Top Hat / Spring Perch Compatibility M / Non-M

[gavronm]

I am trying to fit a used set of Bilstein B8 shocks with H&R springs off a 95 M3 to a 1992 325i. The shocks came with the '95 M top hats (offset to the rear only - pn 31332228345), spring perches (pn 31332227348), and the larger diameter 95 M springs, which fortunately are the same diameter as the non-M springs.

Our race series rules will not allow the use of the offset M3 top hats, and I am trying to figure out which non-M top hats and spring perches I can use. I will not have access to the car for a few weeks and the race is creeping up on me so hopefully someone here knows the answer.

As a '92, our current stock top hat is the one piece top hat / spring perch assembly (pn 31331139436). Later Non-M models use a 2-piece top hat / spring perch assembly comprising pn 31336779613 and 31331135580

All shock / top hat combinations, whether M or non-M, use the same M14 X 1.5 self-locking nut to secure everything to the shock rod. I'd like to believe that means the diameter of the shock rod as it passes through the top hat and spring perch is the same across the board. Can anyone confirm?

I would also like to know if there are any ride height differences between the one-piece assembly we have and the later non-M 2-piece assembly.

All help greatly appreciated.

[M3AN]

My understanding is that everything is cross-compatible and that the B8 shaft will mate to all of the hats you mention.

To be 100% sure, since you have all of the parts you can verify by:

1. Remove an existing shock and install a complete M3 assembly (top hats and all) - this will confirm all top mounting parts will work
2. Secondly, remove the M3 hat from one of your assemblies and replace it with the original one piece unit - this will confirm your B8 shock works with "old" top hats

Assuming all combos work then you know you can use any parts you wish (within the rules).

The two piece units are about 6mm taller than the one piece IIRC.

[gavronm]

Thank you - excellent info and when I have access to the car again with all the parts I will update the thread with the findings.

The two piece units are about 6mm taller than the one piece IIRC.

This is a particularly helpful bit of info as I was already concerned about being too low in the front. I'm going to purchase a set of the 2 piece units to have on hand during the upcoming work weekend when we tear into everything.

[gavronm]

Well, we did the swap last weekend and here is the most helpful bit of information we found - the M and non-M spring perches are interchangeable with the M and non-M top hats. We took the M3 top hat and spring perch off the assembled M3-spec Bilstein B8 and started swapping things around and quickly discovered that the M3 spring perch fit perfectly with the Non-M top hat and vice-versa. Obviously the non-M spring perch works with the Non-M top hat, but the fitment with the 95 M3-spec spring, while probably acceptable, is not quite perfect. Fitment of the 95 M3-spec springs onto the early E36 one-piece spring perch / top hat was also less than perfect, but acceptable.

I can also confirm that the one-piece top hat / spring perch on the '91 and early '92 built E36 cars is indeed shorter than the 2 piece Non-M spring perch and top hat combination found in the later cars. Unfortunately I did not remember to measure but it looked like way more than 6 mm to me, perhaps as much as 16mm.