Some Tips on Rope Seal Installation

Question:

The one car I could never part with is my 1955 Nomad. I restored it 34 years ago, putting it back to bone stock. Every spring I fix the small problems then drive it some that summer.

Oil leakage at the rear of the engine reached an unacceptable level last year, so I tried to replace the rear main seal with the engine still in the car. The results weren’t good.

How do I replace the rear main crank seal on an original 1955 Chevy 265 cid V8 so it’s nearly leak-free? The rubber aftermarket seal seems to fit 1956 on but not the 1955 engine. I’ve installed new rope seals twice, but it still leaks. I’m thinking I may need to pull the engine and then remove the crankshaft to get it right. If you know any tricks I’d love to hear them.

Answer:

It is very difficult to get leak-free or nearly leak-free results when installing a rope seal in a vintage engine without disassembling that engine.

Even under the best circumstances, installing a rear main rope-style seal so it does not leak is part science and part art.

I have found that ongoing rear main rope seal leaks are typically caused by one or some combination of a few different common problems. One is imperfections in the surface of the sealing surface of the crankshaft or a crank that does not spin true at the sealing surface. In these instances, the seal can be perfect but the engine will continue to leak oil.

Another common problem is a rope seal that ends up too short, leaving a fatal gap where the ends should meet. There is an art to cutting the seal to the correct length so that when it is fully installed and all of the bolts are torqued down to the proper specifications the ends of the rope seal touch and actually do seal.

I have also seen where the seal shrinks over time, much the same way that clothing usually shrinks after a while. One way to try to avoid this is to try to “preshrink” the seal by soaking it in oil and putting it through numerous thermal cycles. Even so, it is helpful to put a little extra into your calculation for where the seal should be cut upon installation so that when it does shrink a bit down the road it will form a complete circle around the crankshaft.

Using a good quality seal is also very important. I have heard it said that in the old days rope seals contained a liberal dose of asbestos and this helped them form a lasting seal. However, because of the health risks associated with asbestos its use in the manufacture of rope seals was discontinued nearly 20 yearsago.

A company called BESTgasket (bestgasket.com) makes a rope seal that replicates the longevityof the old asbestos-laden seals.

If you feel lucky and want to give it another shot before removing and disassembling the engine then try using BESTgasket’s “Graph Tite” rope seal.

Another option is to convert to a rubber seal but, of course, that requires that the engine be taken out and at least partially disassembled.

One more point worth mentioning. If you haven’t already, do some investigating to make sure that the oil that’s leaking is, in fact, coming from the rear main seal and not somewhere else. Other leaks that may look like a rear main include a leak at the rear of the intake manifold to block seal, and a leak in the camshaft bore plug or one of the three smaller plugged holes at the rear of the block near the cam bore plug.