Drop Gear Mesh Clearance
Turbo Mini Forums

I have been looking at the meshing clearancies for a while now as the straight cut drops I had were very rattley, and so it would seem are the helical drops. It was my opinion that, although the straight cut drops chatter anyway, that it could be improved on a little by removing or at least using a thinner gasket between the engine and box, thus bringing the primary gear closer to the idler gear and reducing the mesh clearance.

Since I have had the block line bored which has resulted in the crank center line moving 0.003" up into the block, this has increased the drop gear mesh clearance by another 0.003", or there abouts

Looking at the current engine/ box gasket thicknesses, the ones I measured were 0.0091".

As an experiment, I sat the engine on the box without any gaskest or glue. fitted the primary gear and idler, placed a piece of plastigauge verticaly on one of the teath on the top of the idler, and one piece on the oposite side top of the first motion shaft input gear. I then fitted the transfer case with a gasket with the idler correctly shimmed, and bolted it down. Turning the primary gear I could feel the plastigauge in the mesh of the gears. I dismantled and measured the clearancies. It would apear that the primary to idler has 0.004" clearance, and the idler to input gear has 0.007" remembering my block has been line bored.

It would be interesting to see what the clearance was with the engine/box gaskets fitted. It would also be interesting to see what sort of results are given with the straight cut drops.


Im not sure if this is the correct way to measure the clearance between gear teeth, but as an indicator it seems to be OK. I am also not sure if the 0.004" clearance is right, but im willing to experiment and see how it goes

Edited by Sprocket on 10th Jun, 2009.

Is it possible to have too little clearance on these? I'm assuming that sprocket is right and that more mesh is good, but just asking the question

on my complete straight cut assembly, gears, cwp and drops there is just over .040" accumulated backlash ,lots yes , this would be a pain on a road car but not for me,


carl

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What you need to do is messure the clearance between the driving and driven faces. How much back lash/ degrees of free rotation allowed depends on drive train requirements.

Most gear drive trains I have come across do not have the facility to manipulate the meshing clearances, thus making the gears an on condition service item.

The only practicle test I have ever heard of was that you should be able to pass a piece of A4 pater between the gears and it not be damaged (crushed or torn). But that was on a very fine toothed gear system and obviously would not work too welll here.

So long as your gears mesh without fouling in the root of the driven gear and retain clearance at all times what gap you try to set is a personal choice.

Own the day

If your crank has moved up 0.003" through your line boring/honing process, presumably the cap mating faces were machined first (to reduce the bore so it could be line bored/honed).....

The crank being up 3thou and the "front" cap being up by some amount means the primary gear is off centre relative to the main "pink" seal and the half moon seal is no longer compressed as much.

Shouldn't you just face 3thou off the bottom of the block to compensate which would also put the gear clearance back to where it was before (right or wrong...)

Schrödinger's cat - so which one am I ???

dont think there is a problem. Correct line bore procedure is to face off the caps, line up the tool with the block housings so that minimal, if not nothing is removed from the block. That is what was done on the first instance, and then they were honed. This time, as there had been some distortion due to heat. 3 thou faced off the caps and then everything was bored into the block by 0.003" to clean up the distorted housings in the block. Its normal, and 3 thou is fuck all.

There is some compliance in the primary gear seal, and the transfer housing has been machined in such a way to accomodate any miss alignment on the fixing bolts.

As it is, without gaskets, the primary gear seal is pretty dam close in the middle of the seal. if you do the math, 6 thou off the cap and 9 thou less gasket, the half moon experiences 3 thou more compression

Yes, but surely your 9thou gasket will go back in when you rebuild it for real (or have I misread what you were saying about the 0.0091" ????)

Even so, I agree the 3thou or 6 thou is fuck all re. the major assemblies but it has altered you gear clearance compared to design. Machining the block bottom face would "correct" that.

Schrödinger's cat - so which one am I ???