the area between the ring lands would get done though, and you might try looking for some +0.010 oversize rings to what you have already to try and eliminate having big ring gaps


Big ring gaps don’t do any harm.

Using rings made for a bigger bore might prevent rings from sealing against the bore.
Posted By: Allan Gill

Re: Another boring question. - 12/09/19 4:46 pm

I was thinking that part would be a point of discussion, If the gap is around 0.008" then you 5 thou up on a +0.010 oversize. thats if rings are available at .010" over I don't think it will be too much problem in a sloppy bore, if .020" was the next size up then I think you stand a good chance of breaking the ring(s)
Posted By: Ignoramus

Re: Another boring question. - 12/10/19 1:07 am

back in the day there was a process called "micro-peening" it was used to expand up the skirt on a marginal piston ...........they could "grow" them up to 10 thou or so ...i have no idea if anyone still does that , but i know i had it done on a set of A65 pistons and it added quite a few miles to the bore/piston life before i finally had to go the re bore and new pistons route

it is not the same process as what is called shot peening ..........micro peening was done from the inside of the skirt.

might be worth a try to find if anyone still can do that or even knows what you are talking about
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