Re: 50-hour engine test

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Posted by Matt Wilson [] on Wednesday, March 06, 2019 at 12:29:21 :

In Reply to: Re: 50-hour engine test posted by Kaegi [] on Wednesday, March 06, 2019 at 02:22:04 :

Wow, yeah, I've seen you mention doing a lot sustained 3200-rpm driving before, but had forgotten how many miles you said you had put befind the wheel with these engines. All the details of construction you mentioned are things that I have thought about. Add to that the forged rods. Even some of the machine shops I've visited did not believe me when I said the crank and rods were forged and exhaust seats hardened. They are so used to dealing with cast parts, or in some cases, I think they don't really even quite understand the difference between a forging and a casting. Some folks are also surprised that the tolerances were held so tight on such an old engine. For example, the clearances between lifters and their bores are less than 0.001", and the clearance between the piston pins and their bushings are 0.0004" (four ten thousandths) or less.
For some models, crank bearing clearances are between 0.0005 and 0.0015". They think old cars all just had loose tolerances, and that's just not the case. Now I'll agree that some things like the allowable cylinder taper were allowed to go quite big before a rebuild was mandated.

Interestingly, on one of the other forums I visit, someone posted that there were many stories of M37s spinning their bearings in the '50s '60s, with the implication that those failures were speed-related. Someone else who says he has put hundreds of thousands of miles on these engines over the past 60 years responded to that post, saying he has spun a couple of bearings, but not without plenty of warning before it happened, in the form of loud knocking and continued driving despite the knocking. When he took apart those engines, he found that the bearing clearances were excessive (hence the knocking). He has also taken apart a few engines with spun bearings and he said it was clear that the bearings were on their way out and certainly gave warning before they spun.
I think that's the case with any engine, though. Proper maintenance and awareness of the condition of one's engine are certainly key in these cases.

Even if the engine doesn't fail catastrophically, shorter life could be the result of high rpm driving, but I question whether it's shorter by any significant amount. Some people talk as though driving at 3200 rpm consistently will cause an engine that would have otherwise lasted 100,000 miles to wear out in 25,000 miles.

Anyway, one can say I'm going down rabbit holes, but this is an interesting topic for me.

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