Vintage Dirt Bike Q & A

The chain roller / guard on my MT250 is too mangled to use. Of course, originals are no longer available. Is there an aftermarket one that will work? Do I even need it?

I have tried to find an aftermarket part for this too and have had no luck. Mine (1975 CR250M) is still in decent shape and I can still use it. Without it, the chain will be grinding against the swingarm at times and will be wearing that and the chain down .

You may be able to tighten the chain a tad more than usual to prevent that sag and stiffen your rear shock pre-load, but I wouldnt ride it too hard. If you bottom out, you could damage the sprockets, chain etc when the chain gets pulled to tight?

I wonder if one from a MT125 or CR125 would fit. It wouldn't have the roller, but maybe offer some protection.

The '74 CR125 guide had the roller, but later CR125s, MT125s, and MR175s did not, as far as I know. I don't think any of them would fit the 250 mounting holes anyway, but I could be wrong.

If your chain and sprockets are in good shape, your chain will most likely stay on the sprockets no matter what, provided it's not super loose. As mentioned, the chain can flail around and hit stuff, though. My MR's chain stayed on for three hard rides despite the left side of the swingarm being completely snapped off, and held on only by the axle and shock.

Worn chain/sprockets are a completely different story. I let mine go too long, and a couple weeks ago the chain derailed and brought the motor from @ 10K rpm to zero in half a second. The motor seems fine, but the clutch just isn't the same. At least I didn't have to push it home (90 degrees & humid, 7 miles of steep hills), although the chain popped off again on the way back.

If you really want to minimize the possibility of chain problems you might consider retrofitting a modern nylon block-type chain guide, as I did. I welded a new mount on the swingarm for it, but you might be able to fit just the nylon part into your guide frame if it can be straightened enough, or if not, just bend up a bracket from thick steel strip and drill it to fit the existing swingarm mounting holes. This is just a rough example:

http://www.ebay.com/itm/05-06-07-08-09- ... 1690wt_920

Or you could just wait for an MT250 unit to appear on eBay, of course.

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

Thanks for the suggestions. I ended up finding a used one on ebay. Hopefully, that will do the trick.