Got a brand new TT .46 pro in the mail today, at tdc the piston "catches" and stops. It will turn, but it seems like it clicks and stops and then you turn it and it clicks and goes around until you get to tdc again. It is a noticeable stoppage and not a slight bump. what is the problem?

My father an I have about a dozen of these engines between us. the "bump" is normal for this type of ABC engine. Run 2 tanks at a rich setting and it will fly great. MAKE SURE YOU LOCKTITE THE BACKPLATE SCREWS IN ! They have ALL come loose and will go dead stick. TT has been very much behind there warranty, I sent two of mine in after 2 1/2 years and they repaired them with no questions.

chances are if this is a ABC style engine then the "catch" is a good thing. A ABC engine will have a interferance fit at TDC. When the engine is at operating tempeture the fit will be perfect since the aluminium piston and brass cylinder expand at different rates. My nelson, jett,and even my rossi will not turn by hand when cold. bolt it on and rock on

Don't run it too rich (do not 4 stroke engine, run about 500 rpm shy of max lean rpm on break in) or you could cause damge to your engine. Here's why:

That pinch you are feeling is caused by the fact your engine is not a ringed engine, the cylinder sleeve is tapered to insure a proper seal as the engine heats up and expands. Try this to verify my statement, take a torch or heat gun and carefully heat the head of the engine 360 degrees around. Rotate engine and you will notice the ABC "pinch" has decreased a bit and the engine is easier to turn over. IF engine is run too rich, it won't heat up enough for that pinch to decrease, consequently, the pinch may cause excessive load on the engine's connecting rod and this part has been known to break in tight/new engines.


Also, if the pinch is too tight for your starter, apply heat to the engine (as mentioned above) is a perfect way to get a new/tight engine running for its first couple of starts. I do this with all of my Norvels that are very tight when new.

The "click" may also be the clearances in the rod and pins in alignment. As the crank is turned it forces the piston up. Resistance is felt from air compression and metal binding. As the crank/rod assembly forces the piston to TDC the clearances in the three pivot points are at their extreme limit. When the crank is turned further the piston is still 'stuck' and the pivot point clearances change to the opposite extreme to pull the piston back down. The change in direction of the clearances will cause a 'click'.
BTW, it is not very wise to play with it before it is broken in.

Fellows, many thanks for the inforamtion, it is very helpful. I want to make sure I have explained the problem; I am familiar with the compression bump when you turn an engine over. This is a new engine right out of the box, no plus installed. It actually stops at tdc and there is a small amount of actual play each direction and it seems like you're hitting a wall. if you apply enough force you can turn the engine over and get past the bump or obstruction. I took it from the box and turned it over once and felt this and did it again to be sure and then quit because i didn't want to aggravate the problem. it sounds like this is normal. I own two .tt .46's but never a new one and this belongs to a friend and i didn't want to break it in for him until i thought it was okay to do so. In light of all the answers given, am I ok to start it up you think?

Last year a local, brought out his new prop jet with a Jett .90 on it. Was not even bench run or nothing. Could not even turn it by hand. He hit that thing with the starter and took the engine,mount, pipe clean off the plane!

Tailbacker, would've like to have seen that, from a distance, that is.


Jazzy, I think you hit it right, when I took off the backplate I could see the rod at tdc and that was where the click came from. Again, I take it this is normal and I should go on with the breakin as per the instructions.

I distinctly remember the same situation when my thunder tiger engine was new. I never noticed it again after running the engine, so I assume it isn't a problem (it does seem odd though!).

This is not really a "promo" for "Jett" engines but does give me reason to get one for my new F-20. After that guy put everything back together, The power that thing has is tremendous. He had an 11/8 APC.
The kicker here is when he would dial it in. It "honeycombed" the whole front end around it. The only thing that would have made it any better would have been some bad mushrooms. It was really a sight. "Dude!

Go for it!
Break her in!