Excessive Intermediate Shaft Movement

[Woz]

Hello,

I had my engine rebuilt about 18 months ago by the local official Porsche dealership after a cylinder blew. The last month my revs would drop in and out and the car would lurch badly after about 30mins of driving. I took it back to the Porsche dealer who pulled the distributor and looking in the 'hole' they say they can see excessive movemt on the intermediate shaft, probably leading to the timing chains slipping which is causing the problem and that I shouldn't drive the car until this is addressed. They say the engine will need to be taken apart (and reassembled) again which will cost around EUR2,500 just for the man-hours before they even talk about parts. When I had the engine rebuilt (at significant cost) I had a new intermediate shaft installed and new bearings amongst otherthings so I am very suprised I have such a problem already. Now looking Wayne's "How to rebiuld and modify..." book, chapter five, image 3.32, there is a procedure for adjusting the intermediate shaft end-play that is very specific to 1965-68 aluminium 2.0 litre cases (like mine), basically installing gaskets underneath the shaft cover. I'm wondering if the Porsche dealer knew to do this or not and if they didn;t do it, could this have caused my problem. How likely is this? Or is it most likely to be another problem (like what)?


Thanks,

Woz
Porsche 1968 911T 2.0

[Walt Fricke]

Woz

Your early sand cast 2.0 (originally, anyway) engine case has no intermediate shaft bearings. The shaft runs on the aluminum of the case itself. Works fine - my 2.7 race motor is based on that case and that kind of IS shaft. So that's the first thing I'd be inclined to talk to these guys about - what were they doing charging you for parts that won't fit this engine.

The next thing I'd talk about is where they bought a new intermediate shaft. Are these even made any more? Pelican doesn't list them for sale. These shafts have a larger bearing journal diameter, so you can't just slap in the later shafts, which would rattle around. Nor can you just plop in the newer bearings to make up the difference in journal sizes. The journal holes in the case are too large for the later IS shaft journals. If you want to run the later shaft in a sand cast case you have to go through a fair amount of machinist gyrations - either you weld up that part of the case and rebore it smaller, or you custom make some kind of bolt in adapters to hold the bearings.

But your eyes do not deceive you when reading up on this. End play is set by varying the thickness of the shims between the IS end plate and crank case at that point so the spacing of the IS shaft from the plate is within spec. The thrust direction of the IS shaft (when the crankshaft gear is trying to turn the IS shaft under acceleration) is to the rear of the engine due to the cut of the gears. In newer engines this thrust is taken up by a thrust (actually a combination) bearing - a miniature version of the #1 main. But not in yours.

In these engines the IS cover plate is cast iron, and it is what takes up the thrust. How far it stands proud of the end of the case determines end play. This you adjust with shims.

In fact, it is the oil under pressure between the cover plate and the end of the shaft that takes up the thrust. I've used a magnesium end plate on one of these engines, and it has only the lightest impression of the end of a shaft on it, so my conclusion is that the oil, which is under pressure in this area, does what oil does in a bearing - prevents metal to metal contact.

The idea that the chains are slipping is an additional indicator to me that these guys don't know what they are talking about (if you indeed have a sand cast case, and it wasn't highly modified at some point, and all pertinent facts are in your post, etc.). If your chains slipped they would stay slipped, at least until they slipped some more. So your car would be worse and worse each time this happened. Plus more than about a tooth or so probably would cause valves to hit pistons. At some point this is guaranteed. You say the car runs OK for about 30 minutes, and then starts acting up. Well, it didn't heal itself while it was sitting there cooling down.

Now if the IS shaft moves axially, that changes the valve timing because of the angle of the gear teeth. But I don't think it changes it enough (at least within any amount of movement I can imagine without something being drastically wrong) to cause "revs dropping in and out," whatever that means.

So you have a heat related problem, most likely. Your description of symptoms is not quite adequate for remote diagnosis. It could be a simple fix, or it could be a precursor of doom.

I'd take the car to someone else for a second opinion. You can measure IS shaft end play without pulling the engine out of the car. But you have to remove the muffler and the engine mount to get at the three nuts holding the plate on, and to remove the plate. Knowing no more than you told us, that is where I would start.

Now I am assuming axial play. If there is radial play (like a late small bearing shaft was just thrown into an early aluminum case where it just rattles around, or a plugged IS shaft oiling hole has caused problems), you are facing teardown. But it should be on their Euro, one would think.

And this may be something else entirely.

Walt Fricke

[Woz]

Thanks Walt for your detailed feedback. By revs dropping in and out I mean once this problem starts (after about 30 mins driving) the revs counter flys all over the place as the engine stutters, fires, stutters, fires, stutters etc. It's worse under acceleration or at high speeds/revs. How could heat and IS play be related to cause this?

