Squish Gap Questions

I’m looking for some thoughts from the engine peeps here. I recently changed out the top end on my KZ-R1, and was experiencing a noticeable distance from side to side when measuring the squish gap. The two sides had a consistent difference of about 0.04-0.05mm.

I usually measure one side at a time, but due to the difference mentioned above, I decided to try measuring by using one single piece of solder laying across the full diameter of the piston. What happened next confused me even more, as those readings were 0.09mm LESS than when I was measuring one side at a time.

Two questions:

  1. What could be causing the difference in readings from side to side?
  2. What could cause the reading to change that much just by measuring both sides simultaneously?

I always make sure that the ends of the solder are cut squarely, and that the piece is inserted all the way to the cylinder wall(s). I’m using 1/16” 50/50 rosin core solder, which is what I’ve always used.

Any thoughts on what might be going on?

I use two pieces of solder when measuring squish

Because if measuring one side at the time = applies pressure to one side = makes the piston twist/rock in the cylinder bore = gives bigger gap. More piston to wall clearance more squish gap

Two pieces or one long piece will give smaller gap as the piston cant rock. These two pressure points keep the piston aligned with the bore

Solders need to be aligned in line at the piston pin centerline. If one isnt in line it shows up as un even squish. 0.02mm error comes easily. Not sure about how much 0.04-0.05 would require


This is a good explanation but how do actually accomplish this? It would seem like you have to remove the head or am I missing something?

If using one wire per side then you can simply insert via the spark plug hole, position the wire parallel to the wrist pin, and crank the piston past TDC. If using a single wire laid across the piston then it does require removal of the head.

I get the same differences when I’m measuring in my TM KZ10C, I think it will measure slightly different because of the piston play. And since the piston isn’t 100% flat it will move when compressing :grin:

I just measure from the hole with a single wire and confirm the squish by that method since that is how the technicians measure at races.

I set mine at 1.05-1.10mm

related or maybe the result of setting squish… do most people change the port timing to move the powerband to suite the track or just keep it the same? seems like a 1% type activity but no data to support it.

For me, not in a gearbox kart unless switching from sprint to road racing, or vice versa. Gearing changes are a more productive means of adapting for the nuances of a given track.

Very rarely done with KZ because they have timing restrictions anyway. Once it’s right, it’s right. The remainder of tuning you can do with exhaust.

With a mod moto its more common, but still not very common.

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+1 on what Mikko said. Also, you want to use a micrometer mounted on a stand instead of a caliper for these measurements. Last, make sure the head bolts are torqued, in steps, using a torque wrench

My X30 measures 1.02mm and rules state minimum of 0.90 should I put a smaller base gasket to achieve smaller squish, rules state minimum is 0.30mm maximum 0.45mm, currently it has a 0.40mm in it.
If I lower the barrel to achieve a lower squish, it will alter the port timing won’t it?
Is it better to have lower squish more compression which will alter the port timing or keep standard so I don’t alter the port timing?

Last year I had an X30 from a reputable builder, and the squish was set to the minimum legal limit, so I would go that route. Depending on the base gaskets you use they may compress slightly after being torqued for the first time, so double check that you are still legal after making the change.

The difference in port timing will be fairly negligible.

Thanks Evan, that is confirmed what I was thinking would be the best way to go.
That’s a good tip to check base gaskets after tensioning, I wil do that.
Do you know if there is a magic number that the port timing should be for best preformance?

For this season I set the squish in my 175 sse to 0.78mm. Stock combustion chamber shape/ volume. The motor ran fine, (usually ) didnt detonate, used 98 octane pump fuel that we have here in Finland with 4.5% oil ratio. Ran it too lean few times and after 70 liters of fuel opened the cylinder. The head was fine - the piston showed erosion on the edges. Two things : because of my carb adjustments and (maybe ) because I had machined the squish so it would open 0.15mm by the lenght of the squish band towards the chamber. I could see where the ejection stream had hit the chamber. That area was clean. But there was a ring of carbon at the squish band exit area. My reasoning is that this carbon area is fuel that is burning/compressing the mixture in the squish band near the cylinder wall = the edge gas detonates or burns at high temperature. In my mind I would like all mixture to burn at the center of the chamber. I dont want anything burning at the piston edges

So thought that I will tighten the squish so tight that theres no room for end gases and see what happens

