Piston wear limit

BERT · Apr 21, 2021

I am trying to determine whether to replace these pistons and would like advice please. It's a 72 combat. Both bores are in good shape, still showing crosshatch witness marks and both measuring 2.875" top and mid. The pistons both measure 2.868" at the skirt bottom. All measuring was done according to the workshop manual. So there is .007" clearance both cylinders. The fitted clearance(new) should be .0045". What is the wear limit necessitating piston replacement? Should I use these? I searched previous threads and can't find the actual wear limit. Thank you.

kommando · Apr 21, 2021

Wear limit is 5 thou over initial clearance. So you have a few thou left, what is more important is that you do not refit the head detaching pistons with the extended slot under the oil control ring between the extended 2 holes.

marinatlas · Apr 21, 2021

Hi, first check if your pistons have the saw cut in the oil control ring groove , if yes direct in the bin (sorry, if you smoke two beautifull ash tray ..........), then 7 thou is very near of the wear limit , so just hope you are in the first case and buy new pistons....!

BERT · Apr 21, 2021

kommando said:
Wear limit is 5 thou over initial clearance. So you have a few thou left, what is more important is that you do not refit the head detaching pistons with the extended slot under the oil control ring between the extended 2 holes.

Thanks for your input Kommando. I don't understand the refit part of your advice. Could you clarify it- please excuse my ignorance.

Time Warp · Apr 21, 2021

Some of the early pistons have a slot in the groove, the piston heads could detach if you sent the tacho needle around to Made in UK.
You should be able to see from the inside (or remove the oil rings) but those skirts look a little worse for wear, how is the bore ?

BERT · Apr 21, 2021

Time Warp said:
Some of the early pistons have a slot in the groove, the piston heads could detach if you sent the tacho needle around to Made in UK.
You should be able to see from the inside (or remove the oil rings) but those skirts look a little worse for wear, how is the bore ?

Thanks Time Warp. Is this the slot in the groove you refer to?

Time Warp · Apr 21, 2021

I couldn't find my 750's old pistons.
Second picture showing, bottom piston.

Replace pistons or not?

Recently took the head off my 70 Commando to investigate an oil seal leak on the right hand inlet. While there, took the barrells off to inspect the pistons and rings. They are standard size Hepolites with the AE stamp and look good. The do have the long slot underneath the ring. For the...

www.accessnorton.com

kommando · Apr 21, 2021

You want the 2 expansion slots with the ring groove like this. You do not want to refit pistons with a slot in the bottom of ring groove between the 2 expansion slots.

BERT · Apr 21, 2021

Time Warp said:
I couldn't find my 750's old pistons.
Second picture showing, bottom piston.

Replace pistons or not?

Recently took the head off my 70 Commando to investigate an oil seal leak on the right hand inlet. While there, took the barrells off to inspect the pistons and rings. They are standard size Hepolites with the AE stamp and look good. The do have the long slot underneath the ring. For the...

www.accessnorton.com

Got it, thanks TimeWarp and Kommando.These pistons do not have the groove in the bottom of the oilring groove. I could see how that groove may cause a decapitation. Thanks for the explanations. I learned bigtime on that one.
I'm thinking since they are close to the .005" wear limit I should replace them while all apart. These have a "B" stamped on the top along with 063229(RH) and 063228(LH). Are there any fitment issues to be aware of when ordering replacements with the combat engine? The cylinder bores are well within tolerance to go with standard size. Your advice would really be appreciated. Thank you.

Carl H · Apr 21, 2021

BERT said:
I am trying to determine whether to replace these pistons and would like advice please. It's a 72 combat. Both bores are in good shape, still showing crosshatch witness marks and both measuring 2.875" top and mid. The pistons both measure 2.868" at the skirt bottom. All measuring was done according to the workshop manual. So there is .007" clearance both cylinders. The fitted clearance(new) should be .0045". What is the wear limit necessitating piston replacement? Should I use these? I searched previous threads and can't find the actual wear limit. Thank you.View attachment 79342 View attachment 79343 View attachment 79344 View attachment 79341

My Combat had std pistons without the slot , but with a one piece oil ring all it did was foul spark plugs and the bore wear was similar to yours. Get Emgo / JCCpistons with Hastings rings. 4 thou clearance with a careful break in with a well tuned bike. You can't baby them . A 50 mile oil change and a oil tank that has been taken out and cleaned real well. Look over the cam and lifters real good.. Right now Emgo +20 pistons might be hard to find as the major suppliers are waiting for a new batch

Carl H · Apr 21, 2021

BERT said:
Got it, thanks TimeWarp and Kommando.These pistons do not have the groove in the bottom of the oilring groove. I could see how that groove may cause a decapitation. Thanks for the explanations. I learned bigtime on that one.
I'm thinking since they are close to the .005" wear limit I should replace them while all apart. These have a "B" stamped on the top along with 063229(RH) and 063228(LH). Are there any fitment issues to be aware of when ordering replacements with the combat engine? The cylinder bores are well within tolerance to go with standard size. Your advice would really be appreciated. Thank you.

