DT200R Squish

[bobc]

Hi Guys ,
I am fairly new to the forum and apologise for coming straight in with a cry for help, I have a DT200 and live in Cyprus so rely on outside advice and you seem to have more professional bikers used to DT/blaster motors than anywhere.
I have cleaned the barrel & head ready for a rebore next week,I would like to pep the bottom end performance up a bit
After reading tons of info here and elsewhere I have done a squish measurement at the edge of the piston with solder .It is 2mm , that is with the old gaskets (top 1mm and bottom .5mm) and using the old piston , I will check the new piston height is the same or adjust accordingly .
I have seen squish measurements recommended from .9mm to 1.4mm . The engine is standard but will be bored to 66.80mm . I thought .5mm off the head would be a safe option. I will then presumably have to increase the volume of the head to keep the compression ratio within sensible limits. At present it is 20cc. Mathematical help to work out the increase needed in the head volume is much appreciated , I reckon I could maybe work it out but I'd like to be sure ..
Thanks in advance
BobC

 

[joeak47]

First things first...Welcome aboard! Now...Make sure you are checking the "squish" parallel with the wrist pin. In other words the "left" or "right" side of the cylinder right above the wrist pin. This will give you a accurate measurment.
DO NOT measure on the "front" Exhaust port side of the cylinder,or the "rear" intake side of the cylinder. The reason for this is... The piston rocks on the wrist pin,and will give you an incorrect measurement. It will show "squish" being larger than it really is..
2mm (.080 thousandths) is alot. Getting "squish" in the 1mm to 1.25mm (.040 to .050 thousandths) would be optimal. The 20 cc chamber will be fine. I re-chamber blaster heads as small as 17cc (depending on exhaust timing) without issues.
I could go smaller,but I don't see an advantage to this,and want to stay away from detonation.
You can close up your "squish" by using a thinner base gasket under the cylinder. This will also give you a bit more "low end" grunt,because it changes the port timing slightly. The DT200 is a "top end" screamer,so the added low end grunt can't help.

 

[bobc]

Thanks for the reply Joe , I did the measurement side to side . My base gasket is .5mm , the only proper gasket material I have is 1mm , would liquid gasket do the job? or shall I go ahead with shaving the head ?
Bob

 

[best]

I totally agree with Joe.
You could (and perhaps should) sand the head flad with a piece of sand paper on a sheet of glass to get the head flat. This will take a very small amount off the head, perhaps 0.2mm.
If you want to take the thickness off the head it can be done with a file and then the sandpaper if you do not have access to a machine shop.

For base gasket you can use plain paper, cut to the right shape with (Yamabond/Threebond) sealer to make up the thickness you need.
The Yamabond can be used alone if the cases and cylinder are flat if you want to drop this far.

Dropping the cylinder will drop the rpm range, but this modification is reversable, so is worth trying.
Anywhere in the .7mm to 1.4mm squish range will work good for you, just listen carefully for detonation rattle. 1mm is probably perfect.
Like Joe said, I doubt you will have any problem with detonation.

Dropping cylinder makes it more prone to detonation, all other things being equal.
If you drop the cylinder and don't like the powerband rpm, or experience detonation, raise it again and try cutting the head for the same squish.
Somewhere there you will find joy!

Steve

 

[bobc]

I dropped the head and barrel off at the machine shop tonight , should have it back within a week. I decided to shave the head while they were on the job, -0.5mm written in big letters , my Greek is not brilliant so fingers crossed . I can still experiment with base gaskets , I have just made a thin spare one out of thick paper , it is .2mm . This takes me back to when I was 10 years old , my first bike , a 98cc Sun with girder forks , was always getting stripped and I used magazine cover then for gaskets .
Bob

 

[best]

Hi Bob,
The DT200 head is better than the Blaster at 2mm squish (vs 3mm) and a squish angle that more closely matches the crown of the piston. We ran good gas in ours when we could but it seems to run regular low grade gas just fine too.

You can stack the paper gaskets too if you like. I have done this for years and never a blow out problem with stacks under 1mm.
KTM comes with select fit base gaskets that includes a plastic (mylar? cellophane?) thin one.

Steve

 

[Blaner]

sound interesting... I love this stuff

 

[bobc]

Thanks for the info guys, I will let you know how it goes ,
Cheers Bob

 

[Wifesblaster]

Any updates on this???

 

[bobc]

Re: DT200R Squish -update

Bike now rebuilt , head skimmed .5mm, cylinder bored out to 66.8 ports cleaned and bevelled and Wossner piston fitted . These pistons are really well made , I have used wiseco and narita pistons in the past ,the wossner weighs 10g more than the wiseco , about same as oem and is machined inside , an impressive piece of german engineering .
So the spec is now 1.5mm squish , standard gaskets, hy-tech reeds , giannelli full exhaust system .
A few easy runs yesterday to run it in then today had a good blast round the trails here .
The exhaust note is now more of a bark and the bike goes really well ,eager to Go, I am sure there is more to be had but I am happy now and the bike should be still reliable .
Bob