Week or two ago machined the head so that squish matches the piston angle and set the squish to 0.62mm ( contact tolerance for my motor is a bit under 0.5mm) The ejection stream became super strong. And fast - so fast that I now propably have too much timing at very low rpms. I need to retard the ignition at low revs and check what happens. Here I am talking about warm up lap speeds when cruising around the track and warming up the engine - the engine is a bit too harsh. BUT on high revs this thing SINGS. What a beautiful sound. Its like a proper racing 2 stroke like Schwants Rainey Doohan rode. It definately sounds different to the previous angled squish heads. Its sharper than a very sharp thing. And I love love love it :smiling_face_with_three_hearts: After a day took the head off and there was no signs of contact or detonation. Used some toluene to be safe so havent tried with pump fuel. Power wise I dont know - havent dynoed with this squish. I dont know is this a long term success but well see


Fun info, thanks for sharing. What max RPM are you seeing with that engine? Did you notice any shift in the power band after re-machining the head?

Do you have a radiused edge on the squish band where it meets the combustion chamber, or is it sharp?

I would double check your X30 rules, but provided there are no modifications allowed to the head/cylinder and you’re not using some kind of head gasket (spacer), then squish and port timing are always going to be at odds with one another.

Good mark on the radius. I have been thinking how it effects. At the moment theres a radius at the edge - (just ) slightly bigger than in the stock head but not much. I didnt want to change the radius just changed the squish angle to see what happens and where it shoots

I am leaning towards that a sharp(ish) edge, with some minute radius would be best. Just enough that the edge doesnt heat/burn. I dont recall large radiuses on 4 cycle race heads. As the stream is coming in at high velocity - can it stay attached to a radius? I would guess it can to a point but havent tested many radiuses. Big radius might just give space for mixture to not take part with the main combustion near the spark plug

In my previous angled heads the tighter the squish, higher the compression, the lower the rpm range. With stock ignition. Stock tillotson carb. There propably could have been more revs with more timing retard and a bigger carb. And different exhaust

At the moment it seems like this 0 angle head likes to rev. I need to do more laps/testing. Made personal lap record with this head so at least its not completely wrong. Truth to be told the time was in chassis set up - motor had nothing to do with it. Just a nice bonus with a great sound

Evan have you used different heads, different compression ratio, edge radius / sharp edge and if so - what differences have you noticed ?

Base gasket change of 0.1mm only changes port timing duration by ~0.5deg.

Thanks Mark, 0.5 deg is bugger all so It would definitely be better to run the lowest squish possible then.

Sorry for the long delay…I lost track of this post and didn’t mean to blow you off.

I’ve experimented with a couple of off-the-shelf options for the TM, but haven’t really had the capacity to fine-tune some of the details you’re asking about. While I don’t recall specific measurements, I believe all of the head inserts I’ve played with have had an increasing squish gap as you progress towards the center of the piston. I know all of them have had radiused edges.

Truthfully, I need to hunt down a dyno where I can better quantify the impact of very small changes. For the most part I’ve observed that you’re trying to strike the best balance between all of the variables at play. I have one setup with a 4* piston, and one with a semi-flat piston…both run well if optimized, but they require slightly different setups. My current race setup with the 4* piston seems to operate best right on the edge of the cc limit, so I’m curious to open that up a bit and test whether it works any better/worse.

Thanks for the reply Evan !

So far what I have tested different squish shapes need somewhat different timing curves

The head I am using now - the stock ignition is too advanced at low revs (squish at same angle as the piston)

The engine kind of fires against itself when going slow speed. For the same reason its hard to start. The engine water temperature is also very difficult to get up - it takes several laps, curtain up, hand over cylinder etc

I machined stator bolts so I could rotate the stator few degrees and retard the ignition. Now the engine sounds smooth while doing warm up laps and the engine heats up in little over two laps to 50`C. Like stock engine. But now theres too little advance at high revs so engine doesnt pull hard as with stock ignition curve - sounds more lazy/muddy

There should be a lever for ignition advance on the steering wheel - like cars had in the early 1900 :smile:. Retard to start + heat engine up to temp - then full advance when its party time

The previous head (squish gap that opened towards the sparkplug ) wasnt this harsh with stock ignition timing at low revs. I dont recall I ever made note that its hitting against itself like this current head

Does anybody know is there a programmable ignition available on the market to a 2 stroke kart ?