Hopefully somebody can help me identify the size of these pistons as I can't figure it out myself....

The pistons are from a 1980 80" FXWG. Inside the skirt are the following identifiers:

"22124-78" (which I believe is the standard HD part number)
"JC" below a sort of hexagonal symbol
"A1" by itself

Then on the piston crown, it says "0.75" and nothing else at all - which is what is mainly puzzling me. I would have expected to see something like +0.010 or +0.020 to indicate a 10 or 20 thou overbore or something similar - but what does "0.75" mean?

The only reference I can find to 0.75 is with regards to piston rings - I see sets of NOS rings indicated as (for example) "85.14mm 0.75"

Can anyone advise for sure? Some pics below...

Thanks in advance!

 

Just a thought....
A lot of metric stuff would use .75mm as being close to .030" over. Might be the pistons are imported

 

I was thinking that way as well - and the bike is in the UK. However I can't find any piston vendors here or in Europe that offer O/S pistons in metric sizes. They seem to be exclusively in imperial measurements. Most are imported from the US anyway.

 

It looks like you will need to measure the actual bore size and then proceed. If it is in fact .75mm over, and doesn't have much wear, it just might clean up with a hone to .030" as .75mm is just a touch smaller. A very small touch smaller!

 

Here's an ebay listing with a bit of info, same part number but different markings on top. not sure if it helps much.

https://www.google.com/url?sa=t&rct=...ZlEdJy2KjzBAxQ

 

You are right - it's time for proper measurements. The cylinder bores look pretty good with only a little light scoring in one place on the front. So hopefully we can make them work without having to buy new ones.

I will be running the top end over to a specialist machinist later this week. He'll check the bores and pistons and make a recommendation on how to proceed. Can't send the bottom end over yet as I haven't got it out of the bike yet....and it's too cold in the garage to work for long. That will come next though - just waiting on a clutch hub puller to arrive so I can finish taking the primary off.

 

Thanks Robby,

That's interesting - those are marked 2.0 although in a different place on the piston - but if that is mm, then that indicates about a .080" overbore, which I would have thought was way beyond how far you could go.

But if 2.0 means +.020", then maybe 0.75 means +0.0075" - but that would seem a strange bore size as +.005, +.010, +.020 etc seem much more common.

Ah well.....maybe Scott or someone knows the answer. Measurement will tell the truth though....

 

Rough measurements with my micrometer show the cylinder bore to 3.520" - so I guess that's 0.020" over.....

 

I don't believe there is very much metric stuff on a '70s/80 Harley. Having bought a 1974 FX new the only metric stuff I needed for it was a 10mm wrench for the battery terminals. The US Government did pass a metrication law in 1975, but forgot to say when it should be implemented and it is only in recent years that H-D has started sneaking metric stuff onto production bikes! In other words that .75 refers to something other than the bore IMHO.

The markings inside the skirt of the piston are manufacturer's marks, even the H-D part number may not be the finished part number, but the casting part number, unmachined. JC may be the manufacturer and A1 may be the casting mould number or pattern number. I'm a one time production engineer and recognise all such things!

 

About .400" from the bottom of the skirt, measure that OD, and let us know the dimension you see.
Scott