Piston to bore clearance

trident sam · Oct 30, 2013

What is the correct piston to bore clearance using new Hepolite (JCC I believe) pistons with NOS genuine rings. Had a circlip come out and tram line the timing side bore. 850 Commando.
thanks
sam

Matt Spencer · Oct 30, 2013

Buy a eyeglass & a strong lamp .

Hard edge out .

Dont twist the suckers . Precise Fit . Meticulous ly CLEAN .

concours · Oct 30, 2013

.0033-.0047"

This may be of interest as well norton-top-end-break-now-t14414.html?hilit=Piston%20ring

pete.v · Oct 30, 2013

Whatever it is, add .0005 to the drive side. Comnoz says it runs a little hotter over there due to lubrication anomalies.

comnoz · Oct 30, 2013

I fit JCC pistons at .0045 to .0048 inch. They need to be slightly looser than the old original Hepolites.

At less than .0045 you will start to see rub marks and they will require a long break- in.

Getting a person with Norton experience is not a bad idea. Depending on who is doing the measuring the results may differ. Jim

splatt · Oct 30, 2013

Check circlips are 11/16'' not 17mm

Hobot · Oct 30, 2013

If the bores bored to standard over size .040" or a tad under, then rings should fit right out the box with ~.012 gap. I got away fine with some extra power d/t slightly over size 40 over bore in Peel and worn pistons for .055" piston clearance but did have the sides of piston knurled like we've seen on JIm's recent engine exposure. Peel did not make piston slap noises I could detect, like i heard on Jim's at LOP, but did wear off most the knurling in 7000 miles horrendous use. Call TotalSeal rings for a briefing on oil or dry ring install. My hand full of re-ring jobs have all gone smokeless soon after oil seen fully returning to tank. Its the cam/lifter break in that more important/tricky stressing. Much easier for me to put in pistons then slip in cold wrist pins than shoving in both pistons at once with ring compressors, having done it both ways, But IF the pins are not a slip in fit which last late Peel's were not > so risked slamming rods too hard sideways, so back to Jubelee-hose clamp struggle.

comnoz · Oct 30, 2013

If the pins are not a slip fit then put the barrels w/ pistons in place in the oven at 200 degrees. That will give plenty of warm time to set the barrels in place on some sockets over the studs and slide the pins in.

Using hose clamps or even ring compressors is a good way to scratch or break rings. Jim

Hobot · Oct 30, 2013

OH sure Jim sounds so rational to everyone else but hobot, who had to do this in open air shed in the lower 40's upper 30's F so guess how fast heated pistons worked as experts indicate. My most useful and used tool of all times on m/c's is a hair drier, same one to warm bed but never used on hair. Will soon as possible try to over heat Peel then open up to see if pistons fouling then maybe take some off piston ~~till they just don't. IIRC Canaga told me they are in the .055" zone now, which if going by references on boost per bore size is still on the tight side.~~

trident sam · Oct 31, 2013

Thanks for replies .
Will do as you say Jim, 0045 to 0048.

Still haven't found the damned circlip in the case yet :x

sam

pete.v · Oct 31, 2013

trident sam said:
Thanks for replies .
Will do as you say Jim, 0045 to 0048.

Still haven't found the damned circlip in the case yet :x

sam

I have seen bits and pieces nested in the oil ways behind the oil pump on more than one occasion.

phillyskip · Oct 31, 2013

I run .005 piston clearance with the less expensive Taiwanese pistons. They seem to expand more. With the older Hepolite pistons .045 and they DO require some easy breaking in........Skip

concours · Oct 31, 2013

comnoz said:
I fit JCC pistons at .0045 to .0048 inch. They need to be slightly looser than the old original Hepolites.

Can you help us understand WHY? Cast VS Forged? Hypereutectic? The old and not the new? Inquiring minds want to know... :idea:

comnoz · Oct 31, 2013

Well ,first there is the fact that low silicone - like most forged pistons - have a much higher expansion rate so need to be fitted looser. They are tough and somewhat malleable so they are less likely to break. They also conduct heat a little better which can be good or bad.

The higher silicone content in cast and hypereutectic pistons make the alloy expand less, fracture easier and last longer if not abused because they are harder and wear slower.

Then there are other things like the shape of the inside of the piston and the machining on the outside of the piston that will affect how much and which direction the piston is going to expand. The placement of the rings can also have an effect.

Computer modeling can predict somewhat the needed clearance but cut and try is the final word.
Case in point- JE says there Norton pistons can be fit as tight as .002 clearance. I have seen people try it -they will not make it through the first warmup before they seize.

Happy Halloween

xbacksideslider · Oct 31, 2013

Great pic, Happy Halloween backatcha.

concours · Oct 31, 2013

comnoz said:
Well ,first there is the fact that low silicone - like most forged pistons - have a much higher expansion rate so need to be fitted looser. They are tough and somewhat malleable so they are less likely to break. They also conduct heat a little better which can be good or bad.

The higher silicone content in cast and hypereutectic pistons make the alloy expand less, fracture easier and last longer if not abused because they are harder and wear slower.

Then there are other things like the shape of the inside of the piston and the machining on the outside of the piston that will affect how much and which direction the piston is going to expand. The placement of the rings can also have an effect.

Computer modeling can predict somewhat the needed clearance but cut and try is the final word.
Case in point- JE says there Norton pistons can be fit as tight as .002 clearance. I have seen people try it -they will not make it through the first warmup before they seize.

Happy Halloween

So, the JCC slugs are cast? or Forged?

comnoz · Oct 31, 2013

JCC pistons are cast. Jim

RennieK · Oct 31, 2013

Whooooweee Halloweeen! on 1st look I thought we were looking at a bike accident casuality. Great photo! have a good one! Thanks for the photo!

comnoz · Oct 31, 2013

Yeah, that was my youngest daughter as she was heading out to go scare the kids on the spook train.
I may look something like that when she finds out I posted the picture.

trident sam · Nov 1, 2013

comnoz said:
Well ,first there is the fact that low silicone - like most forged pistons - have a much higher expansion rate so need to be fitted looser. They are tough and somewhat malleable so they are less likely to break. They also conduct heat a little better which can be good or bad.

The higher silicone content in cast and hypereutectic pistons make the alloy expand less, fracture easier and last longer if not abused because they are harder and wear slower.

Then there are other things like the shape of the inside of the piston and the machining on the outside of the piston that will affect how much and which direction the piston is going to expand. The placement of the rings can also have an effect.

Computer modeling can predict somewhat the needed clearance but cut and try is the final word.
Case in point- JE says there Norton pistons can be fit as tight as .002 clearance. I have seen people try it -they will not make it through the first warmup before they seize.

Happy Halloween

Jim,

What clearance would you typically use for forged pistons like Omegas ?

sam

Piston to bore clearance

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