Jay Dub

I just picked up a lot of expensive but used nitro engines (.21 off-road) from a variety of manufacturers. The question is, how do I know when an engine has enough wear on it that it should not be rebuilt or invested in? Now, when I talk about wear, I am looking at engines I would use for big events like regional multi day races, Nationals, Worlds etc. and would like to learn about acceptable tolerances for wear on things like the crank pin, con rod, piston sleeve etc. Most of the engines appear to be low mileage, but the piston/sleeve's were all damaged. SO it would appear that cranks, bearings, carbs, rods(?) area all in good condition. Thanks in advance. -J

am

This is were experience comes in. Sorry, but it takes too much time to explain. Happy newyear :-)

22Racer

iTrader: (67)

You need a micrometer to measure the crank pin and a bore gauge for the rod. A mic is pretty cheap but the bore gauge can be very expensive, the one I use I have seen around $400.00 used. I like to see less than .003 total wear or I consider it a backup engine but much less wear if it is a important race. Most often if the crank is not good it is not worth rebuilding cost wise.

Rex

jbrow1

iTrader: (28)

I haven't been involved in nitro in about seven years.. When the engine is apart, if you can push the piston to the top of the sleeve it's too worn for racing. It may run, but will be inconsistent, idle poorly, and not hold a tune. At that point you can replace piston/sleeve, or send it to rayaracing for a sleeve pinch. Use the search function, there's been a thread on here about rayA for many years and it keeps going with happy customers. Actually these questions, could be asked in that thread as well. It's recommended to replace the rod when you replace the piston/sleeve, or get a pinch. I always waited for the second time though and didn't have a problem. Actually by then, the engine has enough gallons on it, you'd be best off to just replace it. You mention some high level races.. I wouldn't go to a Nationals caliber race with a used engine, too much invested and too much to lose. Now say I was going to Stateline for a race that was all weekend long, I'd go with whatever engine I had that I felt/knew was reliable. And I would want a fresh engine that I trust sitting in my box as well for that race. I always had good luck with long engine life, but I never had the fastest (lean) motor on the track either, I took care of them and respected them for what they were. High dollar, high revving, simple engines. Good air cleaners, good tuning, and good fuel. Take care of them and they will treat you well.

Jay Dub

Thanks for the responses. All but one of the engines have less wear than .003 between rod/crank pin. None of them can have the piston pushed through the sleeve or all the way to the top. Two of them do come close though (into the region the button usually occupies).
So, How long would it usually take for a motor to become broken in (for a "Big" event) vs. too much wear? Would I wan't to run a motor until it gets to that 1-1 1/2 gallon range when the tune fattens noticeably and then save it? If so, at that point should I replace the rod (or any other parts for that matter) for reliability purposes? Would one then consider the motor "Big" race ready?

BIGALSRYD

iTrader: (5)

Also very interested in this as I have a mic for full scale engines and would like to look at getting a small dial bore gauge to be able to measure my own rc engines. Is .003" just for rod - crank tolerance, what about little end? And piston to sleeve interference? And finally crank pilot shaft to clutch bell bearings or anything else that should be considered? I'm not after inside secrets, but like OP, curious to know at what point certain parts should no longer be used.

Thanks

nv529

iTrader: (53)

A cheaper alternative to a small bore gauge would be to use gauge pins in .0005" increments as a go/no go gauge for the conrod. Another thing to remember is the clearance between the crank shaft and engine case, this clearance affects the labyrinth seal. If this diametral clearance becomes too large from taking in dirt caused by a failed front bearing the engine will never tune right even if a new front bearing is installed.

22Racer

iTrader: (67)

Gauge pins are an excellent way to check hole size but do not work very well with nitro engines. The bushing in the rod mostly wears on the thrust side making it out of round. Even if a rod has .002 wear a .0005 bigger pin may fit in the hole.
To the earlier question, for a big race I would want an engine with close to no wear on the crank and rod. I would want the engine to have a couple club races on it at least because it can be hectic to fine tune an engine at a big race.

Rex