powdrhound

Any of the local gurus know if the 997GT3/Cup front upper shock mount has the same bolt pattern as the 996GT3/Cup mounts? I have conflicting reports as to whether the triangle bolt pattern (where it bolts to the chassis) is the same. Same say it it while some say it's a few mm different. Thx!

Tom@TPC Racing

997GT3/Cup have slightly different bolt pattern than 996GT3/Cup.
The difference is only a few millimeter.
If you have a 996 C4/Turbo the bolt pattern is the same as all 997 including GT3/Cup.

powdrhound

OK, I'm confused. How could that be Tom? The front shock tower mount pattern is identical on the 996Cup/GT3/GT2/TT/C4S (and 986), yet you say the bolt pattern is slightly different on the 997 compared to the 996GT3/Cup but same as 996TT. I'm not quite sure I follow, either the bolt pattern is the same between the 997GT3 and all the 996 cars or it's not as as it can't be the same between some and not all.

Tom@TPC Racing

I sell and install this stuff everyday for a long time. There are two patterns available for 996/986 and 997/987.

One pattern fits 986, 996, 996GT2/GT3/Cup.

The other pattern fits 987, 997, 997GT2/GT3/Cup, 997C4/Turbo, and 996C4/Turbo.

One pattern top plate will fit into the pattern body but in order to use the slots for camber adjustment the slots must be made wider by grinding which I do not recommend.

powdrhound

Thank you again for the input Tom. Yes, I don't want to grind anything. Where the discrepancy lies is with the fact that the 996TT with 100% certainty has the same exact strut mount bolt pattern as a 996 GT3/2/Cup but that is contrary to what you are saying. Hence the confusion.

Just to clarify Tom, you are saying that the bolt pattern is different between the 996TT/C4 and the 996GT2/3/Cup? Is that correct?

Tom, do you have one of the 997GT3 upper mounts at the shop? If so, could you simply measure the distance from the center of each stud to the other two in millimeters and post the numbers? The studs should form a triangle with equal length sides and one side longer if I'm not mistaken. Thx!

Tom@TPC Racing

The discrepancy has led me to measure the two plates. You are correct that both "patterns" (as we were referring to them as) do share the same measurements with the distance of the legs of the triangle being 118-110-110mm. After confirming these measurements just now I realize the fitment issue isn't the spacing of the three stud alone, it is a compound issue of the positioning of the strut pin top relative to the opening of the body and the exterior shape of the plate causing contact with the strut tower in order for the shock pin top to fit through the strut tower; within the confines of the pin top opening trying the rotate and line up the three studs into the three slots.

The reason that the 996Turbo/C4 can use the "OEM" GT3 hexagon exterior shape plate is because the AWD uprights angles the front shocks to fit through the hole as evident by the "angled" Turbo/C4 plate in this image.


So basically the two different "fitments" are the function of where the triangular bolt pattern is within the exterior shape of the plate and where the pin top of the shock is within the triangular bolt pattern, and how the shock is angled by the upright. So by changing the exterior shape of the plate and the position of the bearing on the plate one could make a true cross-generation plate.

powdrhound

Thank you Tom. I am absolutely not concerned with the angle of the shocks which are totally different on the 996AWD cars. I was simply concerned with the bolt pattern spacing which is identical on all the 996 cars as they share the same tub in the front. For example you can take a 996GT2 suspension and bolt it to a 996TT chassis if you wanted. Irrespective of the shocks, I was simply trying to see if the 997GT3 mounts will simply bolt to the 996 chassis, nothing more, nothing less. Sounds like the answer is NO if the stud spacing of the 996GT3 and 997GT3 mounts is indeed different by a few millimeters. I will measure the bolt spacing of the 996GT3 plates when I get home to compare with what you posted.

Tom@TPC Racing

I was wrong about the different bolt patterns, 996 GT3 Cup does measure 118-110-110mm. It is when trying to install a 997 GT3 Cup hexagon plate on a 996 that it requires the 997 plate to be rotated in a way that the one stud "appears" to be a few mm off. Thats what makes it "appear" the pattern is different but to expand on what I wrote in last post the issue is the external shape fitting flush up into the strut tower dome. That's why the OEM 996 plate is oddly shaped externally in order to fit the strut tower dome. Prior to this evening I thought all along it was the bolt pattern, but I was wrong, it is external shape and the triangular stud pattern within the shape that defines the two fitments as far as OEM
external shape is concern.

993GT

John,
Will try to remember to measure both plates tomorrow... will have both plates and cars here tomorrow; 7.1GT3 & 6GT2
Cheers,
Rob

powdrhound

Thanks Rob. Fantastic my friend!. I have measured red JRZ 996GT3/GT2/Cup plates as pictured below . Measuring via a caliper between the center of the studs, the distances measure 118-110-110mm. The distance between the two upper studs in a horizontal plane is 118mm while the distance between each of those upper studs and the bottom center stud is 110mm. When mounted, there is plenty of clearance between these symmetrical plates and the strut tower sides so clearance is not a factor. As such, I am not sure why the OEM 996 mounts have the odd ball shape. I have used these exact symmetrical mounts on several 996GT3/GT2s in the past with success in both the low camber (as pictured) or high camber configurations where the studs are installed in the alternate holes and the mount is rotated 180 degrees.



