Piston wear limit

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BERT

BERT

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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.
Piston wear limit
Piston wear limit
Piston wear limit
Piston wear limit
 
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.
 
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....!
 
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.
Click to expand...
Thanks for your input Kommando. I don't understand the refit part of your advice. Could you clarify it- please excuse my ignorance.
 
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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 ?
 
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 ?
Click to expand...
Thanks Time Warp. Is this the slot in the groove you refer to?
Piston wear limit
Piston wear limit
 
I couldn't find my 750's old pistons.
Second picture showing, bottom piston.

www.accessnorton.com

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 www.accessnorton.com
 
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.

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

www.accessnorton.com

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 www.accessnorton.com
Click to expand...
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.
 
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 79342View attachment 79343View attachment 79344View attachment 79341
Click to expand...
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
 
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.
Click to expand...
With a Combat cam, you will need to pay attention to the intake valve to piston hitting.
 
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.
Click to expand...
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.
 
Thanks everyone for your help sorting this out. This forum is fantastic.
 
BERT said:
063229(RH) and 063228(LH)
Click to expand...

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.
 
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.)
 
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
Click to expand...

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

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.
 
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