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The Chilton book I have says I can change the rear main oil seal with the motor in the truck. My 38 has a 1961ish l-head. Has anyone replaced the rear main oil seal in the vehicle? Small leak but all other seals have been replaced.

Scott

I haven't done it but I read the same thing. I recently had to take the oil pan and rear bearing block off so that I could insert the flywheel mounting bolts - the engine rebuilder forgot to put them back. Supposedly, you can leave the crank in place and thread the upper portion of the rear seal around the crank. To be frank, that looked to be very hard to do and I had the engine on my benchtop! Possible - maybe, easy - I don't think so.

Yes it is possible. I have done it. It took about two hours (if you don't include the time to clean and repaint the oil pan) and the end result was a leak that was only slightly less than when I started. The military manual calls for using a flexible gasket cement on the ends of the upper and lower seals and also, I believe on the rubber dowel seals. I wasn't aware of this when I did my repair so that may explain the continued leak. Good luck and get plenty of hand cleaner.
When I did my M38A1 engine it was torn apart and on the bench and the result was a good tight seal.

Unless you have a garage type lift it is much simpler to pull the engine and put it on an engine stand and be able to do a very good job of replacing the seal with a clean area, plenty of light and comfortable working position. The engine use a rope type seal. When this seal is properly installed it will hold up for about 20 to 30 thousand miles. On the later "V" type preformed seals make sure the crank seal area is the correct dimension for it. For the best possible sealing you really need to see the full circumference of the journal area the seal rides on and polish it if necessary.

Another point to consider is oil can often leak from the rear cam bearing cavity through the rear welsh plug. This plug often gets knocked loose when the engine assembly screws up by trying to hammer the cam driven gear on the end of the cam. It will leak oil from the exact same area as a rear main seal will. By pulling the engine you can then inspect this welsh plug which is hidden behind the flywheel.