The other day I broke in a new engine for my stampede. This was my first break in using heat cycling. Before this I just ran X number of tanks through it increasing the amount of throttle used for each tank, keeping it nice and rich, etc. For this break in, I marked BDC on my flywheel so that I could drop the piston to BDC after each run. The break in went very well.

I took the stampede out today, fired it up and ran a few more tanks through it. When I finished, I went to put the piston at BDC only to find that the mark no longer marked BDC. In fact it was just about opposite of that (TDC?). Is it likely that the flywheel slipped? Is that normal? Also, is it necessary to set the piston at BDC after each run, or is that only for break in?

Your flywheel should not slip. Pull the engine and ensure the flywheel nut is tight before you run it any further, otherwise vibrations might snap the crankshaft.

OK. I figured I'd do that anyway just to be safe.

Heh, I had an engine snap a crank because of a loose flywheel. Smoke it on as tight as you can, you won't break the crank unless it's already cracked. Just do NOT use something stuck in the glow plug hole to lock the engine while doing so, grab the flywheel itself instead.

only time my BDC moved was on my savage and I landed WOT from a jump(it only got a half foot off the ground when I expected 4-5 and wasn't fast enough off the throttle) [:@]

How did you mark your BDC? Notch the fly-wheel and put a mark on the bottom of the chasis to line it up?

If that's the case, then I would say YES, your fly-wheel has slipped and the nut wasn't screwed on tight enough.

There is no "physical" way for the piston to switch BDC or TDC unless you have had a drastic failure with the internal engine parts.

Good luck and let us know how it goes.

I marked it with a sharpie and lined it up with the center of the carb. I kind of already knew that it was the flywheel, because like you said, the internals would have to be in pretty bad shape for that to be the case. I just wanted wanted to get confirmation that what I was thinking is correct.

Do you guys drop the piston to BDC after each run, or is that just for break in? I'm guessing yes.

each run

I do it during breakin. My engine is mild enough where it won't stick post-breakin so I don't put it at BDC now.

I secure the flywheel with the backplate off and a piece of nylon or hardwood to lock the crank, approved by OS. The devices that thread into the plug hole are known as 'piston punches' for good (read bad) reason.

+1

I agree, takes only a moment to put the piston at BDC.