Note: Not sure what happened to the MGB site but it looks like it had to
revert to a previous Restore Point. I will try again with this new thread
that got dropped.

____________________________

More than one web store selling zero drag seals makes the claim that
the rear rubber crankshaft seal has an RPM killing death grip on the
crankshaft.

Is there any evidence that supports this conclusion? I'm not challenging
the statement or saying it is not true, it's just that I cannot come up
with any logic that supports the statement.

Also:

I noticed that Fun R/C Boats sells the Teflon zero drag seals. They add
a warning to never install Teflons on a clutch equipped boat. Does anyone
know what the reason is/might be for this exclusion?

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Then I think it was Hugh suggested I look at JA's thread:

www.modelgasboats.com/forum/14...-low-drag-seal/

(my reply is in the next post)

Thanks Hugh. Just went thru that entire thread again and I'm not seeing how to
answer the two original questions.

Could the "Death Grip" be one of those R/C boat myths that got started and then
gathered energy because nobody ever questioned the logic?

Part of what thrills me about toy boats is having at a simple understanding of
the mechanics, physics, chemistry, etc. involved. I don't believe I'm trying to
over think these issues and I hope it helps reduce the amount of "rust" forming
on my brain cells.

I do not believe the death grip is a myth & the RPM loss is measurable. There is a post some place on the dynamometer testing used to determine how much RPM a seal with the spring, a seal without a spring, a worn seal & the Teflon seal effects engine RPM. I do not remember the exact numbers from the testing, but I only use the very low drag Teflon seals shown in all the engines. These four bearing engines are using full diameter crankshafts (.4724") out to the tapers on both ends & there is no polishing to make the crankshaft fit without binding!

Understood. But why does a rear rubber crankshaft seal have more friction than
a front rubber crankshaft seal? If it's true, then there must be a reason ???

I posted an answer as to why this happens, but it looks like it disappeared. I'll post again in the morning.

Looks to me like we only lost the most recent posts due to some snafu with the
MGB server. Appears to be back on it's feet now. If anybody sees something they
posted is missing, please post it again.

JA: As always, look forward to reading your explanation. TIA for the repost.

I believe the latter model Zenoah crankshafts are the full (.4724") diameter on one end out to the taper. On the other end the crankshaft is relieved (.4714") after the bearing area out to the taper. This could explain the tighter fit of the seal on one end. I could not find a later model unit in the shop to measure, but there are many older units. Some have been polished down below the .4714" dimension where the inner race of the bearings would sit.

OK, I get it now - crankshaft diameter variations. That makes sense. Thank you.

I've been trying to measure my CY stroker cranks for variations and I'm not sure
I trust my results. A lot to be said for all that nice equipment you keep in your
shop (man palace).

___________________________

The more I think about them, there is a lot to be said for low drag seals.

If a crank is reasonably straight then the seals will outlast rubber by a wide
margin and provide a better air tight seal than rubber ever could.

If a crank is not reasonably straight then the low drags will be damaged fairly
quick and the air leak is going to be evidence the crank is not as straight as
it needs to be.

That's a pretty cool shadetree indicator of crankshaft straightness for boaters
without sophisticated measuring equipment.

I guess the upside about rubber is it compensates for crooked cranks better than
low drags but they still don't provide any additional information about crankshaft
condition.

Not many engine mods have multiple plus points like low drag seals.

I've been trying to measure my CY stroker cranks for variations and I'm not sure
I trust my results. A lot to be said for all that nice equipment you keep in your
shop (man palace).

Any good quality micrometer that measures in .0001" will measure a crankshaft. All stock crankshafts will be .4723" or .4722" to enable easy assembly of the engines. Some other things to consider about rubber seals is the composition of the rubber used, its durometer hardness & tension of the springs if they are being used. Some engine manufacturers remove the springs.