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Old 05-11-2005, 01:47 PM
TurnAround TurnAround is offline
Location: Oregon
Join Date: Jan 2004
Posts: 358
Mein Auto: Z3C, Z3R, X5
Fitting an Interstate MTP-91 Battery to a Z3 Coupe or Roadster...

The Interstate MTP-91 battery, which is supposed to be the battery from Interstate that fits all Z3's and MZ3's, is a half inch shorter than a stock BMW battery. This is not a problem on the M Coupe, because the battery well is in the center of the boot and the battery is oriented width-wise. On the Z3C or Z3R the batter well is on the passenger side of the boot and the battery is oriented length wise. This orientation makes it so that the positive cable lead from the car rests on the edge of the battery well / car body. The half inch drop in the battery height prevents the cable lead, then, from seating properly on the positive terminal post. The following instructions describe a work around for the problem...

Bend the forward sheet metal flange "up" slightly. About half way. You still want some lean to it, or it won't catch an edge on the battery's base. Can be bent with channel lock pliers. "Pivot" your wrist and make small bends. Work your way across the bracket with small movements. If you just "pry" the whole thing foward, you may break the spot welds. Be patient. Slowly bend it up.

Go to your local industrial rubber supplier [ Yes, yes.. some comic sarcasim there, but really.. check around. You may be surprised that your area has a shop like this. Usually they make custom hydraulic and water hoses ]. Get a 1 foot square sheet of 3/8" thick rubber, and a small scrap piece of 3/16" rubber, and a small scrap piece of 1/2" rubber. The 1/2" one will likely measure out to 1/16" less than 1/2". That's normal. and that's what you actually want. The other two pieces should measure out to exact (and need to).

Cut the 3/8" sheet to fit the base of the battery. 7" x 11" rectangle. Now, cut out another inner rectangle inside of it, leaving a 1" boundry all around. This is to dump off excess weight. only the outside edges of the battery touch anyway (look at the battery well). Yes.. you could do this rubber shim with several little pieces, but then they could move around. It's cleaner and more "Stygar / Forbes" like to make the single piece and cut it down to make it light.

Put it down in the battery well. Set the battery in. Unlike the M Coupe, the Z3 Coupe (or Roadster) has plenty of the light grey original BMW battery vent tube hose to reach the new location for the vent (don't need to buy more). Hook up the vent tube kit that you got with the battery. Couple it to the car's hose. Gently stuff it down along side the battery (don't leave it where it could get crimped). Center the battery so the Pos connector will reach nicely.

Put the cross strap over the battery. Carefull.. don't drop the little bolt down inside the well. Place a 1" x 2" piece of the 1/2" thick rubber underneath the left side of the cross strap. This will take up the remaining slack, give plenty of threads for the cross strap bolt to bite, and will provide a very secure locked down fit.

Use the 3/16" thick rubber to make a 1" x 2" piece that will go down where the back end clamp goes. Make a little half round cut out to allow the bolt to pass through to the threads in the car. Without this shim, you could bend or break this particular bracket. It needs "a foot" to help it deal with the fact that the battery is now 3/8" higher in the air.

Put the positive lead on. Note, that the 3/8" pad provides juuuusst baaarely enough for the positive lead to go on squarely and tightly. Why not use an even thicker piece of rubber underneath the battery (after all.. the battery is a half inch shorter). 'Cuz.. if you did this, then the front base of the battery would slip past the spot welded on bracket. It's only "so high".

So, 3/8" inch is the magic number. Any more, the battery slips past the front bracket. Any less, and the positive terminal lead will not fit well.

Without the proper fit on the terminal lead, not only could it lead (arr .arrr.. lead) to a problematic "mysterious" battery drain problem, but, that odd angle fit and the bar resting on the car frame.. and a tens of thousands of miles of minute vibrations... could crack the positive terminal post. Much worse problem.

This combo of rubber parts should work forever. "Interstate MTP-91 infinitum".

The ROadster went well too, although interestingly.. there is no cross strap. Unfortunate, but.. the back end clamp seemed to be enough to apply pressure against the front bracket. I'll have to check it in 4 months. But really, we should all be checking out batteries every 4 to 6 months anyway. I've had a cracked and leaking battery before (no other reason than a defective battery case). Bad enough in the engine bay. But to have that happen in the boot...

Hope this helps others.


'02 Z3 Coupe 3.0i [Hellröt II - Walnuß]
'03 X5 3.0i [GrauGrün - Beige]
'01 Z3 Roadster 3.0i [AlpenWeiß - TraumRöt]
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