Balsacutter

My Feedback: (3)

I bought a NIB Thunder Tiger Pro .46 off eBay. It sure looks like it's never been run, (the needle valve is still in a sealed plastic bag) but turning it over by hand with no glow plug there's about 5 degrees of "play" at TDC.

I've read that new ABx piston/liners "pinch" at TDC, but this seems like a lot of play in a new con rod fit. I'm not a very experienced engine guy, so I don't have a lot to compare this to. Would this be considered within acceptable tolerances for a new engine?

I'd keep the engine if people think it was likely to run OK. It's just going on a high-wing trainer, so I'm not looking for the last 100 RPM. Just a trouble free lifespan before I do that lawn-dart manuever again ....

Whatcha think? Run it or dump it?

blvdbuzzard

Sounds normal to me. There is some movement of the crank and rod when the piston is at TDC.

Dru.

w8ye

My Feedback: (16)

That's the way they are made. Don't worry about it

Broken Wings

My Feedback: (20)

It's not a good idea to measure rod play at top dead center. This magnifies the clearance. Place the piston about one-half way between the top of the exhaust port and top dead center. You will find a big difference.

hdpartman

My Feedback: (35)

This is completely normal all your are feeling is the piston hitting TDC, time to go.

Now as a stated as a novice to these engines, let me share about my hows and where of a ABC set-up like this. I feel that most ABC engines have to be broken in differently than a ringed engine. Remember there is no ring to fit and fill the gaps in the piston and wall of the cylinder. The piston fitting in to a tapered cylinder sleeve is all of the workings of this type of engine for compression. Yes, this type of Engine will not last as long as a ringed engine. It simply wears out the piston and cylinder liner eventfully, but it also mean's that a new piston n' sleeved cylinder can be exchanged very quickly. Oh I for got to tell you, yes it is a bit faster most likely how about a couple of bits faster.

All previous is just standard facts, now break in a new abc engine. Start the engine, it should be running a bit rich, not with fuel flowing out rich just slightly. Now because this piston is going up n' down in to a friction compression, similar to a Diesel. I get my plane in the air for cooling as fast as possible. This is why I would not recommend this type of glow engine to a novice. On the ground there is simply too mush heat being generating to do any good for this type of engine. Keep it rich, the second tank lean just a bit on the top end only not to worry about proper idle just now. 3rd tank lean a bit more. 4 tank of fuel get this thing flying at the correct mixture and enjoy the fun.

Now I know I may be opening a hornets nest but in racing of every type of 2 cycle engines with ABC piston this is it. We are racing and don't have the time to do other ways.
Not throwing old glory days, but yes in the late 60's I was very involved with 2 stroke motorcycles and motor cross racing. Prior to that and again later I raced go karts no not the rental type the racing style
2 stroke motors. I have always bounded back and forth in different types of racing and ABC motor are meant to go fast.

Fly Safe n' Enjoy
Al

Harry Lagman

Perfectly normal, judging by your description. The piston is pinching slightly at TDC, which is normal and is accentuating the con rod big end play.

If you want to feel truly horrifying levels of pinch and play at TDC, turn over a brand new Jett .90!

Your TT .46 Pro should provide years of dependable service - they are fantastic engines.

longdan

As stated above, it's the pinch that lets you feel the 'slop' or clearance. Its always there, just that at TDC, the piston gets grabbed and held. The piston has to be pushed into the pinch point, then pulled away, which means that the gudgeon pin and big end pin move from one side of their clearance to the other, which you feel as you turn it over.

Hobbsy

My Feedback: (102)

When the crankpin is at the top and bottom of the stroke there is more sideways motion than up and down so the oil clearance in the rod ends is exaggerated.

longdan

Its worth noting that when a 2 stroke engine is running, the conrod is pretty much always under compression, which means that the pins don't get whacked around inside the clearance holes. The small amount of clearance is filled with oil, which dampens any radial movement between pin and bush and prevents metal to metal contact, though on brand new engines, this oil may be lacking, adding to the apparent feel of excessive clearance.

Don't turn it over too much by hand until its seen some running time. At room temperature and without any oil in it, it will be very tight, and you can risk rubbing the piston and liner together too much which can affect the performance and life of the engine.