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Carefully take your rings off the piston. Put them in the bore and square them up using the piston then check the end gaps with feeler gauges


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The piston looks pretty fresh + 0.020" , the top ring ( may be dirty oil marks ) has strange black lines on it, the barrel shows a lot of scoring again maybe some oil marks, there is a witness mark from a rusted seizure, how clean was it?
Andy is spot on with his ring remarks. Could be all the trouble was the head seal.

https://s3.amazonaws.com/hires-avia...5e84858c-5d20-4ec2-aac4-21088ae2253c.jpg
I would have the barrel honed and fit fresh rings, gaskets and build it up, the scoring wont really go away and you will burn a little oil but once the rings seat it will be useable.
Look out for a plus 40 piston and consider a fresh bore in the future.

Using a caliper to get the bore size isnt that great , most wear takes place about an inch down the bore, the calipers will register on the unworn part at the top, check the skirt again about one inch down the bore from the top with feelers compare with the bottom.

The HC may mean High Compression, you can check this during assemby, measure the volume of oil the combustion chamber contains at TDC by filling through the spark plug hole with a calibrated syringe, Say 40 cc Chamber , Swept vol , 350 cc. 350 + 40 = 390 total volume, divide by 40 = 39/4 roughly 9.9 to one.
After the test pull the head off again and clean up the oil, you dont want a bore full of oil on start up, the rings should be almost dry to seal correctly.
The head joint should lap up easily, iron heads are usually pretty stable.

I take it you havent done many miles yet, the best thing to do would be ride it, change the oil a few times during the break in and enjoy (hopefully) , then you will know what needs attention.

Last edited by gavin eisler; 01/23/16 9:41 pm.

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The black lines are my greasy fingerprints. There are oil marks and seizure marks, but nothing that I can feel. To the touch, it's smooth as glass.

I'll be checking the fit of the rings tomorrow morning. I haven't ridden it more than ten miles. When I bought it, I changed all the fluids, and adjusted the brakes, test drive it, and found that the clutch was dragging badly. I pulled it apart and fixed it, but took the gearbox cover off and find a bad Permatex seal, as well as a home made kickstarter return bumper glued in with Permatex that had started turning to goo in the case. I cleaned all that out and installed a new bumper and proper gasket and then found the timing was off and the mixture was still too rich after I fitted new gaskets and jets to the carb...and now we're here. I think I'll probably just fit a fresh carburetor to eliminate that as a fault source too.

Whoever put this bike together was in a hurry and too cheap to pay attention to the little things; almost every gasket was a poor reused one glued with too much Permatex, and a lot of the screws were sealed with the stuff too. The guy I bought it from didn't care enough about the bike to find any of the problems I have, or found them, but didn't want to deal with them. I'm trying to justify fixing this bike, but I paid for a running, riding bike and got a barely running rebuild instead. So I'm thousands in the hole already, and now we're talking about machining...


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I added some extra photos to that album that you can link to on the previous page. If you'll notice in one of the pictures the rings are stamped 040 but the piston is stamped 020


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Put each ring in the barrel, one at a time.
Square them off with the bottom of the piston same as if your were going to measure the ring gap only this time hold them up to the light.
The rings should touch for their full circumference, except at the gap, naturally.

If you can see a lot of light under the rings the moron bastard has filed down an oversized ring to fit.
A real cheap & nasty trick but it did get you back on the road, for a while.


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These rings have been corroded, +40 rings on + 20 piston , not good.
Do you own feeler gauges? Your bore is probably badly worn, i dont trust your caliper readings.
The DPO has left you a bodged up mess. Best of luck.


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Originally Posted by gavin eisler
These rings have been corroded, +40 rings on + 20 piston , not good.
Do you own feeler gauges? Your bore is probably badly worn, i dont trust your caliper readings.
The DPO has left you a bodged up mess. Best of luck.
Thanks. Just waiting on coffee then I'll be checking the rings for fit. Pictures will be added, naturally.


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Originally Posted by BSA_WM20
Put each ring in the barrel, one at a time.
Square them off with the bottom of the piston same as if your were going to measure the ring gap only this time hold them up to the light.
The rings should touch for their full circumference, except at the gap, naturally.

If you can see a lot of light under the rings the moron bastard has filed down an oversized ring to fit.
A real cheap & nasty trick but it did get you back on the road, for a while.
Well, as I said before, whoever threw this thing together was cheap and in a hurry.


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All three rings were squared off with the piston about halfway down the bore and checked with a feeler gauge. No light was visible around any of them, and I could not get a .0015" feeler between the ring and bore.

[Linked Image]


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Measure the end gap,it looks too big


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Originally Posted by Andy Higham
Measure the end gap,it looks too big


Scraper ring: .032
Compression rings: .046

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Put the rings near the bottom of the bore and measure them again. The rings are definitely fukced, if the gap when near bottom of bore is much less then the bore is also worn. Max gap is around 0.010-0.015


BSA B31 500 "Stargazer"
Greeves 200 "Blue Meanie"
Greeves 350
Greeves 360
Suzuki GSX1100 EFE "Sorcerers Apprentice"
GM500 sprint/LSR bike "Deofol"
Jawa 500 "Llareggub"
Aprilia RSV Mille "Lo Stregone"
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Originally Posted by Andy Higham
Put the rings near the bottom of the bore and measure them again. The rings are definitely fukced, if the gap when near bottom of bore is much less then the bore is also worn. Max gap is around 0.010-0.015


At 3/4" from the bottom of bore:

Scraper ring: .024
Compression rings: .040


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You will may just get away with the bore. You definitely need new rings. Personally I would get a rebore and new piston/rings/pin/circlips (especially with the corrosion damage). Then you know it will be right

Last edited by Andy Higham; 01/24/16 2:04 pm.

