Does my bike have high comp pistons?

Can you tell by looking/ measuring through the spark plug hole whether high or low comp pistons are fitted??

Le Mans pistons are domed, all others flat as far as I know.

What bike is it?

High domed LM pistons can only be used with LM heads, because the combustion chamber shape in the head also has to be different. If yours is not a LM but has domed pistons then it must also have LM heads. Clues, the bolts that hold the inlet stubs onto LM heads are M6 instead of M8 as for all other models.


I know all that!!!
It’s an 850 le mans engine with 1000 barrels and pistons. Seems a bit “soft” compared to other le mans’ which makes me think they’re low comp pistons. Be nice to be able to check without dismantling and if they are low comp will buy some high comp ones. Be a shame to take apart and find they are high comp! Just wondered if there are any telltale signs through the plug 'ole…

You need an endoscope! Either that or a strong torch and look in to see if the piston tops slope down from the crown or are flat. Best of luck Lemon :smiley:

Presumably if you poked a twig (or similar) in the plug 'ole at TDC and compared the depth of penetration with someone else who knows they defo have HC pistons that might be a clue?

Well you didn’t say that in the beginning so I had to extemporise. Think that’s the right word.

So you had to do something without preparation, or without first thinking carefully about it? :confused:

Maybe Mike you were unenlightened?

I bought an endoscope from Ebay for £5. Just plug it into my phone.

Out guns my twig.

There you go I said that was the way ahead :smiley:

I’d no idea Lemon has a LM with a big bore kit, or I’ve forgotten.

I’m always rubbish at remembering what bikes other people have got :smiley:

Sorry that was a bit too much like self backslapping,

There is a clue in Lemon’s user ID :unamused:

Am I being accused of making an extemporaneus post?

Re endoscopy, had one of those down my throat - not very nice. Looking at a piston top is a far better use!

the 1000 modern barells come as standard with flat top pistons as per Spada but they can to order come with higher domed ones i would of thought wind it up to TDC and look in plug hole or feel with a straw should feel the differance

Yeah, I’ve done top and bottom before - think they shook hands in the middle.

Entente cordiale :laughing:

Not you Lemon, it was Mike H describing his own input :wink:

Self administered endoscopic examination - looking a bit down in the dumps?