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#334458 - 09/20/10 10:44 pm What is the correct piston clearance?
yellow_cad Offline
BritBike Forum member

Registered: 12/08/06
Posts: 448
Loc: Folsom, CA
When my 70 Bonny started smoking all of a sudden, I pulled the barrels and found that the pistons were considerably undersized and badly rubbing the bores. There is no serious damage to the cylinders at this point. When measured one piston had .015 clearance at top and bottom of skirt and the other had .013 at the top and .011 at the bottom. I am measuring the piston skirts perpendicular to the pins 1/2 inch below the rings for top and 1/2 inch above the bottom of the skirt for the bottom.
The manual shows correct piston clearance to be 8 1/2 to 10 1/2 for the skirt top and 4 1/2 to 6 for the bottom. If the piston measures the same at the top and bottom of the skirt, what is the best clearance? Seems like it has to be a compromise. My current bores are 2.857 but I will be honing these just enough to create a good surface (I assume that will only take me 1 or 2 thou bigger). The replacement pistons I have measure 2.852 at the skirt top and bottom. I know I don't want too little clearance but the piston I was running with .011 bottom and .013 top were galled pretty bad from rubbing the cylinder walls. If I know the correct clearance, I could get there by honing. Thanks for any help on this.


Edited by yellow_cad (09/21/10 1:17 am)
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1970 Triumph Bonneville
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#334510 - 09/21/10 8:59 am Re: What is the correct piston clearance? [Re: yellow_cad]
RetroRod Offline
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Registered: 01/19/09
Posts: 701
Loc: QLD, Australia
Pistons are tapered bottom to top. .0046" to .0061" at the bottom of the skirt. RR
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#334528 - 09/21/10 1:00 pm Re: What is the correct piston clearance? [Re: yellow_cad]
John Healy Offline


Registered: 03/24/05
Posts: 8419
Loc: Boston, Massachusetts
The concept of "Piston taper" has changed a lot with the introduction of modern computer controlled production machinery. The days of a straight taper from the bottom of the piston skirt, where the piston would have the least clearance in the bore, to the area just under the oil ring where you would have 0.003," or more, additional clearance is disappearing. What you will find today is the skirt has little, if any detectable taper from top to bottom.

What the skirt has is a nearly straight skirt and a geometric curve providing the needed additional clearance just under the oil ring. If you could measure it you would see that it has as much clearance in the area just under the oil ring as the original linear tapered skirts.
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#334552 - 09/21/10 4:18 pm Re: What is the correct piston clearance? [Re: yellow_cad]
yellow_cad Offline
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Registered: 12/08/06
Posts: 448
Loc: Folsom, CA
So John, what is the best clearance for todays pistons since the manual clearances are for the older style tapered pistons?
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1970 Triumph Bonneville
1973 Commando Interstate

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#334554 - 09/21/10 4:31 pm Re: What is the correct piston clearance? [Re: yellow_cad]
RHall-HCV Offline
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Registered: 07/02/07
Posts: 253
Loc: Charlotte NC
what type of pistons are they? I have some emgo replacements in a bike at .002 that work great and some MAP forged ones in at .003. Too much clearance will just make noise and create blow-by due to increased ring end gap.
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#334557 - 09/21/10 4:57 pm Re: What is the correct piston clearance? [Re: RHall-HCV]
triton thrasher Online   content
BritBike Forum member

Registered: 08/07/01
Posts: 4285
Loc: scotland
Originally Posted By: RHall
what type of pistons are they? I have some emgo replacements in a bike at .002 that work great and some MAP forged ones in at .003. Too much clearance will just make noise and create blow-by due to increased ring end gap.


Piston to bore clearance has no direct effect on ring end gap, except that smaller pistons come with smaller rings.

