Squish Help

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Hooligan
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Squish Help

#1 Post by Hooligan » Sun Jan 24, 2016 7:19 pm

I am in the midst of putting back together the 31K motor I blew up last summer and have run into an issue with the squish measurement. Using the solder measurement method I got the following data:

With no head gasket at all: L - .81mm / .88mm
R - .76mm / .79mm

The issue I am wondering about is that these measurements are almost exactly what one would want to see but this is without any head gasket. The solid copper head gasket I got measures .76mm which would give a total squish of 1.60 - 1.53mm

I also have a three piece metal gasket, which measures 1.02mm. Reading this forum I see people separating these gaskets out to get smaller squish numbers. The centre layer measures 0.28mm, which would give me roughly 1.13 - 1.05mm

Thoughts, suggestions?

Do I go with what I have and accept the larger squish measurements or do I suck it up and have the head skimmed? This is my wife's bike and she really does not ride it super hard. It only gets flogged when I get on it every once in a while. I'm not too worried about extracting every HP out of it, more that it is reliable and runs reasonably well.
'82 RD350LC in '81 colours, OEM pipes, UNI filters
'85 FZ750

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two-stroke-brit
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Re: Squish Help

#2 Post by two-stroke-brit » Sun Jan 24, 2016 8:16 pm

when i had my head modded , it helped the low to mid range power (350LC).
but I would think the RZ is fine with the stock setup .

but the motor ran better for the mod .
( it was stock everywhere else)
350LC,Athena 392 big bore,stock porting,swiss cheese air box,stock reeds,spec11 pipes.(now TSA big bore pipes)
She might not be pretty but she will always be a fun ride


1982 YAMA RD350LC
98KDX 220R
1967 TRIUMPH TIGER
2001 TRIUMPH TT600

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Questo vecchio rz
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Re: Squish Help

#3 Post by Questo vecchio rz » Mon Jan 25, 2016 12:21 am

I've had the head skimmed on 2 of my RZs, absolutely no issues whatsoever, bike ran cooler and noticeably quicker for many thousands of miles. If it's just a stock reliable cruiser like you say, then just leave it as is, but skimming it is just as reliable as stock with no drawbacks. Either way you win.
Banshee (Baja) race bike,+ 2 A Arms,L.E.Ds, Toomey, +4 stroker IMS tank, run flats.
96 GSXR SRAD, Future Yoshimura rep.
85 custom Tri-Z
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JanBros
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Re: Squish Help

#4 Post by JanBros » Mon Jan 25, 2016 6:56 pm

Frits can explain it better than me, so I'll just put a quote I saved from him here :
The higher the squish velocity, the better. Combustion should be as fast as possible (apart from detonation) and squish turbulence helps.
So are there no negative sides to a high squish velocity? In theory no; in practice the ignition may not be up to it.
There is one other possible objection against effective squish. When a gas comes in contact with a solid surface, it tries to cling onto it: it forms a boundary layer. This layer of stationary gas is a fairly good heat insulator. The piston crown enjoys this insulation, as the boundary layer protects it from the combustion heat. In theory the squish flow can blow the boundary layer away, exposing the piston directly to the combustion.
But there is always a far greater danger: detonation. The shock waves in a detonating engine are a thousandfold more effective in shaking the boundary layer loose. So I prefer a good squish that speeds up combustion.

If detonation occurs, it happens in the end phase of combustion, when the pressure in the combustion chamber is high and the temperature of the unburnt mixture in the edges of the chamber is raised by the heat radiation of the burning mixture.
Effective squish flings burning parts of the mixture to the edges of the combustion chamber, igniting all mixture before detonation factors there can reach a dangerous level.
Finally a simple advise for junior tuners: make the squish gap so tight that there is no more room for mixture; if it is not there, it cannot detonate 8)
so best is to keep it at max/under 1mm (or 2x 0.5mm of boundary layer)
if it runs, you can race it !

Hooligan
Posts: 1966
Joined: Mon Aug 20, 2012 1:37 am
Location: Calgary, Alberta

Re: Squish Help

#5 Post by Hooligan » Mon Jan 25, 2016 7:14 pm

Makes me wonder if the original piston melt was a result of excess squish clearance?

I think I will try separating the 3-piece gasket and just use the inner layer. That will put me close to the 1.0mm range.
'82 RD350LC in '81 colours, OEM pipes, UNI filters
'85 FZ750

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two-stroke-brit
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Re: Squish Help

#6 Post by two-stroke-brit » Mon Jan 25, 2016 8:23 pm

sounds like a plan hooli.
350LC,Athena 392 big bore,stock porting,swiss cheese air box,stock reeds,spec11 pipes.(now TSA big bore pipes)
She might not be pretty but she will always be a fun ride


1982 YAMA RD350LC
98KDX 220R
1967 TRIUMPH TIGER
2001 TRIUMPH TT600

Hooligan
Posts: 1966
Joined: Mon Aug 20, 2012 1:37 am
Location: Calgary, Alberta

Re: Squish Help

#7 Post by Hooligan » Mon Jan 25, 2016 8:58 pm

A bit of spray sealant on each side work fine.
'82 RD350LC in '81 colours, OEM pipes, UNI filters
'85 FZ750

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Redline Junkie
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Re: Squish Help

#8 Post by Redline Junkie » Mon Jan 25, 2016 9:11 pm

Pay Slinger to plane, reshape and o-ring the head. He did mine and it looks great! Take the head off three times a day if you want and don't worry about gasket! Better heat transfer/cooling!
1985 RZ350F being restored now!
1987 RZ350 Stock
1985 RZ350F basket case
1985 RZ500 Stock

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Re: Squish Help

#9 Post by (F5) » Tue Jan 26, 2016 5:07 am

I don't think I've ever found an engine that you can skim the head to get the `right` clearance, without going too high on compression. Certainly my 1ua needed a scallop out of the chambers back in the day.

