What is the tolerence on the crank of an OS .46 AX ? - RC Groups
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Dec 08, 2006, 08:06 PM

What is the tolerence on the crank of an OS .46 AX ?

I planted my Sr. Falcon (with an OS .46 AX) hard last week and today got around to disassembling the engine. The crank case was split and the rear bearing was rusty. One of the more experienced flyers had told me prior to the crash that my engine sounded like the bearings were going. I had purchased the engine at a swap meet a few months ago, and while the engine had never been run, it had been sitting in the guys garage for quite some time. Today when I disassembled the engine the front bearing looked fine, but that rear was really rusty. Can this be from just sitting in a garage. The engine only had about 5 or 6 flights on it, and I always used after run oil. I was just curious how the rear bearing could get so rusty while the front looked fine.

My second question is on the crank shaft. We measured it with a dial indicator and it measured a difference of .002. Is this acceptable? I can't find any information that gives the tolerance of the crank.

If the crank is OK I think it will be worth buying a new crank case (I already have new bearings), but if the crank is shot too I'll just keep this for parts.
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Dec 08, 2006, 09:39 PM
Conservative, RC extreme
Hi, I am wondering about how you measured it with a dial indicator? Are you just using a surface plate with the indicator zeroed out then run the crank journal under the indicator tip? When you say ".002 differance" what do you mean? Clearance? If so, I would think that it is excessive clearance for a bearing fit. Imho, just buy a new crank and bearings.

Dec 08, 2006, 10:53 PM
Suspended Account
The crankshaft is under cut to allow the rear bearing to slide up to the bearing mounting journal. The rear bearing should be a light press fit on to the journal. This under cut is used in fitting (sizing) the clearance between the crankcase and crank shaft. The critical area is the sealjust behind the front bearing and the area around the carb opening.

In my race engines I counter bore the case (.2mm offset away from the carb opening) to increase the clearance and yet maintain the seal. More clearance is better other than in the seal area.

Dial indicator = Dial caliper (?)

Synthetic oil + one run + time = RUST!

Dec 09, 2006, 09:43 AM
Chris and Konrad, we disassembled the engine in my friends shop. He races automobiles, so has a pretty complete set of tools. He put the crank in his lathe and mounted the dial indicator on the lathe also, so that turning the lathe by hand we could read the dial as the crank went around. Does that make more sense ? What he measured was the front of the crank, probably where the front bearing goes. And yes, I could just purchase a new crank or a new engine, but we're using this as a learning experience, as if the crash wasn't enough of a learning experience.
Dec 09, 2006, 10:20 AM
Registered User
downunder's Avatar
I had a feeling that's what you were probably measuring seeing you'd had a crash hard enough to break the crankcase. If the DTI was showing .002" on the machined area where the front bearing goes close to where the threaded section begins then that's excessive.

What I do is hold that smaller section (which is the only part of a crankshaft that can bend) in the lathe chuck and put the DTI close to the crankweb because that will really show up any bends because of the length of overhang. In your case that .002" will probably end up looking more like .020". Holding a crank that way then makes it easy to straighten with just hand pressure or by putting some bar stock down the inlet port for more leverage. Get it as close as you can but I usually stop when the runout near the web shows around .001".

As a matter of interest, bends can be checked for without any dismantling other than taking off the prop driver so the machined section can be held in the chuck. If you block the engine itself from turning then rotate the chuck then any bend will show up by the crankcase moving in and out (OK, actually making a small circle at the backplate). I did this with a Webra 1.20 because I absolutely could not get the backplate screws loose without destroying them. Yes the crank was bent. And yes, I straightened it while it was still in the chuck by hand pressure (lots of it ) taking the chance I wouldn't Brinell the front bearing. If anyone ever tries that though then you have to make sure there's a small gap between the chuck and the bearing housing or it won't move.
Dec 09, 2006, 12:26 PM
Suspended Account
It sounds bent. This plus the rust damage and split crankcase make it a good paper weight.

Iron oxide is an abrasive (not as good as aluminum oxide). I suspect that your connecting rod bearings are oversized and your piston and cylinder are contaminated with this abrasive. Throw out the engine not because it is an OS (which is a good reason) but because it has been abused.

How many bones do you crush when you shake hands? Or is it that the Webra 120 has a soft crank?

Dec 10, 2006, 05:36 AM
Registered User
Originally Posted by Konrad
How many bones do you crush when you shake hands? Or is it that the Webra 120 has a soft crank?


Lol..I was thinking the same..especialy when he mentioned Brinelling the bearings
ftw!!! ...Must be all that vegamite, mate
Dec 10, 2006, 10:37 AM
Registered User
downunder's Avatar
Awwww, come on guys
With the crank in the lathe chuck like that then all the load pretty much goes to the front ball race as the pivot point. There's an awful lot of leverage involved

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