Piston failure

Here's the old style (16001) Hepolite. Unfortunately, I didn't know about the oil ring cutout on these. Thought I'd got a real good deal on fleabay.



By jcclausen, shot with C200Z,D510Z at 2007-11-12[/img]

Ouch!! How does the cylinder and head look?

Not a mark on the head or cylinder. Lot of trash in the crankcase. Got a crunched up oil pump. Guess the piston top was just floating and geting hammered against the wristpin bosses. Fred, of Old Britts, tells me this was a common failure of early Hepolite pistons. He called them sliptops. Not knowing of this, I bought these on ebay. The problem stems from the enormous oil ring cutaway.


By jcclausen, shot with C200Z,D510Z at 2007-11-12[/img]

The piston problem is something that was covered in the NOC's 1979 "Commando Service Notes"

This is what they had to say :-

"Don't buy new pistons with a slot under the oil control ring. The slot was deleted to avoid the top coming off the piston when the revs were kept up towards 7000. Of course, the pistons are OK for the man who never goes above 5000 rpm. The latter type are identified as Combat Pistons in the parts list, but the only difference is this slot - the compression height of Combat (non-slotted) and standard (slotted) pistons is identical. ... Atlas Pistons will fit straight into the 750 Commando giving the lower Atlas ratio (a bit more under a Combat head) but use Hepolite Atlas pistos and try to get those without the slot. It's not so critical with Atlas pistons because the combustion pressures are less and generally the chap who fits Atlas pistons doesn't go wailing around at 7000 rpm all day".

This rather old and somewhat dated publication is still worth a look if you can find a copy. It has saved me from learning the hard way on several occasions.

It wasn't under hi revs when it crapped out. I was just taking a putt to breakfast. I suppose the drag race I had with the Harley a few days ago had something to do with it.

Say, can anyone tell me why the post showing the damaged piston the picture is big, whereas the one showing the intact piston is small? I've been editing till my fingers gave out, but can't seem to make the little picture larger.

JimC said:

Say, can anyone tell me why the post showing the damaged piston the picture is big, whereas the one showing the intact piston is small? I've been editing till my fingers gave out, but can't seem to make the little picture larger.

Click to expand...

The first photo address is the actual photo url, the second smaller one is a thumbnail link url. (thumbnail mode enabled)

Thanks, L.A.B. With your help I finally got it.