Wire wound piston clearances

AJSNUT · Post by **AJSNUT** » Thu Sep 06, 2007 4:21 pm

I have found a used wire wound piston to fit my 1952 18S, and remember reading somewhere that these pistons will allow tighter skirt to bore clearances. I have a barrel about to be fitted with a liner and need to specify the clearances that the liner should be bored to. Any info from anyone?
Thanks in advance
AJSNUT

itma · Post by **itma** » Thu Sep 06, 2007 9:34 pm

don`t know about fitting an already used piston, but when new they were selected to the bore.
there should be a letter stamped on the top according to the Niell book.
H= +0.00075
O= +0.00050
A= +0. 00025
M = mean bore size
B= - 0.00025
V= - 0.00050
S = - 0.00075
X= - 0.001

Mean Bore size is given as 3.2490 inch top of skirt
and 3.2494 bottom of skirt

How all that will work out with your worn piston depends on the wear of it I suppose.
even worn at these clearance sit'll need some extra careful running in.

AJSNUT · Post by **AJSNUT** » Fri Sep 07, 2007 9:01 am

itma,
many thanks for that, I am sure the info will be helpful. I have found some other info on the Yahoo group site. This says that the total clearance should be 0.001inch per inch of bore for wire wound, while normal pistons use 0.0015inches per inch.
I think that I will discuss this with the engineering company when I drop the barrel off for treatment.
Regards
Tim

Biscuit · Post by **Biscuit** » Fri Sep 07, 2007 10:04 am

I had my G11 cylinders sleeved so I could used the original wire-wounds. The engineering company wanted the pistons and the manual showing tolerances to complete the sleeving.

They were very interested in the pistons, having never seen wire wound pistons of this type before, Apparently there are diesel pistons (they said) that are wire wound at the bottom of the skirt.

AJSNUT · Post by **AJSNUT** » Fri Sep 07, 2007 5:51 pm

Biscuit, Ray on the yahoo website informs me that there were, in fact, two types of wire wound pistons. One type used tighter clearances, while the other used 'normal' clearances. I dropped the barrel and piston off at T&L Engineering today, and they seemed familiar with the issue and will be boring the barrel to 'normal' tolerances.

Biscuit · Post by **Biscuit** » Fri Sep 07, 2007 6:28 pm

I seem to recall reading that AMC experienced problems initially with these pistons, perhaps that's why there are are two types. Only surmising!.

Groily · Post by **Groily** » Wed Nov 24, 2021 4:29 pm

This might be relevant, from way way back . . .
I thought 2 thou on G9s but am not sure in light of the '2 types' comment. But it seems that T&L knew back then, and I'm sure they still do!

Shelby-Right · Post by **Shelby-Right** » Thu Nov 25, 2021 1:19 am

Measuring the piston 90 degrees to gudeon pin and at the same height will give you the clearance to the " BORE SIZE " .

Groily · Post by **Groily** » Thu Nov 25, 2021 6:53 am

Yup, but it won't tell us the dimensions for the bore which is the question in the parallel thread 'Rebore Piston Clearance'.

I resurrected this old thread in reponse to that new one because it is on point and also highlights the fact that there were 2 versions of the wire wounds apparently.
Wire-wounds run on 'tight', but the Q was 'How tight'. Whereas with most modern pistons we're looking at 5 thou-ish skirt-to-bore, with wire-wounds, half that or less . . .

paulvk · Post by **paulvk** » Thu Nov 25, 2021 10:31 am

I have BHB pistons the the 500 twin, these pistons have lots of numbers on them. Apart from +20 they have 022669 ,W151,L60.
Regards...Paul

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