Vintage Dirt Bike Q & A

Working on a couple of MR175's and I noticed someone mentioning bad crank seals in another thread. When I rebuilt my MT250 the mechanic who bored the head suggested I replace one of the crank seals (I forget which side) because over time they were notorious for drying out.

Now that I am rebuilding these MR's, the question now is do the MR's suffer the same fate? And if so, how can I tell if the crank seal(s) are bad? What kind of test can I do?

If you're rebuilding an MR motor and are splitting the cases, I'd go ahead and replace both crank seals. If not, I'd replace the LH one behind the ignition and then see how it runs. It's kind of hard to make an absolute determination of the condition of the RH MR175 crank seal unless you have a 2-stroke leakdown tester, or the bike's obviously burning gearbox oil.

My own experience has been that the RH seals last a long time and the LH ones don't, not sure if that's typical or not.

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

Whether the right ones last longer or not, I'm not sure, but there is a rational explanation if they do. As you know, right one stays somewhat lubricated (assuming bike is ran once in a while). But beyond actual longevity, the reason it's perhaps more important to worry about the left one is that you can likely do much more damage with an air leak than sucking up transmission oil.

What is involved in replacing the left seal? Can it be done without cracking the case? Also, is there any practical way to tell if its needed?

I think the left seal can be done with the engine on the bike, but not positive. I know it has to come out and be split for the right one though.
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-