You absolutely cannot run a Triumph 650 or 750 twin with common types of piston at 2 thou clearance. It will seize. I think 3 thou is dangerously tight too. The service limits are in the manual and/or in the piston maker's info.
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#334558 - 09/21/10 4:59 pm Re: What is the correct piston clearance? [Re: yellow_cad]
yellow_cad Offline
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Registered: 12/08/06
Posts: 448
Loc: Folsom, CA
The pistons are JCC made in Taiwan. I will be using English or American rings. I had read a few warnings about too little clearance but I do know that the previous .011 I had was too loose. After I hone a little I should be at about .006 or .007. Does that sound acceptable?
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1970 Triumph Bonneville
1973 Commando Interstate

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#334560 - 09/21/10 5:34 pm Re: What is the correct piston clearance? [Re: yellow_cad]
btour Offline
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Registered: 12/18/06
Posts: 3494
Loc: Massachusetts, U.S.A.
Hi YC,

I am having trouble understanding what you intend to do? Are you going to hone the cylinders and go to the next size piston? If so, how are you arriving at those figures? How much do expect to hone off? And are you adept at doing that?
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#334563 - 09/21/10 6:05 pm Re: What is the correct piston clearance? [Re: yellow_cad]
John Healy Offline


Registered: 03/24/05
Posts: 8419
Loc: Boston, Massachusetts
As long as the material the piston is made from and the physical design is the same, the clearances will not change with the way they generate the additional clearance required in the area just below the oil ring!

Hepolite discovered that with modern machining techniques and machinery they could get the required extra clearance under the oil ring by keeping the face of the skirt nearly straight and making, what was called the "taper," in the last 1/4" of the top of the skirt. This has confused more than one Triumph mechanic when they read the Triumph specifications for piston "taper" and measure a new set of pistons and see only about 0.001" of "taper."

The problem comes when some one without the proper equipment, or knowledge, takes measurements at the top of the skirt and sees 0.001" "taper" and what is really there is 0.005" or more. In what is the width of the micrometer face, the side of the piston curves in to create the needed clearance.

The current crop of after market pistons (specifically JCC brand aka Emgo, Careful, Harris, etc) use the same aluminum, and physical design, as the original Hepolites. Thus the bottom and top clearances are the same as Triumph specified in there literature for the Hepolite pistons used since the mid-sixties.

Where I have seen a lot of problems is with is old timers and people who read old service literature not understanding that Triumph pistons went through a change in the mid-sixties. Gone is the 0.0035" clearances used with the lo-ex pistons.

This is where we get into trouble. The old timers, and those referring to older literature, often use .0035" specified for the early lo-ex (low expansion aluminum) 650 pistons. So we often end up with 0.0035" clearance where we need 0.004" to .007" specified for the later Hepolite, or JCC units.

Listen, this isn't something one can write about between phone calls, putting out fires and pulling orders. Piston design is a complicated subject and takes a lot more than a couple of quickly jotted paragraphs to fully under stand.
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#334565 - 09/21/10 6:09 pm Re: What is the correct piston clearance? [Re: yellow_cad]
triton thrasher Online   content
BritBike Forum member

Registered: 08/07/01
Posts: 4285
Loc: scotland
Originally Posted By: yellow_cad
The pistons are JCC made in Taiwan. I will be using English or American rings. I had read a few warnings about too little clearance but I do know that the previous .011 I had was too loose. After I hone a little I should be at about .006 or .007. Does that sound acceptable?


In the absence of any experts right now, [edit: John's here] I believe that's within limits, just loose-ish (which is safer than tight-ish). I'd call 0.005" ideal.

I don't know how much you're taking off but don't try to do with a hone what should be done with a boring bar.


Edited by triton thrasher (09/21/10 6:11 pm)
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#334569 - 09/21/10 6:39 pm Re: What is the correct piston clearance? [Re: yellow_cad]
John Healy Offline


Registered: 03/24/05
Posts: 8419
Loc: Boston, Massachusetts
Quote:
Piston to bore clearance has no direct effect on ring end gap, except that smaller pistons come with smaller rings.


While we are on the same page with most of this I think you miss-read Mr. Hall's statement. I have to agree with Mr. Hall on this one. For a given piston diameter you will get approx. .003" change in ring end gap for each .001" increase or decrease in piston clearance.
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#334577 - 09/21/10 7:49 pm Re: What is the correct piston clearance? [Re: John Healy]
triton thrasher Online   content
BritBike Forum member

Registered: 08/07/01
Posts: 4285
Loc: scotland
Originally Posted By: John Healy


While we are on the same page with most of this I think you miss-read Mr. Hall's statement. I have to agree with Mr. Hall on this one. For a given piston diameter you will get approx. .003" change in ring end gap for each .001" increase or decrease in piston clearance.


