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Alpina diff cover threading


Ian

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I've got an e21 Alpina diff cover that's got a port with a mystery thread pitch.  This is not for the fill or drain plug.  It's on the side, perhaps for a temp sender?  I cannot figure out the thread pitch to plug it.  An m14 bolt is a bit too large, and an m12 is too small.  I can get a 1/2"-13 bolt in and catch a couple of threads, but then it jams as if the hole gets smaller.  So I though perhaps its pipe, like what a lot of temp senders use.  But a 1/4" NPT plug is too large, and a 1/8" is too small.  

Can someone enlighten me to the mystery thread pitches from the late '80's, before I resort to TIG welding or bubble gum to plug this opening?

 

Diff_port.jpg

Ian
'76 M2

'02 325iT

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It's an original Alpina, so I'm assuming it was made in Germany.  Thus my tests with metric bolts first.  The AN option is interesting, I'll have to see if I have a fitting somewhere.  The problem is that it is now on the car, so I can't just drag the cover to a store and plug in everything I can find.

Alpina - 1.jpg

Ian
'76 M2

'02 325iT

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I just repaired a lamp for a friend that had a previously unknown thread to me. It sounds very similar to what you describe and it was 1/4 straight thread NPSM.  There are a lot more plumbing thread sizes than i knew ! I could just thread a 1/2-20  (fine thread) bolt into the hole with a VERY sloppy fit. 

 

c01d-howtoidentifyyourthreadtyped1-digit

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1 hour ago, uai said:

It's for a Temp Sensor

So probably Metric Fine thread
M12x1,5 or M14x1,5

For M12x1,5 you can try a bolt from the Front Brake Caliper
M14x1,5 you could try a Sparkplug

 

Unfortunately, I tried an M14 x 1.5 and it was slightly too large, and the M12 was too small.

Ian
'76 M2

'02 325iT

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Blunt and Paul Wegweiser suggested that it might be British Pipe Thread.  So I went to my local hydraulic hose supply store and we figured out that it might be a 1/4" British Standard Pipe Tapered (BSPT).  That size does indeed thread into the fitting in my diff cover.  However, the threaded portion is 0.6" long, and I can only thread about 0.35" until it stops.  So I’ve got half the plug exposed.  

 

For BSPT threads, is it reasonable to crank half the plug in, or am I still off on the threads?  Anyone have experience with the BSPP (British Pipe Thread, parallel instead of tapered)?  Could this be BSPP?  The option of re-threading is possible, but you can see what Alpina did was sleeve the cast aluminum housing, so I don’t have a ton of material to thread.
Ian

Alpina_Diff_BSPT.jpg

Edited by Ian

Ian
'76 M2

'02 325iT

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If you look at that chart i posted, there are tapered and straight versions of that thread, i wonder if a straight thread fitting would work better.

 

Can you verify if the hole is straight or tapered ?

 

 

Have you tried a 1/2-20  size bolt ?

Edited by Lorin
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If yours is an original (weren't there repops made in eastern Europe) just give Alpina a Call to send you an Oil temp sensor.
(And re-thread accordingly)
Most VDO Sensors are M12x1,5 and M14x1,5
Unfortunately there was a sensor installed during scanning otherwise I could take dimensions from my Cad model (which is 1/100 mm accurate btw).

 

BSPT we don't use but it might be BSPP G1/4"  that's standard and it's between M12 and M14.

 

https://vdo-webshop.nl/de/34-temperaturgeber/s-3/funktionalität-öltemperatur
https://www.rennsportshop.com/de/fahrzeug-technik/anzeigen-elektronik/geber/oeltemperaturgeber.html

Edited by uai
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1 hour ago, Lorin said:

If you look at that chart i posted, there are tapered and straight versions of that thread, i wonder if a straight thread fitting would work better.

 

Can you verify if the hole is straight or tapered ?

 

 

Have you tried a 1/2-20  size bolt ?

Good point.  I can get a straight version of the plug in BSPP (British Standard Pipe Parallel).  It's hard to verify if the hole is tapered as it is on the car.  Worst case scenario I remove the diff cover for forensics.  I'm really hoping the BSPP fitting is the answer.

Ian
'76 M2

'02 325iT

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26 minutes ago, uai said:

If yours is an original (weren't there repops made in eastern Europe) just give Alpina a Call to send you an Oil temp sensor.
(And re-thread accordingly)
Most VDO Sensors are M12x1,5 and M14x1,5
Unfortunately there was a sensor installed during scanning otherwise I could take dimensions from my Cad model (which is 1/100 mm accurate btw).

 

BSPT we don't use but it might be BSPP G1/4"  that's standard and it's between M12 and M14.

 

It's an original Alpina.  Unfortunately, getting ahold of them from the States has proven to be a little difficult.  But I can source a 1/4" BSPP plug locally, so am hoping that is the answer.

Ian
'76 M2

'02 325iT

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