Rod bolt diameter New vs Orig and Junction

[ahh911]

Hi Guys,
It just goes on... 1981 911SC, U.S. Spec.

The rod bolts I bought to replace the original Verbus stamped versions are all of larger diameter than the ones removed, (see point of interest in the attached image copied from another member and edited in red). The original bolts are all identical in diameter at this area and have been machined this way at the Porsche factory I believe to fall into almost exactly the middle of the spec, 0.208mm. It is remarkable how accurate they are to each other. The new "verbus" or T 12.9 stamped bolts all start life beyond the spec, I then took sandpaper and twirled the area down to just above the maximum spec which polishes them off and removes the rings of the original machining, just like the original parts. I'm sitting at .4027 in and the originals were all .4019 in, and I'm above max spec by .0006 in if this diagram is valid. (10.229mm, max spec 10.212mm) For your interest I had 36 bolts to choose from, there were some tapered versions, one well below spec and most above spec, and most with mild thread damage. Don't ask, I received 3 new shipments of these bolts courtesy of damaged threads, then UPS and I filtered them all. Anyhow, I know the fit will be very tight, but I figure the bore has been in service for awhile. Should I reduce the diameter to the maximum spec on this drawing? and I don't believe the original bolts have reduced diameter because of non elastic stretching to any degree since they are all perfectly matched in diameter and centred perfectly on the spec .

part number: 91410317100, Pel has it updated to 91410317101 but I went somewhere else at the time because they specifically mentioned Verbus on the info sheet.

Thanks,
Phil





91410317100

[Flat6pac]

The 12.9 Verbus bolts for the 3.2 get a second 90 degree swing so there is lots of extra stretch.
On all the rods, regardless of CCs they all have to tap in place to seat.
Bruce

[Trackrash]

Maybe time to buy some ARP bolts?

What is the ID of the hole in the rods? I would hesitate to reduce the size of the bolt.
But that may be your only choice. I doubt they would neck down much in size when torqued.

I just measured my old bolts and they are .4017" .

[ahh911]

Thanks Bruce, on the 3.0 SC, there is one 90 degree after initial torque, but it's not the final stretch it's the going in diameter that I want to get reasonably close. For instance, one new unmodified/unsanded rod bolt needed barely a thumb, if that to fall into place, I knew it was the smallest diameter and below the original diameter, this one if not caught could have been a problem. There were 4 rod bolts with a more finished look, meaning less machine rings that all had tapered centre portions (point of interest), they were discarded as the originals were perfectly square in their machining. I haven't driven in the sanded down versions yet (these are the ones that are a little over spec now) because I don't want to drive open the bore, as even the original ones need coaxing to get back in. These are little details that come up that aren't published but I assume are known by the guys that pre-date the internet. ARP is easy I assume, but this OEM rodbolt not from Porsche direct, but who knows, at least looks like it's still a judgement call installation that needs a little attention prior to installation, from another time period where stuff just doesn't snap together. I'll do my best judgement, you can't find this question online, as far as I can tell (1 hour of searching).

Phil

[ahh911]

Gordon, thanks for checking! Your post just came in after I posted that last message. You measured .4017, I measured .4019", now that's pretty cool. If you look, you'll also see that they were machined in that area beyond the first pass cut. (Also the area above it where it doesn't contact the bore is also machined, but to less than tolerance on occasion, but that point doesn't contact anything so I thought it was for weight manipulation). I too am reluctant to go down too deep, but they are oversized by more than 1 thousands of an inch to start with (and vary from part to part) and almost look like they are made to be reduced as the machining rings are still present and are clearly removed in the original bolts. Also, I checked the bore with a ball gauge, all 12 were slightly less than the diameter of the original rod bolts... I don't remember by how much, I can re-check but that was an indicator that guided me to reduce the diameter of the new bolts as well, I thought. I'll get these numbers later. ( I didn't go to the case, and shouldn't have removed the rods either, all indication were of a clean engine, no over-revs (pistons clear, no stretched chains), very clean intermediate gear teeth, cylinders clean, under 100kmiles, I thought I'd look at the rod bearings, if they were bad I'd open the case, they were fine, but now I'm left with newly manufactured rod bolts and bearings that are pretty suspect, had to go to gt3 bearings, but like I posted they are elliptical in design, the originals are not... anyhow, no arp as stretch is difficult with case in, besides, maybe a diameter check would still be in order..)

