X30 Squish Gap

I was wondering what’s the general advice with regards to setting the squish gap on the X30 engine.

Since we are restricted to just one cylinder gasket (at least in my local series) and blueprinting is not allowed, tuning the squish gap is rather limited. The way I see it, lowering the squish gap (by using a thinner gasket) will increase compression, which should result in more power. However, since we can only change the gasket that sits below the cylinder, this will also change the position of the exhaust port.

How will this effect the engine’s power, in combination with the change in compression? I’ve always aimed for the lowest allowed squish gap, however I’m guessing that there are some advantages to running a higher squish gap in certain tracks/scenarios due to the difference in exhaust port’s location.


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Where’s squish now? Go too tight and you could be asking for problems. Also weird set of rules you are running with but interesting at the same time.

Squish is a tech item for a KA100 at .41 minimum. On Yamaha’s we run them pretty tight at aroind .25 but we turn less RPM so we don’t need as big of a safety margin.

When adjusting with base gasket only you are going to run a thinner base gasket to get the squish tighter. In doing so you are lowering the exhaust height and lowering rpm at which peak power is made. If you are on a tight twisty track and need a Broad powerband that might work out. Fast track where you are always very high in the rpm range might not be worth the trade off.

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Thanks for the explanation! Currently my squish is at 0.90mm (the minimum allowed locally). The track that we race at is very tight and twisty, and we only reach maximum for a very short time, so it seems like I’m on the right track.

In my opinion, and of others, “Compression is the Holy Grail”.
Exhaust port height, in degrees, would be a close 2nd.
If I knew the stroke and rod length, I could calculate it. It being the exhaust port height in thousands of an inch versus lowering the cylinder to get the squish height right.
“No blueprinting” as if there are people out there who could detect it.
Is there a minimum head “cc” rule for the engine?
My spreadsheet can give you a piston position versus degrees of rotation.

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Someone pointed out I made a typo in my Post. Wanted to make sure I corrected and pointed out unit of measure.

Thank you, I was typing from my phone and made that typo.

.041 inch is spec for KA. .025 inch is where we set out Yamaha’s but they only turn 14,500 rpm.

Looks like .039 inch (.90 mm) is the x30 spec which is where his motor is set now. Given the RPM range of an x30 I’m not sure I’d be comfortable running it much tighter than it is now. Maybe go down to .034-.035 inch but not much further.

Ideally you want to be able to tune compression, squish gap and port height individually, but you have limited scope here. If you can, you could try to play with copper head gaskets to adjust squish vs moving the ports around. In either case you’ll find that the compression/head volume changes slightly but the stopwatch has final say.

That’s were one part of blueprinting can come in… Even though you can’t avail of it… you would set Ideal port height with correct base gasket, then the head is machined to the ideal volume and squish. Assuming you can machine the head.

In your case, I think all you can do is try different base gaskets, starting at the thinnest that keeps the squish safe-ish for clearance.

It’s like baking a cake, keep trying different things and tasting.

Or just send it, modern two stokes like the x30 have very good tolerances unlike the non-Karting specific engines that we’re more common in the past.

@alvinnunley Glad to hear that as so far I have always been aiming for higher compression! Unfortunately the X30 cylinder and head are stamped, which means that it’s easy to know if the engine has been blueprinted when opened.

@Andy_Kutscher Thanks for the advice, I’m already at the limit in terms of regulations, so I definitely won’t be going any tighter.

@KartingIsLife Really appreciate the feedback. I definitely agree that the X30 doesn’t seem to be too sensitive to such changes. However you’re right and I might try a couple of back to back sessions with different squish gaps.

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So did you try different squish ?

So what’s the min squish gap possible (not allowed) with RON98 for an X30.

I’d say it would be hylomar for a gasket. Try it and report back, I do love a good Guinea pig :grin:

I bet that is thicker than the actual gasket I end up with on most occasions. We have been doing r&d and getting pretty creative for gasket stacks


I read somewhere that someone said the tightest gap is 1% of the stroke… not X30 specific
With the X30 having a stroke of 54mm, and squish gap of 1mm. That’s 1.85%

I was wondering if any KP members did some experimenting with this.

You’ll probably find the limit is more a mechanical one (piston/head contact) than one related to detonation.

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Maybe you could do some testing on your new dyno and share the dyno graphs James :smiley:

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Send me an X30 and I’m on it :smiley:

I was thinking mechanical as in gasket thickness - thinnest being a smudge of hylomar. Pretty sure it wouldn’t make contact even at that but you’d have to make sure by subtracting current gasket thickness from current squish gap to see what the gap would be without a gasket. Obviously there needs to be a gasket for sealing, a smudge of hylomar would possibly be enough to seal. You’d have to assemble it quickly tho else it would harden, then you’d have a thicker than gasket coat of hylomar. Aero grade is more workable.

Are you thinking base gasket, or head?

I figured since it’s an “open” engine for this you could\would machine the parts to make (Keep) the desired port timing and squish clearance. You could even machine the head down to min and use copper gaskets to reach the desired sqush clearance.

Base gasket, there isn’t a head gasket just an o ring seal

Right, so you add one is what I’m getting at…

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What exactly is “squish”? Is it to do with the clearance of the piston crown to the top of the combustion chamber?