1999 25 Marathon 2 stroke

[Dirtrider]

Son-in-laws motor. No compression on the bottom cyl. none,0, I don't see a hole in the piston, it's dark, doesn't look lean. NO CYL HEAD!! What,s up with that design?? I found .030 over pistons available. Is anyone been able to bore these one piece cyl/head assy??? I can't figure why this has 0 compression. What I can see through the plug hole the cyl walls look good. Any ideas???? Thanks G.

 

[galamb]

You are correct - no heads on these - referred to as "headless" or "jugheads".

There are some shops that can bore these out. You may have to look for one. Don't go with someone who "thinks" they can do something with it. A local Merc dealer can probably point you in the right direction.

This model year has what's called a "Mercosil bore" so it can be bored out to a max of .030" over but nothing more. Additionally, when the machinest is in there, get him to check the other cylinder. They have a habit of going "out of round" and ending up with an egg-shaped cylinder (the trade-off for having a cylinder that can be bored. Earlier models where chrome plated and if you toasted the cylinder the motor was scrap).

You may have busted rings - pull either the intake cover or the exhaust cover and you will get a better look at the pistons through the ports. In the end, until you split the block and remove the pistons, you won't get a perfect look inside.