Flipping 901 Gears

[ErVikingo]

Are all 901 gears "flippable"?

For example, can I take a 27:25 "V" gear and flip it over to use it as a 25:27 "R"???

I appreciate your feedback.

[304065]

Thou shalt not flip.

Reversing the gears puts an axial load on the shaft in the wrong direction.

[Plavan]

We flip gears all the time without any problems. We generally flip a ZD 5th to a KA 3rd. It is common practice out here in VARA. No one has had any problems to speak of.

[Grady Clay]

Chad,

I’ll have to agree with John. I do agree it can be done. I don’t think it totally inappropriate on a 1.7 914. With large displacement and high power 911 engines it isn’t a good idea. In fact it can cause some serious wear and failures.

The technical reasons are here: ”Identifying 901/911 Gears versus 915 Gears”.

An additional important reason, not in the above thread, is the possibility of pulling out the nine M8 studs that are threaded into the case and pass through the intermediate plate and nose piece. These commonly pull the threads on the magnesium castings (I Helicoil all of my type 911). Some go so far as to use a nut and bolt arrangement, doing away with the threads in the main casting. I understand this is also true on the aluminum 901 cases. The cause of this may be flipped gears because that significantly increases the force pushing the intermediate plate away from the main casting. It certainly must increase the problem with high torque.

The proper gears are generally available, although expensive depending on ratio and condition.

There isn’t any problem to the gear per se. The issues show up elsewhere in the transmission - in much more expensive places.

Best,
Grady

[Plavan]

I understand the logic, but I have to say that we all do it in VARA and nothing like that has happened (knock on wood) . This is with a whole bunch of 2.0L motors, 2.2, 2.4, and 2.5's. Even on my race 2.7, I flipped gears with no problems. Flipping gears has saved many alot of $$$$. I guess the 901 and 911 boxes must be pretty beefy. We do use the aluminum intermediate plates on the mag cases, but that would not really help with the stud pulling like you state.
I do understand what you are explaining. I'm just suprised with all the flipped gears we all run (and the other's we race with) we have never seen a failure at the track in 8+ years. This is actually the first I heard of it.

[Henry Schmidt]

Chad is right.
I think you guys may be working this issue a bit too hard.
The gear loading may not be optimum but in twenty five years of building 901, 914/6 and 911 not to mention 915 and 930 transmissions "flipping" has never been a problem. In fact we have been "flipping" Ha and Hb gears (2nd gears in a 4 speed) to use as a 5 gear in V8 conversions boxes and no problems yet. I guess 15 years of V8 torque is a fairly good test for this conversion.

Just my two cents worth.

[kenikh]

This makes me feel so much better about the KA (flipped) 3rd going in my car...

[ber911]

I have a flipped 5th gear as 3rd in my 901 gearbox, which has started whining ( only in third) recently and is getting worse. Is this because I have flipped a used gear or could it be another issue?

Rob