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Squish bands
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Posted by: leonard1---------------------

Doing some research I have found many a mention that when milling the head, the squish band angle needs to be recut. Can anyone explain why this is the case if all we are doing is taking some thickness out of the head and not touching the squish band itself

Posted by: IndyMX---------------------

Cause milling the mating surface of the head is going to change the volume of the head, and alter the squish..

Posted by: leonard1---------------------

I understand it changes the volume of the head and the amount of squish, but it wouldn't change the squish angle, right?

Posted by: IndyMX---------------------

I don't know that it would change the angle, but that's not exactly why you have to recut it.. you have to recut in order to maintain the volume.. As far as I understand it that is..

Posted by: _JOE_---------------------

I believe Indy is correct. I'm sure Rich will be by to clear it up though.

Posted by: SpDyKen---------------------

Quote:

Originally Posted by leonard1

I understand it changes the volume of the head and the amount of squish, but it wouldn't change the squish angle, right?

Correct.

You need to check the clearance, both before & after, of what I hear people refer to as "squish," (but I always thought it was the "quench" area.)

You also need to measure the combustion chamber volume, before you modify your head. You then need to machine the combustion chamber to the volume you desire after milling the cyl. head.

Using a professional engine builder who knows what both of these numbers should be is well worth the investment, believe me.

I'd email Eric Gore, if I were you.

Posted by: leonard1---------------------

I know there is a science to all of this but lets see if we can keep things simple and learn by experience. Here in Ozzy there isn't much expertise on kdx's. In my case just milling the head has given me the squish claerance I desired but with too much compression. What would have been nice to know from the start is how much should I have taken off the head without going to high on the compression?

Posted by: whenfoxforks-ruled---------------------

Number 1, if you mill the sealing surface, you must recut the angle and dome. Number 2, by polishing the surface you have ruined the optimal surface. That rough look was about how it should be. There is a specific volume for the head and cylinder. You may have been able to take 5 or 10 thousandths of an inch off without issue.(.127-.254 mm) The amount you took off, I doubt there is enough material to fix it. Maybe this will help, http://www.muller.net/mullermachine/docs/squish1.html

Posted by: leonard1---------------------

That sounds like valuable info that I wish I could have found here earlier but with the limited info available on this forum I suppose I had to find out for myself. I have recently installed a 160 main jet and will try retarding the timing a little to see if this brings any results. Failing this I will try and find a decent engine builder and look at scooping out the chamber and recutting the squish band.

Posted by: _JOE_---------------------

There's weeks worth of reading in older, very informative tech articles in the archives. Like WFFR said, the search function is great.

Posted by: longerdeeper---------------------

Hi Guys, been a while since I have posted and have been meaning to get back to you about how my 220 is after taking then 0.5mm off the head.... it goes great. in combination with the heavier clutch springs adds a lot more grunt, great for lauching over logs ditches etc climbs in the rev range alot quicker and pulls longer.....just completed the 4 round south island cross country series on it. ran mint for all the 3 hour races- just the rider slowing things down!
back to the old questions I run race gas becaus it is not that much more expensive than pump, is more reliable and smells better. When I can't get it the KDX will happily run on 95 octane, I still run it with full synthetic oil at 40:1, the higher compression does not cause any problems.
my 2cents on what has been discussed.. I have played with timing with very little effect on performance, best to just leave it in the middle.
I had a browse throught the technical stuff on cyclinder head machining and I think that if you only took 1.1mm off you will still have the lip around the edge of the head so have not gone too small on the outer area of the squish band and have not touched
the squish angle at all.
I can't see why your bike would not run normal pump gas, as I have siad the guys I know took 1.5mm off the heads were still running pump gas.
changing to a 50:1 oil mix adds a bit of an unknown into the equation, I would initially go back to standard 32:1 and work from there.
My gut feeling is that you might be getting way too lean on the main jet which is giving you the nasty noises you are at WOT, and if you are getting to WOT 50:1 mix is well light on oil. it may be fine for lugging along a trail ride or getting through tight trees so that you don't get spooge and fouled plugs but IMO you are risking a whole lot if you are going to get on the gas properly.

Posted by: leonard1---------------------

Great to get some personal experience. I think the fact that yours is a 220 highlights a few things. Firstly they run a lot less compression than the 200 so you can get away with taking a fair bit off the head and secondly it didn't cut into the squish band. With my 200 after taking only .4 mm off the head it started to cut into the squish band which has left a flat outer section on the band. I hope this doesn't cause any concerns. Since your bike runs fine on pump fuel changing the timing really wouldn't make much difference. I want to play with mine just to see if I can get rid of the detonation.

Posted by: longerdeeper---------------------

I had a thought that the easist way to reduce your compression would be to put a second base gasket in, will change your port height a little but worth a try. Australasin dirt bike mag had an article on Adan Rienman doing this on a KTM200 when he ran it in the Finke Desert race. along with richer main jet to make it last while at WOT for the whole race.

Posted by: leonard1---------------------

I want to maintain my squish clearance so putting a extra base gasket will give me the reduction in compression but at the cost off increasing the squish clearance. This is why lots of people take some meat out of the dome on the head which leaves the squish clearance alone and drops the compression.

Posted by: whenfoxforks-ruled---------------------

Now you are on the correct path. I hope you can salvage that head. Any issues drop Jeff a line and see what he has laying around?

Posted by: whenfoxforks-ruled---------------------

Doubling up on the base gaskets will alter your port timing.

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Squish bands

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