Intermediate gear wear and backlash question

[uber cool]

I'm currently rebuilding my 2.2 T engine and currently checking the condition of the alloy intermediate gear. Question is how much wear is allowable? Its clear the teeth have worn a little with metal particles at the bottom of the teeth. What could cause this wear? Mis aligned chains or incorrect end play on the shaft?

Also what sort of play should there be between the crank gear and the intermediate gear?

I'm probably going to replace the gear, but keen to understand how the wear happened in the first case.

worth pointing out the engine had not run for 10 years and then only recently for 10 - 20 mins on a test bench, I'm fairly sure the metal particles on the gear wheel have only happened recently.

[uber cool]

some photos added and video link





click on image below to play video clip

[356RS]

Well just looking at the picture it seems very worn. The video really makes it look bad. IIRC the backlash measurement should be between 0.017mm to 0.049mm.

[uber cool]

how do I measure the backlash, is it the gap berween the meshing teeth?

I stripped the enigne because of a knocking noise, assumed conrod or wrist bearings, but these have checked out ok. Could this have been the noise?

[BoxxerSix]

You set up a dial indicator off the case using a rigid fixture and measure the backlash perpendicular to a tooth. Rotate Is one way and zero the indicator, then rotate opposite way to indicate backlash play.

Very difficult to measure properly and best done with new main crank bearings if so, but will give you a rough idea regardless. It's thought that the backlash often changes when the case halves are torqued as well.

[uber cool]

ok thanks, if it needs a new gear wheel do you recommend changing the shaft as well?

[Walt Fricke]

I think it unlikely that your intermediate shaft itself is worn. If you replace gears, you should be able to reuse the shaft. It is the gears which wear.

Misalignment of the chain wheels on the cams relative to the chain wheels on the IS causes excessive wear of the chain wheels. But not the IS gears. There really is no way for them to be misaligned.

I am dubious that end play has anything to do with IS gear wear. You can't set it anyway on your engine, and new IS bearings might reduce it. You should be able to tell if the thrust surfaces of the IS bearing look excessively worn, though my experience is thrust surfaces often look bad (coppery)but aren't, really. Unlike a rod or main bearing, which would signal bad things about to happen if it looked that way.

[uber cool]

Done some more checks tonight and intershaft looks ok. No wear on thrust faces and only 0.8mm end play. Which appears to be in spec. I will be fitting new bearings to IS, crank and conrods. Struggling to measure backlash as I have no dial gauge holder! However can get a 0.2mm feeler between gears to remove backlash. IS gear is only bad worn and pitted on one side. Is the normal? Suspect so as it the side that takes the load? Also checked and the case IS gear and crank gear are all 0. Again is this correct. What do you all suggest. Run with it or fit just a new IS gear? I see our host sells from for about $200. is it a case of removing thrust plate, one cam gear then unbolt the IS gear?
Sorry for many questions and thanks in advance of a great forum

[BoxxerSix]

Wear on one side more than the other is associated with normal continuous load while running. One can replace the gear, but may also damage the chain sprocket when attempting to remove the gear with flimsy pulling/press equipment. Be careful, as those sprockets are not cheap to replace either.

Backlash spec, while important, is not nearly as important as proper end play as too much of that can yield misaligned chain sprockets on the IS and camshafts which will quickly accelerate wear on the associated components.

One can pick up a suitable indicator stand for $20-$25 if needed offline. Either use a magnetic base type with a small piece of flat steel locked down to a few of the outer case studs, OR take the indicator arm off and slid it down over the longer case stud near the front of the IS and align/read indicator from there.


And I think you meant .08mm

[uber cool]

Yeah you're right I did mean 0.08mm. So would you change the gear or run with it since the endfloat is good

[Walt Fricke]

Look at the index of posts for today. Someone else (unless it was you) asked about how to get the IS chain gears off of the shaft.

Good advice there. It is harder than most things you to do when rebuilding a 911 motor.

[BoxxerSix]

^ Indeed it is. I've machined special steel plates for use in a press that allow me to evenly heat and immediately press the sprockets off the IS with minimum fuss and damage. I've damaged far too many in the past using pullers, which never apply the force evenly.

[uber cool]

Yes read that post. Seems good advice. Think I'll see if I can strip the shaft before ordering the gear incase I also end up needing a cam sprocket