Qucik Chain Tensioner Questions (2)
 

1. What is the correct type of nut to use to fasten the tensioners down? I opted to use prevailing torque nuts versus the nylocks that also had come with the rebuild kit... any issues?

2. On both tenstioners there is a 2 mm gap between the tensioner and idler arm on the shared pivot shaft (see below). Is this normal?

Thanks,

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Rob - as to what nut to use, I just use a standard 13mm wrench M8x1.25 nut. It isn't going to come off, and I'm pretty sure if it tried to back off, it couldn't come off because the cover plate would prevent it from happening. But it won't come off - there are no forces on it of any significance. I don't use Nylocks to hold the chain housings to the case, either.

That gap isn't going to cause you any trouble. The idler assembly will center itself under the chain, so giving it a little freedom seems a good thing. . And the tensioner is fixed in its location by that nut you wondered about, so it won't care about the gap.

I can't say I have noticed a gap one way or the other, so don't have an opinion on how common it is or what sizes are normal. No doubt next time I am in a 911 motor my eyes will be drawn to it, just out of curiosity.

Your idlers look a bit worn on the sides of the teeth. Did you check for chain gear parallelism? Perhaps out of parallelism might make that gap to the idler larger on the rear side like yours, and smaller on the front (where you can't see it easily)? Just a random thought spurred by your pictures.

Thanks Walt. I did check parallelism when reassembling and all looked good. I appreciate the feedback.

Walt would you be concerned about using those idler sprockets?

re. the 2mm, when I had my engine apart I wondered about that. I made some shims to limit forward travel after finding the center, not sure if that is/was a good or bad idea, seemed like a good idea at the time while I was making spacers to go inside the tensioner cylinders per Jerry Wood.

My concerns about idler sprocket wear would be mostly directed at figuring out why it happened. But if the chain drive and driven sprockets are in the same plane (I guess it isn't parallelism we want, but being lined up on the same imaginary plane and not offset), I don't know as more wear on the idler than on the main sprockets would affect performance.

And I'd not offer much of an opinion based on a photo. I would haul out my stash of used idlers, and maybe a new one, and see how much wear there really was..

Others may have more cogent views of this, though.

I wouldnt consider using Nyloc inside the engine. Wavey washers and plain nuts work well.
Bruce

Walt,

I just inspected all the sprockets and they all show wear. I am going to renew them all along with a new chain. I have a question about the Sprocket support, the bushings have wear and I was thinking about replacing them to. However I was just looking to replace the bushings but I cant seem to find them on PP. It seems as though you have to replace the whole support. Is this true or should I just leave them be and they will be fine?

Andy

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Any decent shop will be able to make you some bushings for those tensioners. AFAIK they aren't an off the shelf item.

andy

For sure Craig Garrett (cgarr on this forum) I am sure can do it, as can Henry Schmidt. But no doubt Andy is right that an automobile machine shop can find bearing material and machine to fit.

If these are the same diameter as rocker arm bushings, you could use those, and they are available. But a proper bushing installation often calls for honing to size after pressing them into place.

They are an odd size, the ID is 15mm and the OD is somewhere around 17.5mm. My machinist made me some out of aluminium bronze IIRC, then I used his press to pop them in and hand reamed them fit the shafts.

There's a thread on here http://forums.pelicanparts.com/911-engine-rebuilding-forum/582903-idler-arm-bushing-replacement.html where a member used an off the shelf 15mm DU bushing with some shim stock to get a nice press fit.

andy

Thanks for the response...Walt would you consider these supports to be worn?

Craig Garrett can indeed do it. He's got my arms right now.

How about sharing a picture of the posts that the idler arms ride upon? You may want to check that surface as well. Its kind of strange that the softer bushing material imparts a noticeable witness/wear mark on the post.

I can't tell from the pictures, and I am not sure I would have a useful opinion even with outstanding macrophotography. It is the kind of thing where I'd look at the wear marks, maybe measure with a bore dial indicator, and check against a spec. In the absence of a spec I'd measure the diameter of the pivot pin.

This is not a part which is high on my worry list. I've never replaced the bearings on this part on my own engines.

There is a belief that the chain tensioner failures through the end of the 2.7s or so were due to there being too little bearing surface. Which is why somewhere in there Porsche increased the length of the bearing surface. Some think this was a complete solution, and the later Carrera pressure fed tensioners were to solve a problem which had already been solved.

The carrier bearing is always under pressure, and that mostly at the same point on the bearing. If the bearing wears some, it is not clear to me that that will be a problem. Only if the bearing wore through and you had steel on steel maybe? Might friction weld and stop moving? Which maybe was the issue earlier? But perhaps if the bearing is "loose" due to wear, that will allow the chain gear to wobble around. Those who have run their engine with the chain housing cover plate off report that it is a scary sight - how much these things bounce up and down.

Not expensive to have two of these rebushed, so with a motor which is new to you, and which seems to have excessive wear on the idler whose carrier the bearings support, having them rebushed would seem to be the way to go. Yes, all these little things add up to a whole pile of money once you are done. But I'd do it. One less thing to worry about.

THX Walt...

Kevin can you send me a PM listing Garetts info?

I agree with Walt that the bushed idler arms are not a big worry. I suspect the bushings look similar to what you see in a used rocker arm. That is, noticeable wear but not drastically so. Usually you can clearly see a worn area in the bushing and the rest of it looks almost untouched. The worn area is smooth and you can still see the cross-hatching (original honing marks) on the untouched area.

The bigger concern would be for someone who still has the older idler arms that are not bushed. They can indeed fail by binding on the shaft. I had that happen to me last September and I thought my engine had taken a more serious hurt, like bearing failure. I got lucky.

This idler arm had to be pulled off the post with a small two jaw gear puller.

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Kevin

You didn't have the idler failure on these cars, did you?

'87 Carrera
'79 911SC Turbo-Look NASA GTS3

I thought the unbushed idlers were only on much earlier 911s.

What were the symptoms of the bushing seizing?

Walt

Hi Walt,

The idler failure was on the '79 racecar. I'm rebuilding the engine this offseason.

The idler arm update was around 1980 I believe? The symptoms were the engine lost power signficantly and when the throttle was closed the rpms & oil pressure would drop like a rock. My low oil pressure warning light is set at 20 psi and it would come on very quickly. I could watch the mech. oil pressure gauge wave like a grandfather clock pendulum!

I found the 1-2-3 chain box with the tensioner in an extremely extended position and the idler frozen in place. Luckily the only resulting damage was the above chain idler arm (and chain box shaft) and some noticeable wear on the cam sprocket & intermediate shaft sprocket wheels. The intermediate shaft may have gotten a pounding too because the thrust bearing location has some grooving on the shaft. I got a complete used shaft that I can make a "new" assembly out of.

The darkened I-shaft sprocket teeth below are not shadows from the camera. They're blued from the extreme pressure by the chain

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Needless to say i'm of the *strong* opinion that anybody using the old style idlers w/out bushings is running a great risk. I would argue that the idlers are more of a risk than the debate between old style hydraulic vs. the Carrera tensioners.

We take the early tensioner arm and install a custom made oilite bushing with a support hat to manufacture an arm we believe functions better that the stock Carrera /wide arm. They are on the shelf ready to ship at less than half the price of a Carrera tensioner arm.


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I used Henry's arms in my 3.0 build and they work flawlessly...