New Pistons for MKIII Grade B Barrel question...

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Tinkerbell

Tinkerbell

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Hi everybody,
I have an 850 MKIII with standard unbored barrels, Grade B. I am doing a top end refurb. I am considering a NOS pair of Hepolite pistons in the old yellow boxes. Is there any consideration for buying new pistons because the barrels are the Grade B size? Are there different pistons for Grade A as opposed to Grade B?
Thanks a bunch,
Kara
 
Do they look like the ones below? If so, they’re not NOS... they’re new Taiwanese. Nothing wrong with them as far as I know, but if you‘re buying them then why not by +0.020” and have them bored to suit?
New Pistons for MKIII Grade B Barrel question...
 
Tinkerbell said:
Are there different pistons for Grade A as opposed to Grade B?
Click to expand...
There were no A or B grade standard replacement pistons listed in the parts book.
If there has been some wear then the barrels may no longer be 'B' grade (3.0320" - 3.0325").
 
Best to talk to whoever is doing the boring first and have him measure the cylinders up and down to see how much wear there is before buying pistons. Usually it is worse than you'd think.
 
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In my experience, if we’re talking ‘wear’ rather than ‘failure’ the barrel wears more than the piston. So, if your standard pistons are worn, your barrel will definitely be worn too.

This is to be expected if you think about it, it’s the piston rings, not the piston, that act upon the bore.
 
As Fast Eddie I had the bores measured all the way up to find they were on the limit of standard and decided to go for +20 rebore. Bought the pistons as above, and took them and the barrels to the machine shop for the rebore.
The pistons were fine by the way.
 
There are 2 tolerances for bores, the finished bore size for a freshly bored barrel detailed in the workshop manual, and then a further limit for wear. If you measure the used bores and they are on the upper edge of the finished bore size they do not need a rebore unless grooved or damaged, you have at least another 5 thou of wear tolerance to go.
 
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