MS660 rebuild question

[1dragon]

Going to put a big bore kit from baileys on my 660. My question is would I be okay deleting the cylinder base gasket ? I ask that knowing you should check squish, but was wondering what normally is found. I also realize that going non oem parts will have an effect on this. Also what do some of you guys use to check squish? I assume if I am able to delete the base gasket that I should use Dirko right? My goal is a strong running reliable saw. Right now the saw runs strong for two cuts in 28" hard oak stumps, then it just falls on it's face like it's running out of fuel. When I bought it I could only get like 120lbs compression on the gauge and did notice slight scoring on the exhaust side, so I figure I'll put the big bore kit in it (good excuse). Thanks

 

[1dragon]

Nobody can help? Maybe I should have just posted in the regular chainsaw section

 

[Canadian farm boy]

With any top end rebuild you should always check the squish. For a ms660 you don't want it any tighter then .020". I use thin soft electrical solder. Make sure when you check the squish you check it in several different locations. I put a small dab of grease on top the piston at the 12,3,6,9 o'clock positions and place a small piece of solder in each dab of grease. Install the cylinder and slowly pull the saw over to compress the solder. Now remove the cylinder and measure the thickness of the solder pieces. This will tell you what you need to do to get the squish you want. Keep the base gasket, remove it or make a different one the thickness you need. Note that when I'm checking squish this way I don't have the rings installed on the piston yet because it's easier to install/remove the cylinder.
You can use dirko, yamabond or almost any other sealant as long as it's petroleum resistant and has an appropriate heat rating.
I've only used the farmertec brand of big bore kits myself and have found that I was able to remove the base gasket.
Make sure when your rebuilding your saw you find out how the original top end got scored (air leak, tuned lean etc.) and fix the issue. Don't want the new top end failing because of a bad crank seal or....

 

[1dragon]

Thank you so much. This information should get me where I need to be.. The one time I tried checking squish was through the spark plug hole and that I could not get good consistent readings. That saw was a good running saw so it was all together. again thank you so much!

 

[1dragon]

So tonight I was looking at my new BB cylinder and noticed a hump on the floor of the intake port. Should I take that out? the old cylinder does not have a hump in the floor.

Also I did check squish with nothing on the base and was right at .020/.019. I figure I should be okay deleting the base gasket. Thoughts?

 

[Canadian farm boy]

Your squish sounds good. Picture of the intake port would help. Make sure all the port openings have nice beveled edges too.

 

[1dragon]

Here's a pic, you can barely see it.

 

[Canadian farm boy]

It doesn't look to bad in the picture. Kinda hard to tell without actually holding the cylinder in my hands. You could try and smooth it out some if you want. Personally I don't think it will hurt the performance much at all leaving it. Ultimately it's your call. Just be sure to check the port beveling. I've seen several aftermarket cylinder that the port bevels sucked and needed to be addressed.

 

[1dragon]

Thank you! I wasn't sure if I should leave it thinking it might be there to kinda act like a venture effect.

I will bevel ports, they look okay but could be better.

 

[1dragon]

Had to add base gasket.test on assembly piston was hitting squish.Don't understand why but here's a pic of compression with base gasket.

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[1dragon]

Couldn't get tapatalk pic up. Ill try it this way.

 

[1dragon]

Well carb kit is next. Tried tuning it and put it in some wood. Stalling out, can't even cut with it. Which I'm not surprised, I knew it would still have a carb problem. Fuel line and crank seals I believe are good, as I pulled the fuel line off the carb on disassembly for the top end and fuel was under pressure just streaming out of the fuel line. Do you think I'm on the right track? I like the compression readings all though 190/200 would have been cool. It will rip the handle right out of your hand unless you use the d comp.

 

[Canadian farm boy]

Your compression will probably increase a bit after the saw has had a couple tanks of fuel run through it and the new rings get seated.
A carb kit is a good idea. The kits are cheap and pretty easy to install. If it doesn't fix your issue your only out a few bucks.
Did you put a new spark plug in when you reassembled everything? Might want to put a new fuel filter in too.

 

[1dragon]

Spark plug is new.fuel filter, yea probably should change that.Lol looks okay but cheap enough and good maintenance anyway

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[trappermike]

Just a note on checking the squish gap in 2-strokes,this is a trick I learned from the Skidoo racing department,it makes checking squish a hell of a lot quicker and easier,because you do not to need to r and r the cylinder!
Just use a piece of soft solder about 4 inches or so long,and bend it in an "L" or whatever shape you need,then simply insert it thru the spark plug hole,when you feel it contact the cyl wall turn the motor over past TDC. I check,front,rear and both sides,squirt some oil in before if you like. I always cut the end of the solder off carefully with a razor so it's not distorted before squishing and measuring. R and R-ing the cyl. is really not neccesary.
In larger motors too tight of a squish can be fatal! Skidoo recommends .060" plus or minus ,010" for racing.(They are larger motors) So I promptly machined the head for .050" squish clearance,a mistake I had to rectify,.050" is actually the ABSOLUTE min.,running less octane than race gas immediately melted the piston crown. I added a .010" cyl. gasket and all was well.