Vintage Dirt Bike Q & A

Ok, so I have this mr250 and the magneto side has a seal that appears to be easily changed but how hard is it to change this seal. The photo is on the clutch. My issue is I donot have the means/knowledge to split the case. I figured I could pull the three screws out pull the bearing out and remove seal but not sure if its that easy, looking for any thoughts on seals to be change.

You have to split the cases to change that seal (Honda P/N 91203-357-003). It usually lasts a long time. I've never had a RH crank seal leak on any of my bikes, though I've had several LH crank seals leak.

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

Splitting the cases is not so bad. I thank Ray for his advice in giving me the confidence to do MR's earlier.

Before you do so, why do you think you need to? Per Ray: "A bad left seal will give you an odd lean condition at first, usually including massive backfiring out the carb. The worst ones will seriously effect compression. A bad right side seal will give lots of blue smoke, and it'll smell differently than the two stroke oil. It'll also get a gunk buildup on the tailpipe. Eventually this one will give running problems too-" You might be able to do the left seal (magneto side) without cracking the case. I have a mechanic, who was working on these bikes when they were new, who replaced the left seal on my mt250 without cracking the case. However, I don't have 40 years of experience.

IF you do decide to crack the case, Key suggestions: Get a good Impact driver, get a harbor freight crank/Bearing/Seal puller. (basically a plate with three slots for bolts to attach to the case and big central bolt to tighten against the crank a push the cases apart. -- watch a youtube video on cracking cases.) Take plenty of photos with your phone, noting where washers/shims go and in which order. I also recommend making a disassembly frame (a 12"X18" rectangle of 2X4's on edge) to place the engine on your workbench so that when you pound on screws with the impact driver, the crank and other protruding pieces are suspended and not being driven into your workbench.

Oh, and if you look at your picture, you can see the punch marks made on the screws to keep them in place. I recommend marking the screws and holes so you get them back in their original holes, so that the punches line back up when you re-assemble.

I had a hell of a time getting those screws removed on mine. Had to basically destroy them to get them out. Had to drill out the peen marks, use a bit of heat and penetrating oil, and knock it pretty hard with the impact driver. Got replacement screws from Western Hills Honda. Other than that and issues getting the flywheel off, my cases came apart pretty easy with a regular two bolt puller I had.