Vintage Dirt Bike Q & A

Just noticed on eBay some people selling crank bearings and listing them as fitting '75/'76 CR, MT, and MR. Not sure how accurate that is.

Well, from a part-number search it appears that at least the con rod, crank pin, and main bearings are interchangeable among the first few years of CR250M and the MR250. If someone with a CR250M and MR250 crank side-by-side took some measurements of crankwheel / bearing shoulder items and the left/right crank ends, you'd have your answer.

Maybe a guy like Joe_Stricklin at the end of this thread, as of three weeks ago:

http://www.vitalmx.com/forums/Old-Schoo ... ne,1155835

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

That's a thought. I found an old thread on here with someone asking the common parts of a CR and MR. Mitchie got on and said the the '75-'76 CRs and MRs are identical down below. Crank, rod, piston, bearings. Sounded like that guy had a '73-'74 CR which Woosh just said are different. Maybe Mitchie can come on here and reconfirm or deny this bit of info? Seems correct since isn't the main difference the porting and the MR's big flywheel?

Found a few CR crank assemblies on ebay last night. Waiting to hear the word go or don't do it. There was a MR crank assembly and rod that apparently doesn't have any cracks or breaks and the threads are in good shape, but I can't tell for sure from the pictures. The crank cheeks are totally rusted up but it doesn't look like the bearing faces are too bad. I have a speed shop not far from home that I send my Triumph cranks off to get burned and blasted and re-ground. Maybe they can do a burn and blast and get all the rust off. They don't do much bike stuff but I know a guy that knows a guy.

I have a good MR250 crank. The threads were a little buggered up, but I rethreaded them to 9/16" fine thread (I think) and it turned out pretty good, I can get pics and a price if you're interested.

Thanks.

Ryan I.

Indeed I am. Just sent you an email. Thanks.

I have a 1976 CR crank and an MR crank and I need to measure them sometime to see if there is any difference or not. I've been told many times they are identical and to the naked eye, they are, but they do have different part numbers. My flywheel will fit on the end of the CR crank. So, despite what I've said, I'm not sure. I suspect if there is a difference, it is in the distance between the ends of the throws.

mitchie,

It would be interesting to weigh the two cranks. My theory with the MR175 is that the crank must have been balanced to work with the stock flywheel, since it vibrates (and revs like lightning) with the small CR125 rotor; I wonder if the same is true with the MR/CR250, and the CR crank is just a bit heavier.

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

It would make sense that they would be balanced differently for the flywheel, but they really don't look much different. Hard to tell from a picture on ebay, but looking at one versus the other, couldn't tell a difference. Couldn't see any evidence of weight been taken out of the crank cheeks in one or the other, so maybe there isn't a difference.

if you compare both cranks at an online partslist you would see that the difference is at the left crankhalf, they have different numbers.

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80 CR250Ra
80 CR250Ra
78 CR250R
80 CR80
82 Bultaco Sherpa 340
78 Bultaco Pursang 250 MK12
76 Bultaco Frontera 370 MK10
76 Bultaco Alpina 250
73 Bultaco Lobito 125 MK6
76 Suzuki RM125A
90 Kawasaki KX250

Been a long time since I even bothered with this bike, but racing season is over and I'm hoping to get something done on it this winter. So I had a question that popped in my head recently....

I got a new set of cases and a new crank and rod. Now I'm wondering if the gearbox can be pulled from the old cases without getting the flywheel off? I haven't had a chance to really look at it and I've never pulled one of these apart, so does anyone know if it's possible to pull the cases apart from the opposite side of the flywheel?

The flywheel on this is rusted onto the crank taper and I've ruined 2 pullers and the stator trying to get it off. I've tried heat, penetrating oil, and impact...nothing. I was under the impression that the gearbox can't be pulled out without splitting the cases...is that true?

you have to split the case to get the gearbox out of it.
the problem is there are some casescrews behind the flywheel you have too screw out.

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80 CR250Ra
80 CR250Ra
78 CR250R
80 CR80
82 Bultaco Sherpa 340
78 Bultaco Pursang 250 MK12
76 Bultaco Frontera 370 MK10
76 Bultaco Alpina 250
73 Bultaco Lobito 125 MK6
76 Suzuki RM125A
90 Kawasaki KX250

Yeah, that's what I figured. Ok, may have a "less than ideal" solution, but may be able to make it work. Thanks.

I have had good luck pulling snowmobile flywheels with patience. Set up the puller, tighten it down (not too tight), and leave it for a few hours. Come back and tighten it a bit more, and give a couple taps on the end of the puller. Wait a few more hours, and do the same thing again.
I have had flywheels pop off as I was walking away from the latest tightening, and even had one that popped off with quite a bit of force so make sure you aren't in it's flight path while doing this.