Hi all,

I am running in an mds 48 and have noticed when its hot the commpression is very low, yet when cold its very strong.
Is this correct? I have noticed this effect on another engine I own which is fully run in, but the MDS difference between hot and cold compression is quite pronounced.

Regards Angus

does it still run o.k.?if so don't worry about it!

Hi, Yes it still runs ok, but requires an electric starter to start it when hot.

I thought the ABC engines were designed in such a way that the piston and cylinder expanded at the same rate to maintain the fit at running temperatures? Is this not the case?

Regards Angus

i have a friend that races (just for fun at the local club) and his good old os has NO compression when it lands flip the props and you can get a good 3 turns out of it

This is somewhat true. When it stops running, the aluminum piston head will start cooling faster than the chrome plated brass sleeve. They can't contract at the same speed because the brass and chrome will hold heat longer than aluminum.

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This is somewhat true. When it stops running, the aluminum piston head will start cooling faster than the chrome plated brass sleeve. They can't contract at the same speed because the brass and chrome will hold heat longer than aluminum.
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Thanks, that makes a lot of sence. So the long and short of it is that its fine....
Thanks for the advice, just putting my mind at rest really.

Regards Angus

littlekicker,whereabouts in hampshire are you?

ABC engines get looser when hot. Here's the theory and is works very well in practice. The piston is made from a mix of aluminum and a little silicon. This lowers the thermal expansion of the aluminum so it is slightly less than the brass sleeve. The sleeve is bored in a taper, tighter at the top when cold. When the engine heats up to normal running temperature the clearance opens up to what is specified for best running.

The ABC idea was originally conceived by Super Tigre for their control line speed engines. An additional feature was when you got a lean, hot run, the sleeve would open up even more, losing compression, but saving the engine from seizing up.

Thanks, I understand now, Im not convinced by ABC engines!! Just doesnt seam to make good engineering sence for maximum performance, at opperating temperature the "blow by" past the piston skirt must rob the engine of power and efficency. Im not clever enough to know if they are more efficent than a ringed engine in this respect, possibly they gain in other areas like reduced friction from the piston to the sleeve? I guess at the end of the day most of the high performance glow engines are ABC construction, but they also are far higher reving than the 10,000 rpm that is used for sport flying on a noise controled site and presumably any blow by effect will deminish with higher RPM???
From an opperational point of view, its a pain to not be able to re hand start an engine at the flight line when it cuts just before take off!!
Thanks for all the advise!
Regards Angus

Well you are definitely in the stoneage with that thinking. For high performance racing engines the 'ABC" set-up makes perfect sense. At TDC when detonation takes place the seal between piston and liner is better than a ringed engine. remember the ring has a split in it. Lower down in the stroke there is some clearance between piston and liner thus less friction than with a ringed piston. In any case, at higher rpm the ring cannot keep up with the rpm and bounces up and down in the groove eventually destroying itself.

Very few of the modern ABC type engines use brass for the liner anymore. Nelson has been using an aluminum liner for about ten years or more, now refered to as "AAC"

Ed S

i thought when an engine has extremely low compression when warm/hot and its difficult to start, the engine was shot, i know in my cars, when they are difficult to start when hot and little compression, its time to change the piston and sleeve. i just see in the heading of the thread it says low compression when hot. An abc engine should loose some compression when hot but be able to start still, without an electric starter. i never noticed a compression drop in a plane, i usually run os engines and they run excellent and usually dont need to be restarted. I do have an mds that i still cant seem to get running, its brand new but has the black head, i think its a .46. very hard to make idle and even harder to keep running, i think i am using too high of a nitro content.

You are mistaking what we call low compression for a worn out condition, which it isn't. ABC engines with their tapered bore are extremely tight at the top, bacically an interference fit. If designed properly, when they are at the correct operating temperature, the clearance will be perfect for high speed running. If the engine stayed as tight as it is when it's cold, you would have excess friction and a slower top end. You would also eventually force the sleeve open at the top causing what is called "bell mouthing."

Thanks to everyone for the advice, looks like im worrying about something thats normal!!!

Very interesting to read all the info, I dont have any high performance glow engines like the Nelson, all the 2 stroke engines I own have brass sleeves, certainly the MDS and Irvines, I also think the TT pro engine I have is brass sleeved too?

I guess I'm a good bit behind the times on whats good in a glow, Im going to stop worrying about it now and just fly them!

Regards Angus