94 Wombat Piston Stamp Question

[Simonkav]

I've got a 94 Wombat that I'm working through a rebuild on. I don't know the history of the bike. I cleaned the carbon deposits off the top of the piston and revealed this stamp. Anyone recognize this? I expected to see no stamp or something indicating 10 or 20 over, etc. It's a Hodaka branded piston. I don't have a bore gauge unfortunately.

Additional phots attached. Your input on overall piston condition is welcomed.

[olddogs]

The piston is not beyond using. Some wet or dry 400 grit lubed with some WD-40 will clean up the scars. What I cant determine is what the .5 is. Someone else will probably recognize it.

Cylinder bores are measured in millimeters, stock being 56mm for a Wombat. Piston oversize's are usually stamped in thousands of an inch. Its definitely not half inch, and I have never seen one stamped in millimeters, as .50 mm would be .020 thousands over. Some cylinders were stamped with + numbers on the top, but not stamped that strong on the piston. Any bike shop or auto parts store should have a caliper behind the counter can and tell you the piston size in mms. You can check the piston in the bore with a feeler gauge good enough to tell you if it needs to be bored.

[Simonkav]

A caliper to the outside of the piston reads about 55.92-55.94 at the skirt. I don't have a screw gauge, just calipers.

[Bullfrog]

Hmmmm, quite unusual -- at least compared to the OEM "ART" pistons I have on the shelf. ALL of the OEM Hodaka pistons I have (made by "ART") have size stampings which are on the order of half the size that your piston shows. In addition, they ALL use the format of "10", "20", or "30" -- what you would expect to indicate "over-size" in thousandths.

I was about to guess that the 0.5 was an indication of over-size in mm (.5mm = .020") AND THEN you posted the measurement, which shows quite clearly that the piston is "standard" - nominally 56mm.

So the 0.5 on your piston is still a mystery to me.

Ed

[matt glascock]

My tired eyes seem to think that the embossed marking reads 6.5 on the second shot of the piston crown when I zoom in on the image. That probably makes less sense.

[Bullfrog]

Yup, I agree, that makes even less sense.

Ed

[matt glascock]

That's how I roll, Captain.

[Simonkav]

Even looking at it in person, I can convince myself it reads 0.5, 6.5 or C.5

If the piston was even 10 over, let alone 20 (.5 mm) as it appears it COULD read, it's safe to say the piston would measure +56 mm, yes? Is it safe to think this is a "standard" size piston?

Thanks

[rlkarren]

I've seen this a few times. It's a standard size piston. The stamping is 6.5, and as I've understood, somehow corresponds with the number stamped on the cylinder deck that is usually preceded with a "+" that is equally mysterious.

Roger

[matt glascock]

Is this all part of the process used during original assembly to match batches of cylinders and pistons for proper tolerance/clearance?

[Simonkav]

Thanks Roger!

I've seen this a few times. It's a standard size piston. The stamping is 6.5, and as I've understood, somehow corresponds with the number stamped on the cylinder deck that is usually preceded with a "+" that is equally mysterious.

Roger

[Bullfrog]

. . . except . . . all the standard pistons I remember had the marking in rubber stamped ink. (Am I just forgetting that metal stamped marks were also used on pistons? I can't rule that out, but I sure don't remember it.)

While calipers are not the measuring tool of choice for establishing piston size when doing a bore job and trying to hit a clearance target within 0.001" (or even tighter tolerance), the caliper measurement certainly can be depended on to tell us that you have a "standard" size piston.
Ed

[junker2k]

I have one just like your piston and the stamp reads 0.5.
Jack

[Simonkav]

@Roger - There is indeed a +6 stamped on the cylinder deck. No clue what either mean.

@Jack - is that a 20 over (.5mm) piston or is it a standard issue 56mm?

-Simon

[Joe Ormonde]

Some are rubber stamped others are tool stamped. 7.5 being the most common. 8.5 is what it probably is. Most likely it`s a Standard size piston. Personally I like to measure with micrometers. I can measure .001 all the way to 6 inches. Telescoping gauges and old school micrometers are hard to beat! Joe Ormonde.

[Joe Ormonde]

Just went through my Model 94 and 95 Standard pistons. One reads 6.5, one reads 7.5 and one reads 8.5. All standard. Joe Ormonde.

[Simonkav]

Ok, thanks for all the input guys.