head gaskets and compression

A few week ago, we were discussing the effects of using multiple, thin head gaskets on compression. I turns out that (according to the Puch manual), the high-torque cylinder for the Maxi Sport 1.5 hp engine requires a thicker (~1.5mm) head gasket rather than the thin, more flexible type (four of which were installed when I received the bike). I happen to have a bunch of gaskets of each type, so I installed the correct (thicker) gasket, replaced the head using a torque wrench and measured the compression at 115psi. Will I cause any damage if I replace this with a single thin gasket in an effort to boost the compression? Will I see much of a boost?



Re: head gaskets and compression

i am not using any head gasket, just 1 base gasket, and i have about 140 psi, on my single speed puch,

the only damage that might happen instantly is if your piston hits the plug, however some people have told me about it causing the engine to over heat and so forth, but i have never had any real problems, however my compression was higher, almost 150, i think my rings are getting really worn, or my base gasket needs to be replaced

Re: head gaskets and compression

High compression ratios pre-heat the fuel mix and so it burns faster. Fast burning is a good thing. But on a air-cooled 2-stroke the head gets real hot and the mix can get hotter than it should.

After ignition, it takes some time for the fuel to burn and produce maximum pressure.. There is an ideal piston position where you want peak cylinder pressure to occur... it is some degrees past TDC.

IF the mix burns too fast, peak cylinder pressure will occur too soon. Power output decreases because the piston is still too close to the TDC. It's kinda like having too much timing advance... You want to start the burning early but not too early.

And I'm not talking about pre-ignition.. that would come later if things get even hotter. You can lose lots of horsepower without any obvious indications of over heating.

So, you can raise the compression ratio but there is a definite point where you lose more than you gain Variables like the head's squish-band width, combustion chamber shape and spark plug location all factor in.. there's no way to tell how far you can go with compression ratio without you doing some experimenting.


As far as compression, 120 PSI is about normal. btw, that PSI reading has nothing to do with your compresion ratio. Compression ratio is the volumetric ratio of (piston displacement + combustion chamber volume) to (chamber volume). Some 2-stroke authorities may use a different formula for different reasons.


As to gasket thickness (thinness).. Aside from damage due to engine overheating, physical damage could come from too little piston-to-head clearance. Measure head /piston clearance at TDC . It's wise to leave about .5 - .8mm clearance because clearance tends to disappear when the engine heats up..

Bolt the head on with your choice of gaskets, put a piece of thin electrical solder wire into the plug hole. Extend it near the edge of the piston. Then move the piston around and past TDC. Pull out the crushed solder wire and measure how thick it is near it's end. Thats your piston to head clearance.

Re: head gaskets and compression

Jason Luther /

other than the piston clearance issue it should be fine. how are the rings/piston. i think the hi-torque motors should have a higher compression shouldnt they? ooorrr that just may be a sneaky way for them to lower the compression to make them legal.-jason

Re: head gaskets and compression

"high torque" would seem to refer to the crankshaft, not the head.

The crank pin might be offset or it's a different crank... then the big end of the connecting rod follows a larger diameter path. That would give you more turning force (torque).

Re: head gaskets and compression

oops.. is it a high torque "cylinder"?

I got no idea what that would mean.. maybe it does mean higher compression ratio and, if everything else is right, the result is higher torque down the line somewhere.

or maybe high torque is allowed for on the head bolts to seal the head/cylinder because it has a high compression ratio?

Re: head gaskets and compression

puch made a couple different size cylinders for their engines, thats what the hi torque refers to, they are usually painted black and have a different port layout than the standard cylinders, they are also made of cast iron as opposed to aluminum,

Re: head gaskets and compression

i also think in the manual that it says no gasket is needed for the hi torque cylinders, although they came with them installed

also on the hi torque thing, the piston is also different, the wrist pin is located in a different area than the non hi torque cylinders

Re: head gaskets and compression

thanks.. larger bore and/or some special porting.

a Google search for "high torque cylinder" (with quotes) turned up nothing but a couple Moped Army posts and "high torque cylinder nuts" for Harleys.

Re: head gaskets and compression

piston? If the pin is in a different place the connecting rod and/or crank must also be different.

Re: head gaskets and compression

yeah the ports are pretty large in the hi torque cylinders, and the transfer ports are set up differently

the crank and connecting rod is the same, theres a difference between the 1 and 2 speeds of course, but as far as i know the connecting rod on a 1 speed with a #22 hi torque cylinder is no different than the connecting rod on a 1 speed with a regular #3 cylinder, at least the half a dozen 1 speeds I have all have the same set up, and they all came with different jugs

the wrist pin is located farther back on a regular piston, as oposed to a hi torque, which has the wrist pin located farther up in the piston,

if you take a standard piston and place it in a hi torque cylinder you may notice a little longer exposure over those ports,

hhhhhhhmmmm sounds like something useful, doesn't it,

Re: head gaskets and compression

yeah.. i realized the bottom end could be the same a minute after i said it had to be different. I pictured a shorter stroke , bigger bore and higher RPM for some strange reason..

As far swapping pistons to get different port timing, it does sound like a really easy way to radically change the power curve but i wonder what you'd end up with ..

Re: head gaskets and compression

yeah its sort of like shaving the skirt of the piston,

i normally run my bike with a 3 cylinder and piston and a 33 head, and only a base gasket, i set my bike up like that, with an entire 33 cylinder and a 3 piston but the piston seemed to be tapping the plug, i didnt have any gaskets at the time, so i went back to my standard set up

i ve got a few gaskets now and I have been porting and polishing my hi torque cylinder this week so I am probably going to try it again in the next few days, i would like to get some new rings though sometime soon

Re: head gaskets and compression

You might look for a different plug.. they make at least one that hides it's nose inside the plug hole.. but there's more of a tendancy for it to foul.

maybe tape or glue cardboard or something to the top of the piston, install your plug and turn it over past TDC and look for an indentation... or maybe a little ball of clay or similar would allow you to accurately measure plug-to-piston clearance afterwards.

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