I don't think I misread so much as miscommunicated. To be precise, the end gap changes with increase or decrease in cylinder bore size, for a given piston ring, as the ring expands until stopped by the cylinder wall. Not that it matters and I hope I'm not confusing the guy with the original issue.
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#334616 - 09/21/10 10:46 pm Re: What is the correct piston clearance? [Re: yellow_cad]
HawaiianTiger Offline
BritBike Forum member

Registered: 06/25/07
Posts: 3415
Loc: Maui Hawaii
I've been using .0035 clearance for years now without a single siezure. I wouldn't recommend this to one of those who like to ride em like they're stolen, though. I've been trying to make my engines as quiet as a Norton for many years and one of the ways is to use the smallest clearance possible. I instruct the machine shop to set the clearances NO LESS than this spec. I'm not lucky here. I just make sure that the carb is a bit rich and the timing is very accurate then break in the motor is stages keeping mind that heat is the enemy. If your engine is making tink tink sounds and smells like burning oil after a ride then you're riding it too hard and risk siezure.
Bill
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#334623 - 09/22/10 12:15 am Re: What is the correct piston clearance? [Re: triton thrasher]
yellow_cad Offline
BritBike Forum member

Registered: 12/08/06
Posts: 448
Loc: Folsom, CA
Originally Posted By: triton thrasher
Originally Posted By: yellow_cad
The pistons are JCC made in Taiwan. I will be using English or American rings. I had read a few warnings about too little clearance but I do know that the previous .011 I had was too loose. After I hone a little I should be at about .006 or .007. Does that sound acceptable?


In the absence of any experts right now, [edit: John's here] I believe that's within limits, just loose-ish (which is safer than tight-ish). I'd call 0.005" ideal.
I don't know how much you're taking off but don't try to do with a hone what should be done with a boring bar.


My current cylinders have no taper or ridge and look as though they were recently bored to their current size (I've only had the bike a short while and no good info from the PO). My problem was that someone installed pistons with way too much clearance. The cylinders do have some very slight longitudinal scratches (just barely able to feel roughness with the fingernail) in a couple of areas so I was only going to hone to improve the surface. I intended to use large three fingered cylinder hone with plently of lube and uniform movement to get a better surface and not to materially change the size. If it is a better idea just to leave the bores as they are, with my clearance at .005, I'm fine with that but I thought I understood that the new rings would do better with a honed surface. Should I hone it or just leave it as it is?
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1970 Triumph Bonneville
1973 Commando Interstate

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#334629 - 09/22/10 1:44 am Re: What is the correct piston clearance? [Re: yellow_cad]
RHall-HCV Offline
BritBike Forum member

Registered: 07/02/07
Posts: 253
Loc: Charlotte NC
The pistons currently being run at .002 are JCC with Rikon rings. These are cast pistons. You can run them tighter than forged. If you are planning on honing to the next size, which i believe would be .020 over your current size, i would suggest boring it because if it is so worn out that you are having to only hone a few thou than the cylinder is no doubt out of round and a few thou will not fix the side to side clearance. TT, for every .001 bore size the ring end gap increases nearly .004. Therefor the tighter you can run the pistons the smaller endgap you will have. For these bikes you should have around .010 top, .012 second. Also the more clearance you run the more rock you get in the piston and rings and the more you twist you put the rings in the more oil and compression goes past them. With a good engine builder and the proper break in procedure, you can run JCC pistons at .002. I promise! I would try not run any looser than .003 if your bore will clean up. Hope it does.
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HCV MOTORSPORTS

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#334642 - 09/22/10 3:50 am Re: What is the correct piston clearance? [Re: RHall-HCV]
yellow_cad Offline
BritBike Forum member

