Vintage Dirt Bike Q & A

i did a compression test, got 110 pounds but steady leak down on the gauge. popped the head and the piston has a section on top right at the exhaust port that is cut away, or perhaps burned away, should the piston be flat across the top? i think itsmelted but im having trouble getting the jug to pop loose so i can replace it with a piston from wiseco. im also ordering a complete carb rebuild kit. thanks for your guys help...
-oliver

Oliver,

You're absolutely correct - replace the top end (piston, rings, bearing, piston pin, clips, etc). I would also hone your cylinder if it doesn't need boring, or at least sand it lightly with 400-grit sandpaper in a circular motion. You're lucky to have a large selection of top end parts available - MR175 owners like myself aren't so fortunate.

I have an assortment of eBay cylinders here with cracked or missing lower fins, so if you're tempted to try the 'crowbar method' to remove the cylinder, I'd advise against it. With the head off, I would spray a ton of PB Blaster, WD-40, or similar penetrant down into the stud holes and let it sit for at least 24 hours, as the stud locating dowels love to rust into the cases/studs/cylinder. Heat up the case/cylinder junction nice and hot - 200 to 250 degrees is fine - which should loosen the base gasket and sealant (if any) pretty well. I use a huge bearing puller on the top fins pushing down onto a plastic drawer pull on the piston crown at BDC and also push up against the bottom of the intake section of the cylinder with a flywheel puller; the idea is to slowly push/pull the cylinder straight up off the cases. One time I took the seat and tank off and straddled the bike, lifting the cylinder straight up by hand against the weight of the bike, trying to rock it as much as was safe - it took a while but came off eventually. Sometimes a combination of heat/PB Blaster/tapping with rubber mallet/levering can work well too, but be careful.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Ya Ray is right the stud locators love to rust especilly when the last person to take the head off dosnt replace the copper washers. I must admit I didnt need the cylinder anymore and it was rusted so I did pry it off with a screw driver. No I did not damage it at all I guess I was lucky. I dont do this normally but like I said I did not need the cylinder anymore. I had to take the nos cr cylinder off of it the other day and it was stuck real good. Spray on copper sealent works real good on the base gasket but it also acts as a glue. I wasnt gonna pry it off so I got out the mallet. There is really not much place to hit it without breaking a fin. I stood on the work bench engine in between my feet and started hitting the exhaust manifold and off the cylinder came. I have a spare cr cylinder and after market manifold for it if you wanna go that route. I will tell you right now its not cheap I found that out the hard way I have way to much into my mt250 half the motor is in a ups truck on its way to rich gagnons house for some port work and a custom reed valve intake am very excited to see what it does. Oh if you do want the cr top end your carb wont work with the manifold it has and then no oil pump ya its a lot of work but im pretty sure its gonna be worth it.