Topic: Choosing MLS head gasket thickness - squish clearance

[macvit7906]

Adding this to high performance section as it's related to more of a performance engine

Hoping to be pointed in the right direction here and verify I'm measuring correctly

1975 cb750F
836cc wiseco kit, stage 4 ported head by M Rieck.

I’m measuring my squish clearance in order to figure out the thickness of MLS gasket to order. I was told to shoot for 0.04" squish.

I zero my gauge at the top of the sleeve. I have a piston in the cylinder without a base gasket.
When at top dead center, the piston does not go over the deck of sleeve. As best as I can measure it is .005-.01” below the sleeve at the edge of the piston near the wrist pin.

When searching the forum I found an example, but the piston goes above the sleeve by a little bit.

My base gasket reads .019” thick.
If I go with a 0.04” thick MLS head gasket that gives me .019 + .008 (piston is below the sleeve) + .04 = 0.067 clearance. Is that too much?

The thinest MLS gasket I can find is from dynoman at 0.03” thick giving my about 0.06” squish at most. Is that ok?

Maybe the answer is to find a thinner base gasket at 0.01"?

To show you how I’m measuring I took a pic below.

[MRieck]

 Matt....use the .010 base gasket with a .030 head gasket. As long as you are below the deck you'll be fine with that combo. You really need a dial indicator to get a good reading.....tough to do with the vernier.

[macvit7906]

Matt....use the .010 base gasket with a .030 head gasket. As long as you are below the deck you'll be fine with that combo. You really need a dial indicator to get a good reading.....tough to do with the vernier.

Thanks Mike. Just ordered a dial indicator. Regardless, I can tell with the naked eye the outer edge of the piston does not come over the deck.

Will go with a .01 base and .03 MLS gasket.

[HondaMan]

You can also improve the squish by milling off the top deck of the cylinders. I typically take 0.020" to also improve the O-ring seals for the 2 oil passages: thicker-than-OEM head gaskets make leaks happen there. The OEM gaskets were 0.028-0.030: thick, modern ones are mostly 0.040" thick. The loss of compression of those O-rings in their shallow recesses lets the oil out. When you get your final head gasket, lay it in place and put the O-rings in, then measure how far above the head gasket they rise. If they are not 0.020" or more, contact me for some extra-thick ones.

[Captain]

 No if's or but's about it and the clearance is .038" min & .040" max   This provides the most gas ejection towards the flame front and without the dwell time not allowing the flame front to access the area at the same time.  Tighter squish (less than .038") is to small for the flame front entry and too large (greater than .040") does not eject the gas enough.  Combustion is also not perfectly symmetrical as it moves away from the ignition point (spark) and so is also arriving at some squish area at TDC and others somewhat later and as long as total combustion is completed by between 15 thru 20 ATDC then you will achieve the highest energy production.

 Captain 
 

[Tracksnblades1]

There's a lot of research on squish and quench. Two completely different terms and events but are represented within the same measurement.

Squish is usually referred to as a clearance measurement to achieve a targeted MSV or maximum squish velocity (expressed in meters per second) for a specific rpm, bore, and fuel. Or simply how fast the squish volume is ejected from the squish/quench clearanced area. Usually directed toward the center of the combustion chamber and sparkplug. Some define proper squish velocity as sufficient to impart maximum turbulence into the compressed mixture to produce a more efficient homogeneous burn without excessive velocities that can adversely affect or extinguish the flame front or disrupt propagation.

Of additional squish/quench interest, there are 1960 published /patented studies on flame/squish trenches extending into the squish areas to aim, direct and impart swirl (similar to a diesel) into the chamber to intensify the flame kernel and provide mutiple predetermined flame propagation paths into the quench/squish area for increased efficiency.

