2 Stroke KTM squish clearance

[380roost]

Do 300 and 380 KTMs have a larger than normal squish clearance ?
I've seen figures around 2 ~ 2.5mm quoted.
The question is why would they do this, is it for ease of maufacture ?
What would be the effect of reducing the clearance, but maintaining the same head volume ?

Regards,
Ken

 

[cujet]

A very general rule of thumb is that larger engines have more squish clearance. For instance, I use 0.35 inches (about 0.9mm) squish clearance on my RZ350 and 1.5mm on my Husky 360. In stock form, the RZ had around 0.060 (1.7mm) and the Husky 2.0mm

Chris

 

[Jman271]

You could do it a few ways,,,I am not a pro, but the first way you could do this is w/ a head mod. You will have to measure stock how many cc's of trapped volume you have in the head w/ a burret tube and water. Shave off amount of desired material from the head surface,(.010 or whatever to attain desired clearance), re-check squish, recess squish if you need to, then, re-cc the head to see what the trapped volume is. Cut into the bowl and remove material to accuire stock trapped volume. I know on my ktm 03 125, the head looks very difficult to cut, I don't know how yours is, but you could also do it w/ a cylinder and head mod. KTM makes a wide variety of diff thickness base gaskets, you could buy a few of them and experiment to get a tighter squish, however, you will alter port timing w/ a thinner gasket, boosting some low end, and possibly not letting the bike rev out as far as it does now. Following the trapped volume procedure, you could also Remove desired amount of material from base of cylinder, then re check squish, etc,,,,The cheapest way to do this is to get the gaskets and check for squish. You will be able to see if you are going in the right direction w/o doing any permenent mods to you jug or head. I hope this info is accurate and helpful, post back-

 

[380roost]

Chris,
The 360 Husky and the 380 KTM have the same (78mm) bore, what difference did it make reducing the clearance by 0.5mm ?

Jman,
Your idea is good, lowering the barrel by reducing the gasket thickness.

The maximum change would be 0.55mm, if I have a 0.75mm gasket and replace it with the 0.2mm one.

I would like to keep the same compression ratio, but I think that the bowl would have to be increased in volume by twice the reduction in head volume achieved, because by lowering the barrel, the top of the exhaust port is also lowered, and this also increases the CR.

Increasing the bowl width reduces the squish width, I think this tends to reduce the velocity, but there would still be an overall increase with the reduced clearance ?

I can live with a little less maximum power, if it runs better down low.

Regards,
Ken

 

[cujet]

380, I really cannot give you the accurate answer you want. I did this as part of an overall modification which included bigger carb, porting, higher compression, better reeds and a better pipe.

I would expect it to provide slighty sharper throttle response and slighty better power.

Chris

 

[BBD]

As you lower the cylinder on a KTM with thinner base gaskets you will raise the compression ratio. KTM has a minimum distance and anything smaller will result in needing race gas.

 

[380roost]

BBD,
I plan to increase the bowl volume to keep the CR the same or slightly lower.

Regards,
Ken

 

[Picklito]

Ken,

Did you ever get the squish figured out on your 380? I just purchased a new '02 380, and measured the squish at 2.54mm (.100"). That's too big. Did you cut the head? How's it run?
Thanks,
Picklito