Thanks.

[Grady Clay]

Woz,

Your description sounds like the ignition points are too close and as the engine warms, the ignition doesn’t function correctly. The tachometer “…revs counter flys all over the place as the engine stutters…” indicates this.


Walt gave you some good advice. Get an independent second opinion.

It strikes me that there may be more than one issue. Cam timing is usually a make-or-break issue, not temperature dependent and not variable.

For your independent inspection, I would start with checking the ignition timing and advance curve. Next would be inspecting the sump and oil filter for metal. Then would be cranking compression and cylinder leak tests. Then would be cam timing check and valve clearance check and adjust.

The functioning of the fuel pump (flow from the tank) and carburetor function are very important.

From your description, there is some question about the competence of the Dealer rebuild. If so, you need careful documentation of the current issues. PAG Dealers have the obligation (via PAG) to do the work correctly. I don’t know the consumer law in UK but here most States here will find that improper work a breach of contract or something similar even if not covered by specific consumer law.

As you pursue diagnosis and documentation, you will find a lot of help here. Even more important, you will find significant help with all the technical details of the repair to proper functioning. I hope that that will prove easy. If it takes another rebuild, the Forum can help you (and even the Dealer) have the process turn out correctly. Even a Porsche Dealer is welcome to ask questions on this Forum.

Best,
Grady

[lindemans]

It sounds like the access play is actually too much backlash; I wonder if the shop checked this while replacing the intermediate shaft.

Very weird to confront a customer with this after having taken the entire engine apart themselves.

It very much looks like the shop overlooked something during the rebuild. Show them Wayne's book for a warranty re-do ....

[304065]

Woz,

I concur with Walt's advice, I am unpersuaded by the dealer's diagnosis.

First of all, you can't tell anything by looking down the distributor hole with a flashlight. You can see the distributor drive gear, but that's it. If they are trying to tell you that you have a worn distributor drive gear which is causing the timing to vary, well, that's a different matter entirely. What does the gear on the end of your distributor look like? You can see a picture of mine here: Before there was BHKZ: SWB engine electrics

And now to the intermediate shaft. Wouldn't a '68T have a magnesium bore for the intermediate shaft? Wear in the bore could definitely be an issue, but this sort of thing is routinely checked at a rebuild. So too is the intermediate shaft gear backlash: if there's enough slop in the gears from excessive wear OR the shaft moving around in the worn-out bore, it should be immediately obvious.

And what's all this about the timing changing? Has anyone at the dealer taken the very short time to hook up a stroboscopic timing light and see if the timing is walking around? This would identify distributor drive gear wear, but not chain slop. Has anyone removed the chain box covers and inspected the chains to determine whether they are in the correct positions? I would think that if the intermediate shaft was moving in the bore there would be a great deal of slack in the chains that would be taken up by the tensioners-- it would look the same as stretched chains, with the tensioners bottomed out, except that it's not chain stretch. Surely you could tell.

It's also possible to pull off a valve cover and mount a z-block with a dial indicator to record timing events. Wear doesn't come and go, as Walt says. . . it should be evident from every rotation of the cam that something's not right.

I agree that a second opinion is absolutely imperative before commencing a high dollar rebuild process. The tach jumping and the miss suggest an ignition problem to me.

[Woz]

Thanks all for your feedback. I am taking the car to a second porsche specialist (but non dealer) in the next couple of weeks for a second opinion. I'll let you know what they say!

Thanks again.

[Grady Clay]

Woz,

John posted:
[QUOTE]Originally posted by john_cramer
And now to the intermediate shaft. Wouldn't a '68T have a magnesium bore for the intermediate shaft? Wear in the bore could definitely be an issue, but this sort of thing is routinely checked at a rebuild. So too is the intermediate shaft gear backlash: if there's enough slop in the gears from excessive wear OR the shaft moving around in the worn-out bore, it should be immediately obvious.

The question he is asking is VERY important.

Your ’68 and all prior 911s didn’t have shell bearings for the jackshaft. That wasn’t too much of an issue with the aluminum cases (although they benefit from the update). Starting in ’68 with the magnesium case it became an issue. No competent rebuilder would not retrofit the shell bearings. This is an issue with the ’68-69 mag cases. By ‘70 the engines had two shell bearings supporting the jackshaft.

(To make this complete, in ’69 Porsche added one shell bearing – there should be two.)


With help of the Forum, I recommend you devise an investigation plan for your second opinion. Not only will this make effective use of the shop’s time but you want to gain the correct information so you can proceed to a solution.