Quench, can be an aid in detonation control by preventing the spontaneous combustion of the end gases. Effective quenching, lowering or maintaining the temperature of the end gasses, within the squish/quench clearanced area, as they are subjected to the intense radiant heat of the advancing flame front can provide some resistance to detonation. Quenching occurs by the end gases held in close proximity to the cooler surfaces within the squish/quench area. Ideally these end gases will be completely burned as the crank angle provides additional clearance after TDC. Even the flame front speed can be calculated for different fuels. So you can be sure the piston has moved down enough to allow the flame front to advance into the squish/quench area and not be snubbed out. Low octane gas burns faster...😩

There are several squish/quench calculators online. Some even calculate increased crankshaft loading from squish clearances. Nothing is free. While there is some agreement in the maximum squish velocity, actual build velocity is very secretive in the winners circle. Turbocharged, Supercharged,  Alcohol, Nitro, Nitrous Oxide, brings more to the equation. Some NA engine builders would hand scrape the pistons for clearance after they lightly contacted the head. Two stroke turners even allow for the aluminum cylinder's vertical expansion and have different squish angles, stages, and percentages for different fuels.

Squishy Squishy Quenchy Quenchy...

[macvit7906]

You can also improve the squish by milling off the top deck of the cylinders. I typically take 0.020" to also improve the O-ring seals for the 2 oil passages: thicker-than-OEM head gaskets make leaks happen there. The OEM gaskets were 0.028-0.030: thick, modern ones are mostly 0.040" thick. The loss of compression of those O-rings in their shallow recesses lets the oil out. When you get your final head gasket, lay it in place and put the O-rings in, then measure how far above the head gasket they rise. If they are not 0.020" or more, contact me for some extra-thick ones.

Thank you sir. That's a good tip, I'll remember to do that.

I ended up going with the 0.040" NO LEAK MLS head gasket from CycleX after speaking with them. This allows me to use the dowels and rubber seals as well. Just have to make sure there is enough clearance. The MLS gaskets do no compress, correct?

[HondaMan]

You can also improve the squish by milling off the top deck of the cylinders. I typically take 0.020" to also improve the O-ring seals for the 2 oil passages: thicker-than-OEM head gaskets make leaks happen there. The OEM gaskets were 0.028-0.030: thick, modern ones are mostly 0.040" thick. The loss of compression of those O-rings in their shallow recesses lets the oil out. When you get your final head gasket, lay it in place and put the O-rings in, then measure how far above the head gasket they rise. If they are not 0.020" or more, contact me for some extra-thick ones.

Thank you sir. That's a good tip, I'll remember to do that.

I ended up going with the 0.040" NO LEAK MLS head gasket from CycleX after speaking with them. This allows me to use the dowels and rubber seals as well. Just have to make sure there is enough clearance. The MLS gaskets do no compress, correct?

The ones that I have removed were a hair over 0.032" upon removal (where they did not delaminate in the process).

About squishy and quenchy things: keep in mind that the 750 engines built before the K4 all are swirl-charge types as well. That's what all those ridges and guide angles are in the chamber. I removed them in my K2 head in 2006 (and recessed the valves and milled back the head) to a full hemi design. The increase in midrange torque between 3500-6500 RPM was immediately noticed, too. In top gear (I push a Windjammer around with mine) I was never able to 'roll on' from 50 MPH in top gear, always dropped 1 or 2 gears first to jump on things. After this, the bike rolls on so well that I actually stepped up the countersprocket to 19T (had to clearance the rubber cushion on the sprocket cover post and trim the teeth tips all 2mm to fit it) and now use the 5th gear as my Interstate-only overdrive.

[scottly]

No if's or but's about it and the clearance is .038" min & .040" max 

 Captain

A tolerance of +/- .001" is unrealistic for an assembly like this, much less holding it over all four cylinders. 

[MRieck]

 Kawasaki 600 factory race specs from about 20 years ago recommended squish of .032"

[turboguzzi]

Kawasaki 600 factory race specs from about 20 years ago recommended squish of .032"

in my 500, im in that range... makes sense to me, 500-600cc, bore is smaller, the squish band is narrower, so you need to tighten up a bit to get same squish effect. my gpz750 was more like .035

[MRieck]

Kawasaki 600 factory race specs from about 20 years ago recommended squish of .032"

in my 500, im in that range... makes sense to me, 500-600cc, bore is smaller, the squish band is narrower, so you need to tighten up a bit to get same squish effect. my gpz750 was more like .035

Exactly. I wouldn't try it with a Hayabusa.