Phil

[ahh911]

Gordon,
I measured everything again, numbers are relative to each other as micrometer feel plays a role, so absolute is not essential, and I'm not practiced enough in the art:

Bore of interest: 0.4018"
Orig bolt removed from engine Diam of interest: 0.4020"
Diameter on average that I have after sanding: 0.4026"

Two randomly selected brand new untouched rod bolts Diam of interest: 0.4040", 0.4039"
Smallest diameter brand new untouched rod bolt Diam of interest: 0.4015" (It slipped into the rod bore without any help at all, this one was easy to toss out)

So should I shave some more off? I think the ones from the factory can vary quite a bit so some manipulation seems necessary, just to what degree. Any idea what a good diameter value I should be aiming for?

Phil

[356911930]

Hi Phil,

From the original drawing it looks like the tolerance is listed as:

10.212mm or 0.4020 inches as a high diameter of the Rod Bolt

10.201mm or 0.4016 inches as a low diameter of the Rod Bolt

I would think that the diameter of 0.4018 inches would be the ideal Rod Bolt diameter.

A Rod Bolt diameter of 0.4016 inches to 0.4020 should be acceptable.

Best of Luck,

Rahl

[ahh911]

Hi Rahl,
I'm not even sure about the source of the drawing and what it's application is, to be honest.

I'm going to ask my Dad first, but my plan unless someone thinks otherwise will be to slightly exceed the existing delta between the rod bolt diameter and bore diameter. So the original parts seem to have a rod bolt with +.0002" diameter than the bore, using the same sort of measurement feel. The original bolts need tapping in, as expected but not too hard, I will slightly exceed the original diameter by another .0002" to take into account any bore enlargement over the initial factory installation, probably fantasy. It's not easy to take an accurate measurement, but it looks like I'm aiming for around .4021" with my measurement tools and technique. With the micrometer it's very sensitive to pressure and easy to be off by .0002, So everything will be relative to the old bolts, a touch bigger than the originals, that's all.

Phil

[ahh911]

I ordered a set from ARP, then contacted them directly, the diameter of interest I received is .4015-.402”. , if it can really be .4015 it will slip in without any interference I'll be back where I started or worse as there will no locating contact between upper and lower halves, and I'll be hundreds less in pocket. I may be reduced to re-using the originals by the sounds of things.

Phil

[ahh911]

Looks like I wasn't the only one with the oem 911 sc bolts, read the last statement by op, he talks about some being hard to get on and some can be jiggled. Quality is an issue with these. Posted in 2020.

Stuck connecting rod bolt

[ahh911]

Update, for what it's worth:
Went with the OEM replacement bolts that I had bought. Used a drill press to spin the bolts after protecting the threads, several different strip widths and grit of emory cloth/sand paper, brought down the needed areas to around .4020", test fit them for a similar snugness as the original bolts for each bore, then put them together with the old bearing shells and used a straight surface and a dial indicator to check that rod and cap flat edge were aligned to within .00025", if not, removed the bolt on the offending half, then figured out it was either too tight or binding a bit, replaced it and all six are clean on the straight table, no wobble at that point as well, and big end roundness seemed well controlled. I do not recommend this if you don't have a lathe as it's very time consuming (there are two holes on either side of the bolt where you could hook it up in a lathe, I don't have one), I had plenty new OEM rod bolts where they were oversized and clearly meant to be brought down to size and I didn't want to run in circles ordering new/different bolts when it was manageable, but time consuming. (it's also important to bring the smaller diameter area into spec as well, though it's dead easy as it's a .1mm tolerance, otherwise it may contact inside the upper area of the cap)

Phil