Piston/head squish height

[Bonz]

I’m putting my ZRX1200 back together and wanted some advice on squish height (distance from piston top to quench surface on head). I don’t have any concerns with piston/valve clearance given the valve pockets in the pistons.

.040” is a good number (leaves just a bit of margin) I have put together from info I have read about bikes in general with steel rods and was wondering what you guys think?

Them from a curiosity stand point, what is a good number to aim for if it was an XS1100 when going for a safe build but still wanting to get as much compression as you can? Would numbers be the same or close for either bike?

Thanks guys!

[Bonz]

Geez guys, no one has any thoughts or experience?

This place is dying on the vine!

[jetmechmarty]

Geez guys, no one has any thoughts or experience?

This place is dying on the vine!

It sure is. There aren't many contributions any more. Keep the rally going. That's out best shot.

[3Phase]

It sure is. There aren't many contributions any more. Keep the rally going. That's out best shot.

I quit taking apart my Yamahas and the XJ1100 is running great. I haven't even torn into the head to see what the tapping noise is, it starts with one kick cold so I just ride it!

Columbo's on the "Meh..." list. The engine's in great shape, the frame not so much. The '79 Special's on the same list for the same reason.


For the squish height, what is the height now and what are you trying to do?

(Captain Obvious) If there's too much squish it will disturb the fuel/air mixture boundary layer on the piston crown, then the piston will burn up or kiss the roof of the combustion chamber on overrev.

If there's not enough squish then you have a safe, dependable, long-lived lawnmower engine that slurps fuel and leaves carbon everywhere if you try to make power with it. (/Captain Obvious)

[TomB]

As a safety check I always nip the head down with a couple of nuts without the gasket in, spin it over - if nothing hits it certainly won't with the gasket in.

On my XS I use a paper thin base gasket and 0.9mm copper head gasket, so upped compression, needs a good battery and no carb/petcock issues or you burn holes in pistons (damhik)

Best I can offer

[Bonz]

I am waiting on piston rings to arrive for the ZZR 1200 pistons I am putting back in the engine. 3 good pistons from the engine, 1 new used piston sourced for a ZRX forum friend.

Unfortunately munched an oil rail ring last fall and scored the cylinder wall (nikasil plated) and gouged the piston. Best I can tell is the used rings probably didn’t have as much tension and at higher rpm a ring may have fluttered and caught an edge, so to speak.

Deck height (piston in the hole distance) with the block on the engine at that time was .009”-.010” without base gasket (.010” additional is what the base gasket would add).

Cleaned up the replacement block on a flat surface and 320 grit paper, deck height is .006” w/o base gasket.

Head gasket (MLS) measures .034” uncompressed, each of the 3 layers is .010”. I figure .030” compressed.

.030” head gasket + .006” under the deck is .036”. I feel like I need to put the base gasket back in to give 046” piston to head clearance (squish). Based on same measurement criteria, engine was running .040” clearance for almost 5,000 miles before the oil ring rail broke.

I guess going lower than .040” would seem to be pretty tight, wanted some reinforcement that “yep, it would be too tight for comfort”.