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Originally Posted by Andy Higham
You will may just get away with the bore. You definitely need new rings. Personally I would get a rebore and new piston/rings/pin/circlips (especially with the corrosion damage). Then you know it will be right
Should I just get it bored for .040 and get the same sized piston and rings then? I've seen cylinder hones before, but I'm not sure if you'd recommend them. I don't know anyone locally that is familiar with these bikes that I'd trust with the work, and I hesitate to mail the barrel anywhere.


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Honing is only to rough out the bore so the rings bed in, it not for taking a worn +20 thou bore out to +40, it needs boring on a machine and then honing to give the crosshatch pattern so the rings bed in to suit the bore. If you hone out a worn bore which is belled out in the middle then it stays belled out when it should be straight.

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Originally Posted by kommando
Honing is only to rough out the bore so the rings bed in, it not for taking a worn +20 thou bore out to +40, it needs boring on a machine and then honing to give the crosshatch pattern so the rings bed in to suit the bore. If you hone out a worn bore which is belled out in the middle then it stays belled out when it should be straight.
Gotcha. A friend of mine who got his KZ750 machined is sending me the info for a local shop he trusts so I'll give them a call tomorrow and see what the damage is. I'm correct that I'll need to go to 040 and get the appropriate piston and rings though, right?


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I'm not sure if this is helpful or the best way of doing this, but if I lie the barrel sideways and pave the piston inside without the rings and take a feeler gauge measurement at the 12 o'clock position, here are my readings at the top of the barrel and the bottom:

[Linked Image]

[Linked Image]


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Make SURE to get the piston/rings before you have anything cut.

Make SURE the manufacturer supplies the needed cylinder/piston clearance, and ring gap. These numbers do change from manufacturer to manufacturer.

Members here may have experience with several brands but every brand is different. The OEM specs mean nothing today. Materials have changed so make SURE you have the parts in hand, and the numbers on paper before you head to the machinist, and make SURE he can/will do exactly what you need done.
Lots of machinists will rely on what they know or how there machine performs, and disregard what you ask/spec out.

It's obvious that your bike was resurrected to make a fast sale. You have the opportunity to correct all the poor work, and make a very nice bike out of it so attention to the smallest detail is important now.

Now, I am curious as to the condition of the bottom end. Since the bike sat long enough to suffer a rusted bore, chances are not good that the bottom is pristine.
You might just want to consider a total rebuild.

What a scope... From poor jetting to a total tear down.
Like a gal with make up, wig, padded bra, girdle, high heels, false teeth, contacts. You have no idea what you might have under all that.

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You nearly got the piston clearance right, pistons are wider at the bottom than the top, clearances must be taken round the lower skirt not the top ring area, insert the piston upside down and check the skirt to max wear zone about an inch from the head joint.
before you bore out, get the new oversize piston, the machinist needs the new piston in his sweaty palms to get the fresh bore clearance correct.
Zombie makes a good point, at this point its " what you dont know cant hurt you, but later it might bight your ass"


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Originally Posted by Zombie

What a scope... From poor jetting to a total tear down.
Like a gal with make up, wig, padded bra, girdle, high heels, false teeth, contacts. You have no idea what you might have under all that.
I know what I paid for her, that's for damned sure.

I've got an engine marked HC with a 66-483 connecting rod. My spares book doesn't list this rod though, and I'm not sure if it's long or short. Anyone got a clue?

As for the bottom end...

The clutch plates needed a lot of grinding, but the bearings were good and is the crank sprocket and primary chain. The timing gear train looks great, and the connecting rod is solid. There's play side to side, but up and down nothing at all. Granted, speculation is no replacement for solid facts, but I'm uncertain whether it's necessary at this point. But i didn't think any of this would be, soooo...


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Originally Posted by gavin eisler
You nearly got the piston clearance right, pistons are wider at the bottom than the top, clearances must be taken round the lower skirt not the top ring area, insert the piston upside down and check the skirt to max wear zone about an inch from the head joint.


And it's clearance front to back.


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You should take measurements from the thrust faces of the bore IE; front & back of bore as it sits on engine. Judging by the rough measurements you have given I would say buy a new +040 piston and get the barrel bored to manufactures spec. I have just rebored a BSA single which when measured at the top & bottom thrust faces looked in spec but when measured with a internal micrometer is was 11 thou oversize in centre of bore. I would take your barrel to engineering shop to get a accurate measure. Also make sure you get the right piston as there are ones for long & short conrods! yours should be long rod but best to check as someone might of changed rod/crank. Dave


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The number on the rod is not a part number its a casting number, they are sometimes numerically close to the part number. Generally a con rod part number may include the complete big end and matching crank pin, hence the difference.


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Originally Posted by gavin eisler
You nearly got the piston clearance right, pistons are wider at the bottom than the top, clearances must be taken round the lower skirt not the top ring area, insert the piston upside down and check the skirt to max wear zone about an inch from the head joint.
before you bore out, get the new oversize piston, the machinist needs the new piston in his sweaty palms to get the fresh bore clearance correct.
Zombie makes a good point, at this point its " what you dont know cant hurt you, but later it might bight your ass"
The gap at the max wear zone at an inch from the top is .0012" and it's the same at the bottom.

And that's .0012 front and back.

Last edited by SpencerAZ; 01/24/16 5:55 pm. Reason: correction to data

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