Registered: 12/08/06
Posts: 448
Loc: Folsom, CA
Originally Posted By: RHall
The pistons currently being run at .002 are JCC with Rikon rings. These are cast pistons. You can run them tighter than forged. If you are planning on honing to the next size, which i believe would be .020 over your current size, i would suggest boring it because if it is so worn out that you are having to only hone a few thou than the cylinder is no doubt out of round and a few thou will not fix the side to side clearance. TT, for every .001 bore size the ring end gap increases nearly .004. Therefor the tighter you can run the pistons the smaller endgap you will have. For these bikes you should have around .010 top, .012 second. Also the more clearance you run the more rock you get in the piston and rings and the more you twist you put the rings in the more oil and compression goes past them. With a good engine builder and the proper break in procedure, you can run JCC pistons at .002. I promise! I would try not run any looser than .003 if your bore will clean up. Hope it does.


RHall, my bore is not worn out at all as a matter of fact, it appears to have only a few miles on a rebore. The bores look pretty good, have no taper or ridge, and both measure 2.857 pretty much every where on both bores. Someone just screwed up by installing pistons with from .011 to .015 clearance. My basic question is with a good bore (except for some very minor scratches described previously) and apparently the correct clearance (.005), should I hone the cylinders to breakup any glaze caused by the previous rings or will new rings seat fine if I just repiston and rering it?
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1970 Triumph Bonneville
1973 Commando Interstate

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#334653 - 09/22/10 7:05 am Re: What is the correct piston clearance? [Re: RHall-HCV]
triton thrasher Online   content
BritBike Forum member

Registered: 08/07/01
Posts: 4285
Loc: scotland
Originally Posted By: RHall
TT, for every .001 bore size the ring end gap increases nearly .004. Therefor the tighter you can run the pistons the smaller endgap you will have. For these bikes you should have around .010 top, .012 second.


Sorry this is a bit of a side track, but I don't get that. By your approximate formula, if you reduced your bore size by 0.003", a 0.012" end gap would be reduced to zero. But you can't run with zero clearance so you would open the gap up by filing the ring end.

The ring end gap must be such that it never closes completely when the ring gets hot and expands circumferentially. The correct safe end gap must be a function of circumference (directly related to bore size, of course) and thermal expansion of the ring, with a slight complication of the hot cylinder expanding away from the ring. You can't run safely with significantly smaller ring gaps just because your bore is a couple of thou smaller than mine.
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#334654 - 09/22/10 8:12 am Re: What is the correct piston clearance? [Re: triton thrasher]
RetroRod Offline
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Registered: 01/19/09
Posts: 701
Loc: QLD, Australia
And I must say also that I've always checked the fitted end gap prior to assembly, and adjusted accordingly by filing. And that is what everyone does surely? RR


Edited by RetroRod (09/22/10 8:13 am)
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#334660 - 09/22/10 10:21 am Re: What is the correct piston clearance? [Re: yellow_cad]
RHall-HCV Offline
BritBike Forum member

Registered: 07/02/07
Posts: 253
Loc: Charlotte NC
yellow_cad, if the bore is straight what i do is lightly hone with 280 grit just to de glaze the cylinder. Do check your ring end gap as the proper way to fit rings is to file fit them. (thank you for agreeing RR). I would not run over .004 if you are really that scared. new pistons shrink in there first few heat cycles so what you end up with if you go with .005 will be .0065 before long. break in with about 5 short heat cycles on straight 30w non syn oil and you should be fine. No matter what TT says mine is running safely at .002 with the same style JCC pistons. TT grab you a cylinder and rings measure the end gap then hone .001 and re check. The more you can tighten the bore up the better. If you dont agree then please explain why, dont just say it wont work.
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#334661 - 09/22/10 10:30 am Re: What is the correct piston clearance? [Re: yellow_cad]
RHall-HCV Offline
BritBike Forum member

Registered: 07/02/07
Posts: 253
Loc: Charlotte NC
Yes just lightly hone it to de glaze it and ride it. It will be a little on the loose side but will work and you will make everybody on here happy that you went to the looser side. Haha you will be fine at .005, not ideal for me but will work ok for a street bike. Please check ring end gap and let us know what it comes up, im just curious. Hope some of this arguing helps.
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#334670 - 09/22/10 11:49 am Re: What is the correct piston clearance? [Re: RHall-HCV]
triton thrasher Online   content
BritBike Forum member

Registered: 08/07/01
Posts: 4285
Loc: scotland
Originally Posted By: RHall
No matter what TT says mine is running safely at .002 with the same style JCC pistons.