[PeWe]

What about a 1005cc and 900cc? 

[macvit7906]

You can also improve the squish by milling off the top deck of the cylinders. I typically take 0.020" to also improve the O-ring seals for the 2 oil passages: thicker-than-OEM head gaskets make leaks happen there. The OEM gaskets were 0.028-0.030: thick, modern ones are mostly 0.040" thick. The loss of compression of those O-rings in their shallow recesses lets the oil out. When you get your final head gasket, lay it in place and put the O-rings in, then measure how far above the head gasket they rise. If they are not 0.020" or more, contact me for some extra-thick ones.

Thank you sir. That's a good tip, I'll remember to do that.

I ended up going with the 0.040" NO LEAK MLS head gasket from CycleX after speaking with them. This allows me to use the dowels and rubber seals as well. Just have to make sure there is enough clearance. The MLS gaskets do no compress, correct?

The ones that I have removed were a hair over 0.032" upon removal (where the did not delaminate in the process).

About squishy and quenchy things: keep in mind that the 750 engines built before the K4 all are swirl-charge types as well. That's what all those ridges and guide angles are in the chamber. I removed them in my K2 head in 2006 (and recessed the valves and milled back the head) to a full hemi design. The increase in midrange torque between 3500-6500 RPM was immediately noticed, too. In top gear (I push a Windjammer around with mine) I was never able to 'roll on' from 50 MPH in top gear, always dropped 1 or 2 gears first to jump on things. After this, the bike rolls on so well that I actually stepped up the countersprocket to 19T (had to clearance the rubber cushion on the sprocket cover post and trim the teeth tips all 2mm to fit it) and now use the 5th gear as my Interstate-only overdrive.

Got the cycleX "no leak" MLS gasket. It is exactly 0.040" thick. The no leak gasket allows me to use the 8 dowels and rubber sleeves the extra holes are used for.

I put some dowels in and began measuring to see how far above the cylinder deck the dowels protrude. As best as I can tell they are about 0.031-0.034" above the deck (i just have a measurement range because of my crapy vernier). That being said, do you think it's safe to use the 8 dowels, if the MLS gasket crushes to 0.032" ?

came up with an idea to use some feeler gauges to measure better so I'll try that when I get home. As long as all dowels do not protrude more than 0.032" above the deck than safe to use them?

[PeWe]

My CB750 K6 with billet block 71.25 mm pistons, MLS 72 mm have only the 2 o-rings for oil feed going to head. I did that just for sure.
I had plan to not use any o- ring since the MLS seal around all oil holes.

Bone dry after 15.000 km and no oil smoke. Dynoman Cometic MLS.

Both cylinder and head have flat and very smooth surfaces as an MLS must have. Check so rivets will not be located on the mating surfaces.
It did on my stock cylinder with 836.
I drilled small dents fir the 4 rivets where contact head and cylinder.

[macvit7906]

My CB750 K6 with billet block 71.25 mm pistons, MLS 72 mm have only the 2 o-rings for oil feed going to head. I did that just for sure.
I had plan to not use any o- ring since the MLS seal around all oil holes.

Bone dry after 15.000 km and no oil smoke. Dynoman Cometic MLS.

Both cylinder and head have flat and very smooth surfaces as an MLS must have. Check so rivets will not be located on the mating surfaces.
It did on my stock cylinder with 836.
I drilled small dents fir the 4 rivets where contact head and cylinder.

If found out today that the rivets are too thick and not allowing the gasket to seal fully. Can I just cut off the rivets and use the MLS gasket without the rivets?

I don't see why not, as the dowels hold the gasket in the proper orientation

[cbr954]

Yes I take the rivets out as well as they held my cylinder head off as well.  The rivets just hold the layers together, the dowels keep everything aligned anyway.