Vintage Dirt Bike Q & A

I am having the rod replaced on the crank of my 1974 CR250M. Does anyone know what the measurement should be between the bearing surfaces?

I got two sets of washers in the rod kit and the mechanic working on the crank wants to make sure the width measurements are correct.

Thanks,
Jim

_________________
JimN
74 CR250, 75 CR250, 77 CR 125, two 83 CR 480s
74 TM125, 79 RM100

If that's a factory pressed crank, I was taught to measure the outside thickness then to rebuild it back to the same thickness. That way you can make sure the thing is the same thickness all the way around. That makes it easier to true.
Or, the spec book says big end axial clearance of 0.2 to 0.4 mm. Service limit is 0.6 mm.
Enjoy! Jon

Jon,
one possible problem with the crank is that it looks like someone has worked on it before. The crank pin is flush on one side of the counter weight and a bit inset on the other side. Not sure if it should look like that or not. Anyway we picked up the new bits including rod pin and we're trying to get what you call the thickness measurement or if available the distance between bearing race on each counter weight or each side of the crank.
The kit came with different thicknesses or crank pin washers which would indicate that rod end play would be different depending on the distance between the two bearing races. This seems like it would be a critical measurement but I don't have access to anything more than an owners manual and that doesn't seem to get that far into an engine rebuild.
Thanks,
Jim

_________________
JimN
74 CR250, 75 CR250, 77 CR 125, two 83 CR 480s
74 TM125, 79 RM100

Hey Jim,
I've got to ask if the fellow rebuilding this has done these before. Those measurements from my last posting are the clearance measurements he will need. Whether those spare thrust washers are extras for another application or for two different versions of a crank for the one model bike I don't know. He ought to be able to tell which ones are right by comparing them to the old ones. I am just a little worried that he might be a car machinst doing something new to him. Make sure he knows how to true the crankshaft - that he has suitable truing equipment, at least V-blocks, a pair of dial indicators and if he's really old school - a lead hammer. If he doesn't true it really well your bike will vibrate like a Maico. Sorry Maico guys, I couldn't resist that one.

Yeah he's done these before but he's been working on later model bikes for a while. His name is Al Holley and works out of a shop in Reseda California. He doesn't have a manual for this bike and asked if I could get the measurements. Maybe I just misunderstood what he was trying to say to me. Anyway I will give him the measurements you gave earlier.
Thank you,

_________________
JimN
74 CR250, 75 CR250, 77 CR 125, two 83 CR 480s
74 TM125, 79 RM100

Haha, Isn't that Jim Holley's dad? I am humbled in the presence of someone who really knows what they are doing. I hope he doesn't see the lead hammer line.

You must me in good hands.

Enjoy,
Jon

Yes that is Jim's dad. You should see his shop...full of pictures of Al's racing days, Jim's racing days, Al's grandkids and friends. Really cool seeing the history and even cooler talking to Al, he is a really nice guy. He's only doing the crank for me and another mechanic named Chaz s doing the rest of the engine. Chaz did my 83 CR480 a few weeks ago.

I have a little down time since I had shoulder surgery for a torn labrum. Hope to get into shape for the last CALVMX race in November although it will likely be on a 77 CR125.

Cheers,

_________________
JimN
74 CR250, 75 CR250, 77 CR 125, two 83 CR 480s
74 TM125, 79 RM100