Intermediate shaft assembly question?

[milspam]

I have been gathering parts as they come available for year for a 2.7 liter air-cooled engine build and need to purchase an intermediate shaft assembly. The question is, does it make a difference between size 0 or size 1 intermediate gear assembly as long the crankshaft timing gear number matches? The main reason for my question is not only wanting to know the difference if any , but there is a vast price difference at times between the two sizes. Also, since I've spent a lot of cash to get the block work done, I just can see buying a used one and having to replace all the gears anyway. Thanks of any knowledge sent my way.

[Flat6pac]

What is stamped on the case. That’s what I would match to
If you can’t match the case there is a little list of combinations that work
If you buy a good used match to the case, why would you be changing gears
Bruce

[milspam]

Flat6pac - where on the engine case is it stamped to specify intermediate gear size? I know engine case has matching case halves, but never knew the gear size was stamped somewhere on the case. Like I said, I've been gathering parts for years to build a correct model year engine as I was not lucky enough to have a number matching motor as per COA. Any information is appreciated.
Thanks, milspam

[KTL]

What Bruce is referring to is in the pictures below. This is taken from the '84-'87 911, 930 Tech Spec book




I disagree with the note that indicates the shaft and bolted gear are one part. People have changed the gear on these shafts over the years. Some like to take the steel gear from the 964 shaft. Also, Pauter and JB Racing make straight cut I-shaft gears and that tells me their gears are being bolted to shafts that have had their original gear removed.

[Flat6pac]

The stamping is next to the crank
Pulley on the left case half
Bruce

[milspam]

Thank you all for your experienced input to my question. I've learned some interesting and detailed information that was unknown to me. What would be the benefit of having straight cut gears on the intermediate shaft and the crankshaft as apposed to factory/stock type? I really love these forums and the knowledge you can gain from others, its like going to Porsche University. Many thanks to all.

[KTL]

The straight cut gears are both steel (most original gears on the shaft itself are aluminum) which provides some added strength. But the aluminum gears typically haven't demonstrated a problem with strength. The downside to the straight cut gears is the noise. Common angled or helical gears like you see in transmissions are purposely cut that way to reduce noise.

[Walt Fricke]

Straight cut gears should avoid introducing axial loads on the shaft. The axial load face of the flanged IS bearing tends to show copper come rebuild time. Same as the #1 main, which is subject to clutch release loads.

Porsche used these in its racing engines, and my supposition is that there was less gear drag as a result.

Having had an aluminum IS gear strip its teeth, with the immediate result of breaking 12 rocker shafts, color me doubtful as to the long term strength of aluminum in this application, at least on a race motor spun above 7,000 rpm, unless perhaps replaced frequently with a new one - say at each rebuild.

The 964 steel gear approach is by far the least expensive approach for durability in a race motor.

[scott automotiv]

We have an input shaft showing some signs of wear on the bearing at the gear end of the shaft. When plastigauged up with new bearings it shows a clearance of 0.006". We feel this is far too much. As you cannot purchase different size bearings, are the shaft bearing diameters all the same when supplied new to match new bearings or do they also change with the size code. What diameter should the bearing on that end of the shaft be? We appear to have a code 1 gear fitted.

[Walt Fricke]

Like most, I've just put in new bearings and not worried (though perhaps I should). As you note, the bearings only come in one size. The early motors didn't even have a bearing, the case material being adequate for the purpose apparently, though machining these early cases to accept bearings is a normal part of making a race motor from a sand cast case.

Since there is only one size bearing, it would seem that all the IS shafts are the same diameter (when new) at their bearing surfaces.

Coating the bearings could close up the clearance a bit?

Unlike, say, rocker shafts or rod journals, which can show visible wear or at least measureable wear, I've never seen/felt wear on the steel IS shafts, so it never occurred to me to measure, much less Plastigauge.

The little white spec book for the 78-81 gives this information:
Bearing 1, crankcase bore - shaft B 27.5-27l521 W 25 - 24.98
Bearing 2, B 26.5-26.521 W 23.98-23.967

IS shaft clearance is given as +0.030 to +0.084.

All in mm. I confess I don't know just what German words B and W stand for. Bohr? Breit? Weite? So I can't make sense of that.

Volume I of the service manual shows, via a diagram with numbers pointing to the space between shaft and case (this is for the cases with no separate bearing shells) a clearance of 0.1mm/0.0039"
The columns these clearances appear in say "Clearance press fit from" and "(+) or (-) to", which I don't claim to understand - press fit? Must apply to some other dimensions in the columns.

Alas, the followup Volume III doesn't mention any of this, at least not that I can find.

0.006" does seem like rather a lot of clearance for a rotating, pressure lubricated, shaft. Do measurements give about the same clearance as the Plastigauge?

[scott automotiv]

Thanks for reply Walt.

We measured our shaft at 24.9862mm and I managed to get a very good supplier to take a new shaft out of packing and measure bearing surface dia and this measured exactly 25.000mm, which is 0.0318mm down on the new shaft. Allowing for the new shell bearing to have a clearance, exact size of these is unknown at present, this is where we are seeing the 0.006" presumably.

We have ordered a new shaft at vast expense as it comes complete with all sprockets and gear, which we were not considering replacing as the engine has only completed about 300 miles since the last 'Reputable' engine builder put it together.

[Walt Fricke]

Checking this shaft for wear (assuming you can't tell by looking at the bearings - mine always looked fine other than copper on the thrust surface of the one) and the shaft for a visual clue to lack of oil or foreign matter, I've never seen checking this on any list of things to check. Our host Wayne's book on rebuilding these engines doesn't mention measuring the shaft. Same with Bruce Anderson's book, with a two+ page engine builder's checklist complete with quite a lot of measurements to make.

Sounds like a good thing to know when sourcing a used shaft, which should be more economical than through Porsche. Its business practices don't always quite line up with those of owners of older cars - witness the fact that you have to purchase a whole gear set to replace the dog teeth if you are using Porsche parts and not aftermarket.

At the cost involved for a new unit, perhaps a flame sprayer could bring it back to the needed diameter? Just calls for machining the (over)sprayed surface back to smoothness and spec.

[HaroldMHedge]

Checking Google translate.
Bore is Bohrung which has been shortened to B.
Shaft is Welle which has been shortened to W.

Wayne's How to Rebuild & Modify Porsche 911 Engines 1965-1989 book lists the following information on page 206.

Case Bore for Intermediate Shaft
Outer thrust bearing (bore)
Outer thrust bearing (shaft)
Inner bearing (bore)
Inner bearing (shaft)

Intermediate Shaft Clearance
Intermediate Shaft Axial Play
Wear Limit