Vintage Dirt Bike Q & A

After fighting to get this thing running, I finally pulled the exhaust and cylinder head off last night. Well, of course it won't do more than sputter: 1) half the studs are broken off; 2) NO head gasket!! 3) front of piston appears seriously chipped with resulting scarring on front of cylinder. Only good thing is the bike is completely dent free and beautiful (Of course I already knew I need to rebuild clutch basket and have already rebuilt carb and dressed points). It is the only motorcycle my wife will let me have for the forseeable future, so I have to make do or do without.

Here's my dilemma: Searching for pistons and cylinders, everyone seems to believe Wiseco pistons come in .020-over increments. Maybe that used to be, but the new online Wiseco catalog indicates .050-over increments. I found an e-bay old Wiseco piston that is .080 over I might bid on, but is it safe to bore these cylinders that much?? It would be cheaper than buying a new .050-over Wiseco. I found one cylinder that seller says needs to be bored. I found another that is already bored .020 over. Of course if I use mine, I'll have to drill and tap the studs and bore it also. (I believe my Clymers manual tells me not to bore these cylinders as they are chrome plated, but obviously, that is not an option.)

So, I'm thinking maybe buying the cylinder that needs to be bored, buying the .080-over piston and taking them to the machine shop. Maybe putting my cylinder on the shelf for next time and keeping my eye open for a piston in the future that is .040 over or .060 over. Of course, I'll need a head gasket too.

Any suggestions other than parting out the beautiful money pit??

Meant to also say it is a 250 MR. Of course it may be a 256 when I'm done with it. LOL

.020 is inches. .50 is in millimeters. They are virtually the same. Wiseco changed their measurement standards to metric a few years back to better represent what the manufacturers were using.

That .080 or +2mm piston is a 4th (and last)oversize. Why reduce the lifespan that much if you don't need to. Only bore it as much as is needed to clean it up and make it fresh.

That cylinder is not plated. Honda started that with the 78 model CRs.

Thanks. I may need to go .040 over if I keep my cylinder.

Anyone know the difference between a 338 wiseco piston listed for 75-76 models vs. the 337 listed for the 73-74 ?? The bore looks to be the same. I have found a great deal on a 337, but I'm assuming it won't work since wiseco lists the part number for 76 as a 338.

The topic of differences between the pistons you mention has been covered earlier.

My observations with the pistons out of my 74 and 75 CR250 engines shows the ring locator pin in a slightly different position.

The 75 cylinder gained an extra transfer port on intake side and the pin is located to stop the ring ends from popping into the new port.

_________________
'84 Honda CR500
'84 CR125
'85 KX125
'83 IT490
'82 Maico 490
'80 YZ465
'75 Husky CR360 GP Mikkola
'75 CR250
'72 DT2MX
'91 XR200

Anyone know if the MR cylinder had this same port added?? And, if it did, whether I can bolt on a 74 CR cylinder instead. I don't know if my exhaust header would fit with a CR cylinder, since it has a higher exhaust port height than an MR.
Thanks, Mitch

hey Mitchie, look on e-bay for Honda MR. just found an engine with a little over a day left and no bids. starting at $5.00. Says it has "good" compression. "blew my finger off of head when I kicked it over." Missing timing cover and flywheel looks very rusted but, it might have top end your looking for. I am tempted myself just to have spare parts. I found this in the motorcycle section (not parts) and entered honda MR for a search. came up in the e-bay store section under the 2 mr 50's.

Mitchie,
the 75/76 CR250 had the extra port added.
There is no way you can swop the 75 CR cylinder onto the 74, ...base mounting stud spacing is way out........later cases are completely different even though look identical at a quick glance.

75 exhaust port exits on opposite side.

As far as I know the 74 CR cyl will fit 74 MR

_________________
'84 Honda CR500
'84 CR125
'85 KX125
'83 IT490
'82 Maico 490
'80 YZ465
'75 Husky CR360 GP Mikkola
'75 CR250
'72 DT2MX
'91 XR200