Originally posted by mkirsch
Okay, here's the skinny on why new engines are "squeaky tight" at TDC:
ABC engines have no ring to seal the gap between the piston and liner. They rely on precision machining to provide as tight a fit between the piston and liner as possible.
Piston liners are actually tapered, wider at the bottom, and narrower at the top. This is because the top of the liner gets much hotter while the engine is running, and expands more than the bottom of the liner. When the engine reaches proper operating temperature, the cylinder liner has expanded to the point where the sides are now perfectly parallel.
You do NOT want to turn the engine over by hand until it "loosens up." That's a bad thing to do. All you're doing is wedging the piston into the liner and wearing the bejeezus out of both. The best thing to do is get the engine running and warmed up as quickly as possible. Hand starting new ABC engines is darned near impossible, so you will do well to borrow someone's electric starter for the first few starts if you don't own one.
You do not want to run the engine blubbering rich. It won't get hot enough to expand the liner, and the same thing will happen as if you turned the engine over by hand. Tune the engine for maximum RPM, then richen it down to blubbering rich, then lean it back out until it just starts to run smootly.
Whoever told you to turn the engine over by hand until it loosens up at Tower should be fired immediately.
Hi mkirsch,
I usually remove most text from a long post to save the server disk space but I left yours because of the valid info.
I'm getting back into R/C and many things have changed.
I never recall having an engine that was so tight.
This one was tight even and the bottom of the stroke so I called Tower hobbies and they told me to add more oil and turn it by hand until it starts to loosen up.
I did that with no glow plug installed.
Then I got my electric starter and held a rag over the top and it spun like it was free.
It seems OK now and I have not run it yet.
With the glow plug installed I do hear that little squeak at the top.when I try to turn it by hand.
I have nice free loose rotation now excep t when i'm getting close to TDC.
But it goes with no problem with the electric start.
BTW,
Is this the engine that has the bad rep for developing air leaks around the carb?
Thanks to you and the others for the nice educational postings for us newbies getting back into this great hobby