The OEM 997GT3/GT2/Cup plates are pictured below. You can see that the shape is identical to the JRZ 996 plates pictured above. If we can confirm that the spacing is an identical 118-110-110mm, then that will confirm that the 996 and 997 mounts are identical and interchangeable.

powdrhound

I don't believe this is correct either Tom. Have you physically tried to bolt the 997GT3 hexagon plate into the 996 strut tower without success? I've been using the symmetrical hexagon 996GT3 JRZ 118/110/110 plates for years and there is no interference issue with the symmetrical hexagon shaped plates fitting into the 996 strut towers in both camber configurations. IF INDEED the hexagon 997 plates have a 118/110/110 spacing, then there is no question they will be proven interchangeable. Again, I was unsure of the stud pattern measurement of the OEM 997GT3 plates hence starting the thread here. I had previously read one of your posts a while back where you stated that the 997GT3 bolt pattern is different and that in order to bolt it to the 996 tub you would have to grind out the slots. That is what started this confusion on my part. At this point I guess the verdict is still in the air as you seem to have measured 996GT3 and TT mounts in post #6 but NOT the 997GT3 plate, unless I misunderstood. In other words, did you measure the 997GT3/Cup plate as 118-110-110 also?

Looks like we will have it settled once and for good when Rob measures it tomorrow.

pete95zhn

I have 997 GT3's upper shock mounts on my 996. The bolt pattern is the same. They will space shock tops about 10 mm outboard / side. (IIRC, maybe it was 15mm. Can't measure just now, the car is 200 kms away)

However 997 GT3 Cup's strut brace will not fit 996s, because 997's body's installation slots have been rotated to a different angle.

Tom@TPC Racing

Yes I have, with a criteria being I cannot widen the middle hole where shock pin top goes through.

Tom@TPC Racing

I have provided some incorrect info. For that I apologize. It was not my intention to misinform. It was in good intention from my installation experience and from mail order support to shops/car owners over the years as to what fits and what doesn't fit with or without grinding.

To make amends, here are photos I took this morning.

Here is a 996GT3 with the OEM plate at narrow pattern. Noticed the two curves on the plates for clearance. Left front.




Using a 996GT3 Cup tub to model, here is a 997 aftermarket plate at "wide pattern". The plate has the exact same 118-110-110 stud pattern and exact same exterior hexagon shape as OEM. Left front.




Same 996GT3 Cup tub with same 997 plate at "narrow pattern". Left front.

View from top with plate at narrow pattern.

Notice the bearing boss is contacting the strut tower dome and this tub had the middle opening severely widen to accommodate.


To sum things up, and to answer,the original question bolt patterns are the same. For the OP, putting a 997 plate on a 996 will fit with full adjustment range if you widen the middle hole. Some of my customers do not want to widen the middle hole buy grinding/cutting, thus some plate makers including OEM offer two different shape plates to accommodate.

powdrhound

Hey Tom,
Thanks again for the pictures and no worries about making a mistake with regards to your previous posts. We all do it, I know I have.

Based on everything posted up at this point, it is unequivocally clear that the shock mount bolt/slot pattern is identical (118-110-110) between the 996 and 997 cars. It is also clear that the hexagon shaped 997 plates do not cause any interference between the plate and the shock tower itself when mounted anywhere within the range of the slots.

The last bit of information that has been posted above is with regards to the clearance between the center bearing boss and the shock tower itself when you are at the "high" camber configuration, or what you are referring to as "narrow". As you can see in your last two pictures Tom, when the plate is in this "narrow" configuration AND the plate is additionally pushed all the way inboard in the slots to maximize camber, the plate bearing boss just comes into contact with the strut tower. I have looked at my car and that is also the case with the JRZ symmetrical hexagon 996 plates, identical case. You can also see in the pictures below that in the max camber "narrow" configuration you have to widen the strut tower center opening to make room for the top of the shock shaft. However, there is really no reason to make a huge hole up top, you just need to grind the center opening slightly to clear the shaft (not the bearing boss). The dome shape of the strut tower is what gives it it's strength so the less you take away from it, the better. Whoever opened up the shock tower in the pics you have above where they cut away a large portion of the upper "dome" simply went way overboard, no reason to go that drastic. I will add that you should never enlarge the opening leaving it with sharp corners as that is where stress cracks will propagate from, the opening should always be round/oval (think back to what brought the end to the De Havilland Comet at the dawn of the jet age). If you look carefully at your picture above, you can already see a stress fracture forming in the sharp corner of the cutout. NOT GOOD. Whoever cut that open like that needs to take a basic metallurgy class.

Finally, looking at the geometry of the 996GT3/Cup plates and the orientation of the bearing within the geometric mounting patters, it is IDENTICAL for both the 996 and 997 plates assuming you are comparing each in their respective "high or low" (narrow or wide) configuration.

With that said, I am willing to go out on a limb and say that if you take the asymmetrical OEM 996GT3/Cup plate and mount it in the high camber "narrow" configuration, you will find that the bearing boss is also just barely making contact with the strut tower wall when the plate is pushed all the way inboard in the slots. As such you ALSO will need to grind out the top of the shock tower opening slightly to clear the strut shaft even with the 996 plate if you max out the camber with it. There is simply no way around it.