Regularly they have huge clearances to be safe.

My advice: Use a new std yam gasket and don't touch it.

Unless it becomes yours, in which case send it to a tuner, or buy a burrette and measure the compression ratio. Going too high will detonate and kill the pistons. Large squish; abhorrent to theoretical efficiency as may be will not cause any issue.

Now, down to the garage to remove the heads from the 1970 stinger engine on my bench as I can't live with it going into the bike with the 2.2mm squish I've measured (honestly true story, right now).
496 Cheetah. TSS PVs, PWK35s, Ignitech, RGV(ish) chassis

Hooligan
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Location: Calgary, Alberta

Re: Squish Help

#10 Post by Hooligan » Thu Jan 28, 2016 12:25 am

Good point on the compression issue.

I tried drilling out the rivets holding the 3-piece gasket together but as rivets they just spin making them a PIA to drill.

Lost patience with it and said "screw it" and went with the solid copper gasket, which was the original plan.
'82 RD350LC in '81 colours, OEM pipes, UNI filters
'85 FZ750

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motoholic71
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Re: Squish Help

#11 Post by motoholic71 » Thu Jan 28, 2016 2:48 am

That's your problem right there you should bin the copper and go with the stock gasket. why would you put a stone age copper gasket on an engine that has a sophisticated bullet proof multilayer Teflon gasket really beats me...

btw squish check with solder is made with all the gaskets in place and bolts torqued down to spec, otherwise is irrelevant...

A new stock gasket is the only sure way for a reliable engine, later on you'll find your problem is either oil pump related or carb lean/jet clogged/air leak on intake side or air filter side and those are 99% responsible for 2 stroke failures; can't beat facts!
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Questo vecchio rz
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Re: Squish Help

#12 Post by Questo vecchio rz » Thu Jan 28, 2016 4:00 am

motoholic71 wrote:That's your problem right there you should bin the copper and go with the stock gasket. why would you put a stone age copper gasket on an engine that has a sophisticated bullet proof multilayer Teflon gasket really beats me...

btw squish check with solder is made with all the gaskets in place and bolts torqued down to spec, otherwise is irrelevant...

A new stock gasket is the only sure way for a reliable engine, later on you'll find your problem is either oil pump related or carb lean/jet clogged/air leak on intake side or air filter side and those are 99% responsible for 2 stroke failures; can't beat facts!


I'm inclined to agree on alot of what you say.
The stock set up is practically bulletproof in my experience, under virtually any condition.
Unless one is designing ones interpretation of a ultimate competition engine, just skim it, use a factory gasket and enjoy it until the damn thing just wears out. . A engine built and maintained anywhere close to original specs, I can't imagine it ever self destructing, the cause, more often than not, would arise from one of the scenarios you described.

Not opposed to or diminishing more in depth squish design or O rings etc.. but Id trust the stock Yamaha design for reliability 7 ways to next Sunday, on your average RZ.
Banshee (Baja) race bike,+ 2 A Arms,L.E.Ds, Toomey, +4 stroker IMS tank, run flats.
96 GSXR SRAD, Future Yoshimura rep.
85 custom Tri-Z
RZ/YZR bike(project)
86 VFR750 RC24 Merkel replica (project)
Royal Enfield Bullet 500

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motoholic71
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Re: Squish Help

#13 Post by motoholic71 » Thu Jan 28, 2016 5:20 am

Right? If he drops in a brand new stock gasket and torque down all the bolts to spec, he can pretty much forget the damn thing exists and it wont blow top ends either.

I would check if the vacuum operated tap doesn't have a leak on the actuator line or a blocked gas filter, or even a simple bent fuel line, a stuck thermostat, broken water pump gear... but not the squish nor the ignition on a stock engine-more likely you win the lottery!
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Hooligan
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Re: Squish Help

#14 Post by Hooligan » Thu Jan 28, 2016 1:21 pm

The 3-piece gasket is about .015" thicker than the copper gasket.

I went copper for the reusability aspect.

The squish measurement was made with the head properly torqued.

The engine didn't seize. It ate a hole in the top of the piston, which should be detonation related.
'82 RD350LC in '81 colours, OEM pipes, UNI filters
'85 FZ750

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JanBros
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Re: Squish Help

#15 Post by JanBros » Sat Jan 30, 2016 8:01 am

Hooligan wrote:The 3-piece gasket is about .015" thicker than the copper gasket.

I went copper for the reusability aspect.

The squish measurement was made with the head properly torqued.

The engine didn't seize. It ate a hole in the top of the piston, which should be detonation related.
doesn't detonation "eats" into the piston's right at the edge and never in the middle ?
a hole in the middle is from the piston getting too hot probably from running too lean (air leak ?) or bad engine cooling.

just my humble opinion :smt002
if it runs, you can race it !

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