What are the best 850 pistons ?

What are the best pistons for a stock 850? There’s been a lot of discussion about poor pistons and circlips that come out, etc. But I’m not sure what the conclusion is !

And regarding the the bad ‘self removing’ / ‘engine destroying’ circlips how does one identify them?

Are these the good ones?

Well it depends on the profile of the groove in the piston, the 750 JCC pistons that come with the round wire circlips have a square groove so this pic shows the best circlip for them. The round wire ones go in the bin.



For the 850 that are around currently I cannot answer as I have not inspected the groove.

I know there is a round / square groove issue, and a need to ensure round clips in round grooves etc, but I also thought there was a theory about long tangs causing them to pop out (due to resonance / vibration etc)?

A mate recently rebuilt the top end on his original mk3. Stock pistons had done 30k miles and still looked good. Bore was a little worn though, so to be a good boy he fitted new plus 20s. It seized up a few miles after completion.

He doesn’t want it to happen again.

Hence the question...

If a piston has round grooves I would only fit these circlips. If there is no slot for a tool to remove them cleanly I would mill a slot or if no mill then a well held dremel.

I'm very happy with my map cycle billet pistons.

Why JE 880 pistons, of course!

I went with the cast JCC pistons Comnoz recommended when he rebuilt my head, they came with the square profile clips shown above and colour band oil ring. .0045 clearance suggested, but this tightened up slightly after I had them coated

Comnoz recommends JCC Does he...?

Anyone have a source for JCC pistons in Blighty ?

Wassell's if you can stand the faux dusty brand new boxes.

Fast Eddie said:

Comnoz recommends JCC Does he...?

Anyone have a source for JCC pistons in Blighty ?

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Can't comment on UK supply, but Jim sent them with Hastings rings, not sure if that's standard fare with the JCCs or a swap out he makes

He also changes out the circlips.

I like the thought of Jim Scmidt's light 12 to 1 comp. pistons and longer con-rods. With methanol fuel, that might be a good way to go. I've never had a circlip pop-out in use, but I never compress them enough to take the tension off them through deformation.

Ok, ok, ok.... I see I need to clarify my question a tad. I’m sorry for any confusion, but when I said ‘standard 850’ I guess what I really meant was ‘standard 850’. No long rods. No methanol. No 12:1CR.

I should have also specified a U.K. supplier.

I see that depending on size, RGM do new Hepolite, NOS original Hepolite, JP & GPM pistons.

Andover obviously stock them, but I don’t know who’s they are, does anyone know?

What are folks recommendations for standard spec 850 pistons from a U.K. supplier please ??

It’s not a trick question !

kommando said:

Wassell's if you can stand the faux dusty brand new boxes.

Click to expand...

Does that mean new ‘Hepolite’ ?

Are they OK ?

acotrel said:

I like the thought of Jim Scmidt's light 12 to 1 comp. pistons and longer con-rods. With methanol fuel, that might be a good way to go. I've never had a circlip pop-out in use, but I never compress them enough to take the tension off them through deformation.

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Well I do too, although I seem to manage fine with 10 point something as a compression ratio with JSMs other pistons, but as a response to fast eddie's question, this response makes no sense at all!

Fast Eddie said:

What are the best pistons for a stock 850? There’s been a lot of discussion about poor pistons and circlips that come out, etc. But I’m not sure what the conclusion is !

And regarding the the bad ‘self removing’ / ‘engine destroying’ circlips how does one identify them?

Are these the good ones?
View attachment 16333

Click to expand...

Good evening together,
I was in contact with JCC and they informed me that they deliver metrical circlips with their Norton pistons. Unfortunately these circlips are for 17mm bores, which is too small for our 17,447/17,442mm pins. What you need to do is using 11/16" circlips for these pins apart from using Hastings piston rings and you are on the safe side.
How did I find that out?
For some reason I had to dis- assemble the cylinder- head and while I was doing that I thought it would be worthwhile to also remove the cylinder checking the JCC- Pistons which I fitted a while ago. I found that the cirlips were very loose in their grooves and continued investigating.
This is what I found: The grooves were to big for the metrical circlips in a way that these could freely rotate in their grooves. With the results of my findings I contacted JCC and they were really so kind to tell me the story.
Apart from these 2 issues the pistons are near to perfect. Before I fitted them I measured them and found absolutely equal measuring values in every way. Even the weight was equal down to only 2 gramms, which is tolerable.
Now I am using MAP's billet machined pistones which are a lot lighter and also very, very good quality- wise.
So my conclusion is: Use the JCC- pistons with Hastings- rings and the above mentioned 11/16" circlips for normsl road use. Racing and fast road use is a different matter.
Best Regards
Klaus Monning

Fast Eddie said:

Ok, ok, ok.... I see I need to clarify my question a tad. I’m sorry for any confusion, but when I said ‘standard 850’ I guess what I really meant was ‘standard 850’. No long rods. No methanol. No 12.1CR.

I should have also specified a U.K. supplier.

I see that depending on size, RGM do new Hepolite, NOS original Hepolite, JP & GPM pistons.

Andover obviously stock them, but I don’t know who’s they are, does anyone know?

What are folks recommendations for standard spec 850 pistons from a U.K. supplier please ??

It’s not a trick question !

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For a standard road bike, if you could get genuine NOS Hepolite with whatever clips they came with, why wouldn't you?

But I still don't understand if your friend actually had the barrels rebored to +20? You don't mention that, or if the reborer actually bored to the required clearance specificed for the air cooled engine, or used modern water cooled numbers?

Has a tear down revealed a circlip problem, or a clearance problem?

Fast Eddie said:

Does that mean new ‘Hepolite’ ?

Are they OK ?

Click to expand...

Hello once again,
Wassels, New Hepolite etc., and all the other brands (besides GPM, these are Italian pistons and also very good) are all JCC.
Best regards
Klaus

Many thanks Klaus and Steve.

Steve, off the press update is: the barrels were bored to plus 20. He used NOS genuine Hepolite plus 20 pistons.

He currently has the barrels off and is going to get things measured up to see what’s what with regards to clearances etc.

They picked up good and proper! Looks like a clearance issue IMHO.

If the bores are ok and the plus 20 bore can be salvaged (if they were bored too tight, there is a good chance they will clean up nicely at plus 20) then I think he’ll use NOS Hepolites again. If he has to go to plus 40, he’ll need to decide on another brand.

kommando said:

Wassell's if you can stand the faux dusty brand new boxes.

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