Piston wear limit

Frankie17 · Apr 22, 2021

your bore measurement of 2.8750" is actually 4 thou oversize standard bore would be 2.8746" based on Original Hepolite information
the B grade pistons from factory would be used if cylinder was found to be a couple of thou over tolerance when initial boring / hone were performed on the block

as you can no longer source B grade pistons fitting a standard piston sized for bore at 2.8746" will increase piston to bore tolerance

a set of standard pistons will most likely measure 2.8742" depending on manufacture

Bernhard · Apr 23, 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 in them.)

This is one of my big bugbear, having to throw away what would be perfectly good pistons (without slots) all that would be required is to recut the ring groves wider and fit suitable wider rings. But getting those rings made up was a major stumbling block, there isn't nobody who would make a small batch to special order.

BERT · Apr 23, 2021

Frankie17 said:
your bore measurement of 2.8750" is actually 4 thou oversize standard bore would be 2.8746" based on Original Hepolite information
the B grade pistons from factory would be used if cylinder was found to be a couple of thou over tolerance when initial boring / hone were performed on the block

as you can no longer source B grade pistons fitting a standard piston sized for bore at 2.8746" will increase piston to bore tolerance

a set of standard pistons will most likely measure 2.8742" depending on manufacture

You are talking tenths. 2.875-0.004-0.005=2.866 which would be the wear limit size of the piston. Please correct me if I am wrong.

Time Warp · Apr 23, 2021

BERT said:
You are talking tenths. 2.875-0.004-0.005=2.866 which would be the wear limit size of the piston. Please correct me if I am wrong.

Frankie17 said:
your bore measurement of 2.8750" is actually 4 thou oversize standard bore would be 2.8746"

Yes a difference of 4 /10ths (0.0004") or just under a half 'thou.
I still run a standard bore in the 850 but with new different grade NOS AE's.

BERT · Apr 23, 2021

Bernhard said:
This is one of my big bugbear, having to throw away what would be perfectly good pistons (without slots) all that would be required is to recut the ring groves wider and fit suitable wider rings. But getting those rings made up was a major stumbling block, there isn't nobody who would make a small batch to special order.

I won't throw them away, they may fit my chainsaw.

BERT · Apr 23, 2021

BERT said:
I won't throw them away, they may fit my chainsaw.

I am glad you guys are not working in the bank doing currency exchanges...be careful of your decimals!

Bernhard · Apr 24, 2021

BERT said:
I am glad you guys are not working in the bank doing currency exchanges...be careful of your decimals!

You must have a very powerful chainsaw !

1972Combat · Jan 3, 2022

Hi, quick question.

1972 Norton 750 combat
My bores measured (before hone) at 2.875" - cylinder wall variation of less than 0.002" (dial gauge)
Piston skirts measured both at 2.869"
Rings were all pretty much worn past 0.014"

If my bore measurement after I hone measure at 0.001" or 0.002" more, would that still fall under acceptable wear limits for the piston? From what I understand, piston wear limit is 0.009" from the bore size?

Edit* Ring lands or ring groove clearance shouldn't be more than 0.006"? Mine are consistent at 0.002" (top and middle rings)

lcrken · Jan 3, 2022

1972Combat said:
Hi, quick question.

1972 Norton 750 combat
My bores measured (before hone) at 2.875" - cylinder wall variation of less than 0.002" (dial gauge)
Piston skirts measured both at 2.869"
Rings were all pretty much worn past 0.014"

If my bore measurement after I hone measure at 0.001" or 0.002" more, would that still fall under acceptable wear limits for the piston? From what I understand, piston wear limit is 0.009" from the bore size?

Edit* Ring lands or ring groove clearance shouldn't be more than 0.006"? Mine are consistent at 0.002" (top and middle rings)

I haven't seen any specs for ring side clearance for Commandos in their sevice manuals, but the typical number for it in most similar engines with conventional rings is .002". For performance engines anything over .002" would mean piston replacement. Like Jim said, at .006" they are too worn to re-use.

Ken

SteveA · Jan 3, 2022

These pistons are soon to have their 50th birthday. Probably more than their intended service life regardless of mileage

I don't think you need to debate their replacement, other than to be sure you get the right ones and check your clearances.

What else might need replacing? Big end shells and rods. Does the crank need a grind? In any case split and clean crank, fully inspect, and install new studs and nuts.

Fully inspect the main bearings. Fully inspect the cam. Replace if necessary. Anything else can be changed without re-splitting the cases.

Dan1950 · Jan 3, 2022

Frankie17 said:
your bore measurement of 2.8750" is actually 4 thou oversize standard bore would be 2.8746" based on Original Hepolite information
the B grade pistons from factory would be used if cylinder was found to be a couple of thou over tolerance when initial boring / hone were performed on the block

as you can no longer source B grade pistons fitting a standard piston sized for bore at 2.8746" will increase piston to bore tolerance

a set of standard pistons will most likely measure 2.8742" depending on manufacture

.4 thou not 4 thou

Piston wear limit

Frankie17

Bernhard

BERT

Time Warp

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

BERT

BERT

Bernhard

1972Combat

lcrken

SteveA

Dan1950

1974 MK II Roadster

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