7R case throughbolt hole mystery and ?

Walt Fricke · 03-05-2010, 04:51 PM

I have a 7R case I am converting into a jig for installing bushings around the throughbolt holes instead of shuffle pins. In doing so I realized that the left case half had steel inserts ~1/2" deep around all its holes, while the right half had no such thing (and could be drilled fairly easily). You can see in the pictures. I didn't believe this at first, but a magnet proved it to be so.

Just out of curiosity, is this normal? Granted, one seldom pays much attention to these holes - I never did.

While on the subject of through bolts, remember where Anderson (or someone) shows how the 917 (935? 2.8RSR?) used titanium case through bolts of a narrower diameter, but also used spherical washers (a double washer, one concave on one side, the other convex) to reduce the stress caused by the fact that the throughbolt ends were not precisely opposite each other? I thought I could find the reference easily, but am having trouble.

But that's not my point. These washers are not all that expensive, $2.40 each set for a 13.5mm ID. Has anyone tried these?

Walt

Flat6pac · 03-05-2010, 04:55 PM

Walt, I ve never noticed anything like it, I miss the use or application because they dont support anything and everything else is still magnesium..
Bruce

Walt Fricke · 03-05-2010, 06:02 PM

Bruce

Assuming you, another experienced mechanic I discussed this with, and I are right, and this is not part of the factory setup no one notices:

Then my guess at this point is that the machinist who installed the anti-shuffle bushings on this case (where I got the general idea, though Cgarr has done this I see) drilled the stock bolt holes out so he could through drill/ream into the other case half with everything buttoned together. Then he installed these to keep the bolts where they belong.

Walt

But that's a guess. They are well and truely in there tight.

kenikh · 03-05-2010, 06:11 PM

This is a good mod - the RSR shuffle bolts were a flush fit for this purpose and were titanium, no less. The holes were reamed to tigh tolerances and the bolts were just short of an interference fit.

Remember that oil moves through these passages, which is why the "shuffle bolts" had flutes in the sides.

As for the steel inserts, this seems like a nice way to get the strength, durability and clearance for an perfect fit.

Walt Fricke · 03-05-2010, 07:19 PM

I found the Anderson reference I was thinking of: he shows the spherical washers, along with three styles of through bolt - early coarse thread, later finer thread, and a "special aircraft quality bolt" used on more recent (as of 1996) race motors. These were one bolt size smaller in diameter, he says, but the larger diameter center portion he says was to stop harmonic vibrations. Not to serve as a case shuffle stopper. No grooves apparent on these larger diameter bits.

I, too, remember reference to titanium, which Bruce does not mention. The 2.8s used the mag case and were pretty highly stressed, so some kind of shuffle prevention would seem to be likely.

kenikh · 03-05-2010, 09:28 PM

They are titanium. I have held a NOS set in my hands.

jcge · 12:31 PM

Walt - here's the 917 assembly - Detail A shows the spherical washers and thread locating detail.

Center diameter increased at the case parting line - not sure if impedes or allows oil flow in this application

(C) IME 1972

John

Walt Fricke · 03-09-2010, 10:56 AM

John

Ludvigsen is about the only reference I have that has any technical stuff on the 917. He says (page 567) that the 917 through bolts (at least on the initial version of the engine) used Dilivar case throughbolts. Also that the head bolts (not studs) were Dilivar, and jacketed so the cooling air would not reduce their temps as much.

Walt

jcge · 02:40 PM

Walt - a great reference for the 912/917 engines is the technical paper that Hanz Mezger delivered to the Institute of Mechanical Engineers in February 1972.

"The development of the Porsche type 917 car" Proc lnstn Mech Engrs 1972 Vol 186 2/72

You can get it here

But back to the spherical washers, according to DIN 6319 D

For a regular M10 nut

ID = 10.5mm
OD = 21mm
Height of assembly (washer and seat) = 6.4mm

This is thicker than the standard washer, so you'd need to consider thread engagement (and oil sealing).

Alternatively you could use a DIN 6330 hex nut which incorporates it own spherical base in conjunction with a spherical seat (DIN 6319 C).

Perhaps more or equally importantly is the detail design of the transition from the shaft to the threaded portion of the bolt....the image of the 917 assembly above provides some initial direction.

I've not personally tried implementing this....but food for thought.

What specific problem/failure mode are we trying to solve ?

John

Walt Fricke · 03-09-2010, 08:05 PM

Thank you, John

Actually, I don't think I will have a problem to solve. I believe the 917 got this treatment because a) the throughbolts ran at a bit of a diagonal through the case - I don't know this for certain, but it was given somewhere as the reason for this I think, otherwise why bother, and b) the bolts were thinned for weight savings to the point where the uneven stress of having a slight diagonal began to matter.

I've not heard of a 911 througbolt giving problems. I am beginning to suspect there may be a slight offset, but the bolt must have enough strength reserve that it is not an issue.

But these spherical washers intrigued me from the first time I saw a reference 25 years ago. They look like an interesting solution to some problems.

Then again, if alignment and perpendicularity are issues, perhaps one could solve them more simply by milling the washer surface at an exact, if slight, angle, so they were boresighted, if you will, on both sides.

I had considered the thickness issue, recalling how the washer on later motors (like what I am building) requires a bevel. I didn't know about the DIN 6330, though.

And I had wondered about sealing - the stock washer has an ID bevel to help the O ring do its job.

Walt

Walt Fricke · 03-09-2010, 08:23 PM

Oh, I see what you mean about the detail where the bolt enters/emerges from the hole: it is at or almost at the diameter of the hole. That would solve any sliding sideways due to angularity issue? Though if there were a slight offset, there would be bending just below the wider part of the bolt? Maybe that would be more acceptable than at thread roots or whatnot?

Walt

jcge · 02:01 PM

Walt - I'm pretty sure there is no offset - no designer in his right mind would do that ...surely...
What is it that makes you suspect an offset ? cylinder clearance ??

Early flat washers were....flat, and later had a chamfer on their OD for clearance...

John