With a Combat cam, you will need to pay attention to the intake valve to piston hitting.

kommando · Apr 21, 2021

BERT said:
These have a "B" stamped on the top along with 063229(RH) and 063228(LH). Are there any fitment issues to be aware of when ordering replacements with the combat engine? The cylinder bores are well within tolerance to go with standard size.

The B means nothing when fitting new pistons to a used bore, it was used at the factory as production boring was not as good as one off boring on tolerances so they got the pistons made to 2 sizes and measured the bore and picked the piston size.

Combat pistons are just 750 pistons, the extra compression comes from planning the head. You will need the bores honed which might add a thou to the clearance.

As Carl says check the piston valve clearances.

L.A.B. · Apr 21, 2021

Norvil Motorcycles - Tech Talk

BERT said:
These have a "B" stamped on the top along with 063229(RH) and 063228(LH).

See factory manual, Technical data, Cylinder block, Grade B.

BERT · Apr 21, 2021

Thanks everyone for your help sorting this out. This forum is fantastic.

L.A.B. · Apr 21, 2021

BERT said:
063229(RH) and 063228(LH)

Also, they are the part numbers for the bare pistons.

The piston assembly part numbers are 063339 (RH) and 063338 (LH).
Confusingly, these are marked 'Standard' in the '72 parts book but should be marked 'Combat' as they are the stronger type.

063348 and 063349 in the parts book marked 'Combat' are the final (strongest, no slots) fitted to all 750s from late '72.

batrider · Apr 21, 2021

Check the ring grooves in the piston for wear. My piston to cylinder wall clearance was good but the ring grooves were well worn so I had to replace with new std. pistons. (The other reason was that they had the big slots in them.)

BERT · Apr 21, 2021

batrider said:
Check the ring grooves in the piston for wear. My piston to cylinder wall clearance was good but the ring grooves were well worn so I had to replace with new std. pistons. (The other reason was that they had the big slots

batrider said:
Check the ring grooves in the piston for wear. My piston to cylinder wall clearance was good but the ring grooves were well worn so I had to replace with new std. pistons. (The other reason was that they had the big slots in them.)

Both of these pistons show a consistent. 006" gap with the top compression ring. The other 2 rings on both pistons show less.

BERT · Apr 21, 2021

L.A.B. said:
Also, they are the part numbers for the bare pistons.

The piston assembly part numbers are 063339 (RH) and 063338 (LH).
Confusingly, these are marked 'Standard' in the '72 parts book but should be marked 'Combat' as they are the stronger type.

063348 and 063349 in the parts book marked 'Combat' are the final (strongest, no slots) fitted to all 750s from late '72.

Yes, the parts list I have show the same numbers. Maybe these pistons weren't correct, as they clearly show the stamped numbers on the photos

L.A.B. · Apr 21, 2021

BERT said:
Yes, the parts list I have show the same numbers. Maybe these pistons weren't correct, as they clearly show the stamped numbers on the photos

The piston assemblies (with those pistons) are 063338 and 063339 as the piston assembly has a different part number to the bare piston which generally wasn't available separately and why 063228 and 063229 aren't in the parts book.
It's the parts book description that they are Standard (only) and not (also interim) Combat that's isn't accurate.

jseng1 · Apr 22, 2021

BERT said:
Both of these pistons show a consistent. 006" gap with the top compression ring. The other 2 rings on both pistons show less.

at .006" ring land clearance - they are junk and will burn oil.

Piston wear limit

BERT

kommando

marinatlas

BERT

Time Warp

.......back to the 70's.

BERT

Time Warp

.......back to the 70's.

Replace pistons or not?

kommando

BERT

Replace pistons or not?

Carl H

Carl H

kommando

L.A.B.

Moderator

BERT

L.A.B.

Moderator

batrider

BERT

BERT

L.A.B.

Moderator

jseng1

Similar threads