karolh

The crank on a stock non recoil start engine has a solid crank pin and after having 2 pins shear without abusing the engine I really have my doubts on the reliability of the design as I have heard of others that have suffered a similar fate.

Karol

Anthoop

Do not have doubts about the design..as in the hollow crank pin...the Hyper .21 and Macstar .28 use a hollow crank pin no matter if pull/roto or bump start, there is not a crank with a solid pin available....I have never sheared a pin nor seen/heard of it ever happening.

So the design works but that does not help you eh?!

A brief search of Google only turned up a similar result posted by you?...on rcgroups.
The Force .28 is an engine that has been around for a while....so maybe there is a rogue problem with something else given that you have sheared two pins now...who knows...

Maybe the crankcase has been machined incorrectly misaligning the crank and sleeve....maybe the starter pin/spring are worn excessively (doubt that would cause the pin to shear tbh)....maybe the back plate and/or starter shaft are made incorrectly or have excessive play..etc...

If you have some pictures of the damaged cranks then that may be useful.