I’m in favor of simple & cheep first to more complex & time consuming approach later if necessary. Here would be my sequence:

1) Look in the sump and cut apart the oil filter. Save anything found. A coffee filter works well to collect debris.

2) Check the ignition timing curve with the engine running. If any question, pull the distributor and investigate on a distributor test machine. A heat gun there might duplicate your running situation.

3) Cranking compression and cylinder leak. This will eliminate (or confirm) if there are more serious issues. While I wouldn’t expect any problem here from your description, this absolutely should be dome to eliminate this as an issue.

4) Check the valve clearance and cam timing. Look for rocker arm shafts out of place and tensioner support shafts moved.

5) Next is to remove the muffler, engine mount and related. This is only slightly more complicated but keeps the engine in the car. This gives you access to the jackshaft cover plate. You can measure the total (unrestrained) end play and how it is restrained with the cover plate. (Note that with shell bearings, the cover plate & shims no longer determine the jackshaft axial movement.)

This also allows measuring the shimmed position of the cam sprockets. Here you can measure the cam end-play. Most important is visual inspection of the sprockets and jackshaft driven gear.


A major problem with a non-shell thrust bearing jackshaft and mal-adjusted cam sprockets is if the jackshaft is pulled toward the oil pump. This cause the jackshaft to eat into the case at the chain sprocket for the 1-2-3 bank.


Lets collectively devise a good test plan for Woz’s situation. If he goes to someone with a well thought-out and Porsche detailed plan, he will get better diagnosis.

Best,
Grady

[Walt Fricke]

Well, WOZ said he had an aluminum case. He did say "like the '65-68", so maybe he has what most of us would call a mag case. That fits better with his being charged for a new IS shaft (though they really don't wear, do they? The gears do, the case without bearings does, and the bearings do, but the shaft?).

My bet at this point: resetting the points or something fairly simple in the ignition (John's thought) and he's back on the road. And the shop was good enough at rebuilding, but had someone less skilled doing the troubleshooting.

Had the shop not talked about IS end play, and he relayed that to the forum, I doubt it would be on anyone's list based on his symptoms.

Walt

[andrew15]

Keep in mind that early 68 models had aluminum cases - magnesium ones were introduced mid-production for MY 68.
It's very possible that Woz has an aluminum case, but then the puzzle of a new intermediate shaft comes up again...
AM

[lindemans]

If the problem was caused by worn intermediate shaft bearings; it is very coincidental that it bangs now straight after rebuild and not earlier before that or after many miles more after rebuild.
That is weird.

Statistically I would look for a cause associated with the rebuild.

WOZ indicates in his posting that the intermediate shaft was replaced by another one. That requires backlash testing always; if the gears don't match properly, play in the cam drive is exactly what you build in.

Ask the guy who rebuilt it whether he did a backlash test, from his frowns you may tell whether he even knows what that would be .....

[Woz]

Thanks all for your help. I took your advice and sought a second opinion. I found another (non-franchise-Porsche) garage that specializes in classic Porsche road and race cars. The mechanic has 30+ years experience and after driving the car and looking at the service history, his first impressions were that the engine does not need to be stripped down to correct the problem (which he believes can be addressed through working on the ignition/distribution topped off with some carb tuning). It's in there now and should be ready this week. Fingers crossed he is right! I'll also try and take some pics of the great racing porsche's he had in there including a 935 racer that he says was Paul Newman's car in 1980 (although I'm not so sure about that) and some early RS's.

Thanks Again,

p.s. my engine has an Aluminium block.

[Woz]

Got my car back today. Only had time for a 40 minute drive but so far so good. The engine runs very smoothly. As promised, the mechanic didn't need to open the engine but performed the following tasks: removed, inspected and adjusted the carbs (found that the 'float' in one had been incorrectly installed). Replaced the distributor parts (rotors etc.), installed Lumenition ignition unit, replaced the ignition coil, adjusted the injection and installed new Magnecore 80 electrosports ignition cables (which I gather are pretty good!). I also had the ride balance, steering alignment and break pedal adjusted which makes it feel better than ever. Early days but so far so good. Thanks again for all your help.

[Woz]

And here are some pics of the cars in the garage "Modern' Garage" based near my home town of Grenoble. Including Paul Newman's 935 from 1980 (now with white paint job, was black I think before).









Apparantly the one below is an early RS (didn't quite catch the year) of which there were only 13 produced. They had just done a complete restoration:



Puts my lowly 68 T in the shade but the mechanic was just as excited to work on it having a passion for old Porsches! He even offered to buy it (no chance).

Cheers!