I'm pleased to hear your bike runs well at that clearance, but I think it's bad advice to give to people asking about piston clearance. You would be asking them to risk a seizure.

Quote:
TT grab you a cylinder and rings measure the end gap then hone .001 and re check. The more you can tighten the bore up the better. If you dont agree then please explain why, dont just say it wont work.



I'm not denying the obvious geometric effect of a larger bore giving a larger ring gap on the same ring. I just don't see how any significant reduction in ring end gap is safe in a bore that is 0.003" smaller diameter than standard. You get a small bore, leading to a small gap, which you file out to a safe gap. If you filed 10 thou off the ring, you have reduced the circumference from 8.782" to 8.772". That's a reduction of about 0.1%. The circumferential expansion will reduce by a similar proportion. Where is the benefit of the minimum safe gap now being 0.1% smaller than before? It's within my hand filing margin of error!

If my sums are wrong I wouldn't be very surprised, but can you see what I mean?




Edited by triton thrasher (09/22/10 12:13 pm)
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#334674 - 09/22/10 12:17 pm Re: What is the correct piston clearance? [Re: yellow_cad]
Dick Harris Online   content
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Registered: 01/07/05
Posts: 1520
Loc: East Bethany New York
WOW,.002 clearance!! I have felt things tighening up a bit at .0035 which I figure the very least that I go with on a 650. Like the "feller" says, "what ever works for you" Dick

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#334683 - 09/22/10 12:53 pm Re: What is the correct piston clearance? [Re: Dick Harris]
phantom309 Offline
BritBike Forum member

Registered: 11/01/05
Posts: 1346
Loc: spfld vt
Got mine at .002 but but thats another story by itself.All aluminum and niksil bore.Just took a look at them and look like the day they went on. Also broke it in per Robs method


Edited by phantom309 (09/22/10 12:55 pm)
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#334689 - 09/22/10 1:38 pm Re: What is the correct piston clearance? [Re: yellow_cad]
RHall-HCV Offline
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Registered: 07/02/07
Posts: 253
Loc: Charlotte NC
Haha thanks Tim! There are many ways to do things. Im going to keep running tight clearances but that doesnt mean you have to follow me. But at the end of the day most have been passed by that #309 bike on the track and its quiet and hauls!
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#334696 - 09/22/10 2:24 pm Re: What is the correct piston clearance? [Re: yellow_cad]
btour Offline
BritBike Forum member

Registered: 12/18/06
Posts: 3494
Loc: Massachusetts, U.S.A.
Originally Posted By: yellow_cad
My basic question is with a good bore (except for some very minor scratches described previously) and apparently the correct clearance (.005), should I hone the cylinders to breakup any glaze caused by the previous rings or will new rings seat fine if I just repiston and rering it?


YC,

So what you are saying is that you have another set of pistons and they fit with that clearance with a light honing? If that is true than I would say go for it. I thought maybe you were maybe trying to do the math to see how much you would have to hone out and it looked like too much to have a true bore, unless you were really good at it.

Anyway, now is the time to consider what rings to use. Especially the oil ring.

I found the above discussion interesting, especially that RHall gets it at .002. I hear what Tim has, the aluminium barrels with the nikasil, is great.

Tim, do you have to use a diamond hone with those?

I posted a chapter from Irving's, "Tuning For Speed", which was all about pistons and rings. But only two people were interested, so I took it down.

I will just mention that he describes how one can run closer tolerances. But it involves heat cycling, than carefully reading the piston, and filing it. I dunno how RHall is getting away with it, unless he has everything perfectly square plumb and true, and then the rough hone, and light oil, and short heat cycles is accomplishing the same thing. Or as he says, the pistons today do actually shrink after heat.
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