Piston wear limit

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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
 
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.)
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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.
 
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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
Click to expand...
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.
 
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.
Click to expand...
Frankie17 said:
your bore measurement of 2.8750" is actually 4 thou oversize standard bore would be 2.8746"
Click to expand...
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.
 
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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.
Click to expand...
I won't throw them away, they may fit my chainsaw.
 
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)
 
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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)
Click to expand...

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
 
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.
 
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
Click to expand...
.4 thou not 4 thou
 
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