Squish measurement and RPM's

[Watty]

Just a quick question here...

I'm running an ADA head on top of a 61x cylinder that is at 83mm with Sudco pistons.

I've measured the squish, which comes up within spec of .040 no problems, but upon inspecting the pistons, I notice there is a small ring halfway around the outer piston top on the exhaust side. I was thinking that at high rpms, the piston is just touching the squish band on the dome. Does this sound feasible? The mark, is very fine, and I can't catch a nail on it, but it's there as clear as day.

I have a friend who is on 83mm with a Blowsion head, and he has the exact same marks. Both of us replace little end bearing whenever we pull the engines down.

Surely there isn't that much movement when the engine is running high rpms to take up all the .040 clearance?

 

[#ZERO]

I seriously doubt the piston is touching the squish band on the dome and it's more likely detonation hot spots.

Do you have any pictures that you can post?

 

[Watty]

Yeah, thats what I thought.

I don't have any pics at the moment, but I'll get some next time I have the head off.

As I said though, it a thin line about .5mm wide that goes around the exhaust port side, kinda like a smile but does not extend past the gudgeon pin area. I'm hoping it's no deto, as I spend a LOT of time dialling my engines in! It could well be from a previous lean condition though. I really need pics I think!

Cheers.

 

[SN760]

Sounds like detonation.

Some years ago I was running 650s with no rev limiters, when the pump would unhook these motors would hit 10,500 rpm. With the squish band at .040 I would get some piston to head contact, had to widen the squish band to at least .055

Running a rev limiter I would not think .040 clearance would be an issue unless you have a loose crank or rod bearing.

Also any time I have seen piston to head contact the contact area would show up all the way around the top of the piston.