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The original Stovebolts had main shims until some...
These engines have cranks with no flange – like m...
This block was done in a CNC mill, but it can be ...
You not only plan the process, but make certain t...
The finished and installed seal looks and functio...
Old Iron: Sealing Stovebolt Chevy Engines
You know that modern lip seals can be installed so as to be all but entirely dry in many engines. They are actually NOT, by the way. All seals require some amount of lubrication and so at least a small amount of seepage – enough to lube the lips – is required.
By Doc Frohmader
Machine shop staff tend to forget their capabilities. They get into a groove that works and then resist change. This not only precludes improvement, but fails to address problems with new solutions that actually work.
Case in point: Leaking rear main seals on vintage engines with rope seals. The Stovebolt Chevys (inline sixes 235 and 261 cube engines) are a good example. I have been working on a series of articles for these engines, addressing possible solutions and improvements, and found one seal solution that is pretty slick. Not only that, but it illustrates the point of thinking about old problems with new technology and quality machine shop engineering.
Let’s say your customer can’t stand the thought of an oil spot on the garage floor and wants a DRY Stovebolt.
You know that modern lip seals can be installed so as to be all but entirely dry in many engines. They are actually NOT, by the way. All seals require some amount of lubrication and so at least a small amount of seepage enough to lube the lips is required. Granted, you can have a VERY limited leak (the occasional spot or just where you have to wipe the seal area and pan off a little once in a while), but almost never a completely dry engine. In the best cases, something relatively common these days, the leakage is about the same as the evaporation and you get what is for all intents and purposes a dry seal.
The Stovebolts first had shimmed mains, including the rear, until sometime in 1955. One camp claims that the machine operations on the cap were done separately from the block before the caps were installed and align boring occurred. That would mean that the alignment of the seal groove in the block and cap was hit or miss. I think all these engines were machined for the crank bore and the seal groove together just given the most economical and rational means of doing the machine work. If you consider the machine operations required to do them separately, it makes no sense. In terms of leaks, what I think happens is that the shims move the parting lines apart so the inside diameter of the seal groove is no longer round. For rope seals this is not a problem, but for lip seals it certainly is.
Starting somewhere in 1955, these engines stopped using shimmed main bearings. If you have shimmed bearings you can’t use the lip seals with any confidence as the shims can offset the lip seal enough to cause problems. The only way to correct this is to have the block align BORED so the main bores and rear seal groove are all correctly sized and centered for use with the later non-shimmed bearings and lip seals. It’s probably easier and cheaper to go with a later block.
I am convinced that a properly cleaned and prepped block that does NOT use shims can use the lips seals without a problem. I would suggest installing them dry in the bore, rotated slightly so the parting lines between the seal halves and the parting lines for the main cap and block are not aligned and use a tiny dot of silicone on the seal ends and a VERY thin coat of silicone between the cap and block at the seal area.
For that matter, a rope seal properly installed can also give a pretty good seal. But it means finding seals that are any good (original asbestos or the new Best Gasket GraphTite material work the best), and having the experience to finesse the seals. Forming, and especially cutting the seals perfectly and getting the cap on and tight without seal material wedged between cap and block, takes skill and experience so don’t fool yourself. For a shimmed block rope may be the best OEM type solution if not the only one unless you re-machine and eliminate the shims.
Now, then, let’s say you are serious about a DRY engine, you have an early block, any shimmed block, or even that you want to go high tech, and money is available to make that happen. There actually IS a rear main seal, made on modern design standards, that will dry this engine up. It is readily available, and once the machine work is done it is simple to install. The only problem is you have to machine the block for it. What is required is to bore a 4.751˝ diameter pocket, .375˝ deep, in the rear of the block and main cap to fit a one-piece lip seal.
It can be done on a mill tall enough to allow the block to stand under it, it can be done with a mill that has a rotating head so the block can be rested on the table, it can be done with the same boring rig used for align boring, or it can be done (with careful setup) with a conventional portable boring bar. No matter how it is done, it is a precision machine operation, it requires someone who knows how to use the equipment properly, it means accuracy so the seal gets the proper press fit, and you only get ONE chance to get it right or you have to start looking for a replacement block.
For all practical purposes, though, it’s really best to do this in conjunction with align boring (on the shimmed main blocks) as the setup is the same and the separate operations cost more than the combined deal. After looking at this a while I’ve concluded that it is most probably something someone with a restored car will do when the original block needs to be retained and the owner is willing to spend the money to do the align boring and seal conversion. I will say that it is a slick setup that many of us will like simply because it really resolves the leaking issues, leaving little chance of rear main seal problems in just about any of the Stovebolts from 1937 through 1963.
This process was done at Industrial Machine and Engineering in Brookings, SD. However, I can’t see why anyone with decent equipment and a little common sense can’t replicate it. While it is certainly a different solution, it uses modern seal technology and basic machine operations to completely change the design of the rear seal and resolves a persistent problem. When it comes to doing quality work and making your customers happy, isn’t this something that will enhance your reputation and add to the machine operations you make your living from? Isn’t it exactly the kind of thing you should be considering and offering?
If you want more detailed information about this particular conversion, it can be found at www.webrodder.com where the Stovebolt Chevrolet engines are a current build project.