Large Model Association Forum

Hi,
Is there anyway to check for a bent crankshaft on a Zenoah 38? I've just acquired a second hand one which keeps shaking my throttle servo loose at full throttle,you can see the servo vibrating.
None of my other three 38's do this.

Sean.

I straighten lots of cranks, i have access to all the best gear for this, but the 38 can be a pain, its also possible that if its taken a whack, the large flywheel is twisted, double jeapordy, undo the mag, bring the magnets round, trap paper between the magnets and the mag, tigten up, remove the paper, turn the shaft, does it bind? somethings wrong, bacause the taper on the shaft for the wheel is so far from the bearings, and its unsupported, it bends at the taper section, taking the flywheel with it, i have skimmed the wheel when i have got a crank something like, but its not something i like to do, if its a simple displacment, then its easily doable, IF you have the correct gear, of all the cranks i do, the 38 and its flywheel have the worse repair rate,

Hi Sean,
You can do a quick check by using feeler gauges to measure the air gap at the magneto, while rotating the prop shaft through 360 degrees. It should be the same throughout the full turn.
If this is ok then it's possible that the mag flywheel front face or prop adapter is damaged/distorted. You need a clock gauge to check these items properly, but you could get a rough check by mounting a decent size (4" - 5") spinner back plate. Then either with the engine in the model, or clamped to the workbench rotate through 360 degrees and look for any run-off of the back plate to a fixed point i.e. the edge of the bench or the model cowl.
Hope this is of help. Any problems you can contact me.
Thanks and regards.

Hi Alan,Ken,
Just had a look at the gap between the flywheel and i can see its out just by looking at it. I assume that the engine requires a new crankshaft?

Sean.

Not always it COULD just be an alignment problem, at the worst, the flywheel could have had a crack, either way, that motor had to be cheap, the fkywheels are £44 (ish) and to mount a new flywheel, it has to be a good crank, ive even on a rare occasion, had a bent flywheel, and a straight crank, but not often, i would strip the thing, and get it to either me, or Ken, i am very busy right now, with shop work, i only get dinnertimes to play with the crank de bender unit( either 41/2lb or lump) i would get onto the seller, and get some pennies back, i have also bought quite a few petrols in the past, the look on folks face when i get a vice (to grip the mount) and a Dials Test Indicator out of the car is always classic

Could you give me your address please Alan and i'll send the engine over to you. I have contacted the seller and i'll see what he comes back with?

Sean.

As per our PM Sean, see what the guy comes up with, for me, unless it was very very cheap, i would get my pennies back, and next time, do the simple checks both me, and Ken have mentioned, before parting with the hard earned, if you wish to strip it, and send me the bits i have asked for, to check it out, let me know,

Yet another crankshaft assembly overhauled,

I look forward to putting it back together

Working fine now, thanks Alan.

great to know its running well again, the flywheel had had a bang, twisting the wheel, and the tapered part of the crank, this one was salvagable, i managed to skim the flywheel true, many are not, if anyone buys a zenoah 2nd hand, undo the mag, and drop it onto the flywheel, then turn the engine over with no plug in, if it binds--walk away, saves me a job, and you some money!!!! i might add here, that if the crank is a baddun, i dont charge for my time, just the postage back to you, its bad enough having to buy new parts without my taking a cut!!!!