Trans gear flip. Can I do it? Options

[Han Solo]

I'll probably get more eyes on the thread in the Garage.

I've got a spare 901 trans purchased about 2 years ago. No idea how many miles are on it or what year car it came off of. What I do know is I need a different gear set for my Prepared '73 914 autocross car. I've read through the gear set charts and I believe the flipped 3rd and 5th will get me where I need to go.

So today I got that trans on the bench, drained the fluid and removed the rear cover. Those of you who know what they're looking at, please let me know how 1st gear slider and dog teeth look in the photos.

My initial observations are that those look pretty good. I noted that someone had installed a magnetic drain plug. That had done it's job and there was about 1/2" of metallic goo stuck around the inside of the plug. The gear oil came out pretty clean with only very slight traces of micro particles.

I just read Bondo's thread on trans disassembly and it's pretty intense. There's also no mention of the 3rd and 5th gear flip so I've got no clue exactly what that procedure is.

914 transmission disassembly

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[oldie914]

The gear cluster on the intermediate plate, ring/pinion and case are a set with clearances adjusted during assembly. If you take a transmission apart and don't see any R&P or bearing problems, you can reassemble it and preserve the original adjustments. If you exchange major components, you need to reset R&P adjustment.

Pinion depth is the 1st thing checked when rebuilding a transmission and requires special tools (expensive). Rough pinion depth is established with shims in the gear stack next to the roller bearing. Fine adjustment is made with various thickness gaskets between the case and the intermediate plate. This means that you can reassemble a transmission with the same major components and with the same thickness gaskets and pinion depth setting will be OK. The gasket between the intermediate plate and the nose has no adjustment function; it is always about 0.20mm thick.

The second thing checked is the preload on the differential roller bearings. It is adjusted through various thickness spacer rings on the differential carrier spigots under the roller bearings. It is measured by checking the turning resistence of the differential carrier. If you do not change differential carrier, case or bearings, this adjustment is preserved.

The last thing checked is the backlash between the ring and pinion gears. It is adjusted by changing the spacer rings under the differential bearings so as to move the ring gear toward or away from the center line of the pinion shaft.
Rings need to be changed in pairs in order to preserve the bearing preload. To measure backlash, you need to fix the pinion shaft and attach an arm with a dial indicator to the differential carrier and measure movement to a fixed point on the side cover. Backlash adjustment is preserved if you do not change components involved.

I hope I didn't tell you how to build a clock when you only wanted to know the time.

[Han Solo]

The gear cluster on the intermediate plate, ring/pinion and case are a set with clearances adjusted during assembly. If you take a transmission apart and don't see any R&P or bearing problems, you can reassemble it and preserve the original adjustments. If you exchange major components, you need to reset R&P adjustment.

Pinion depth is the 1st thing checked when rebuilding a transmission and requires special tools (expensive). Rough pinion depth is established with shims in the gear stack next to the roller bearing. Fine adjustment is made with various thickness gaskets between the case and the intermediate plate. This means that you can reassemble a transmission with the same major components and with the same thickness gaskets and pinion depth setting will be OK. The gasket between the intermediate plate and the nose has no adjustment function; it is always about 0.20mm thick.

The second thing checked is the preload on the differential roller bearings. It is adjusted through various thickness spacer rings on the differential carrier spigots under the roller bearings. It is measured by checking the turning resistence of the differential carrier. If you do not change differential carrier, case or bearings, this adjustment is preserved.

The last thing checked is the backlash between the ring and pinion gears. It is adjusted by changing the spacer rings under the differential bearings so as to move the ring gear toward or away from the center line of the pinion shaft.
Rings need to be changed in pairs in order to preserve the bearing preload. To measure backlash, you need to fix the pinion shaft and attach an arm with a dial indicator to the differential carrier and measure movement to a fixed point on the side cover. Backlash adjustment is preserved if you do not change components involved.

I hope I didn't tell you how to build a clock when you only wanted to know the time.

That's all good information. What I'm hearing is that unless you're trained transmission mechanic, you're best just using the original stack, case and intermediate plate.

Yesterday I pulled the stack out of the spare transmission (with the bad 1st gear dog teeth, slider and syncro). Mainly just to see how the other gear sets looked and to help with re-assembly on the '76 trans shift forks and rods. The syncros and dog teeth on that stack look okay but not as good as the '76.

Everything went back in well but unfortunately the threaded end of the pinion shaft got mushroomed a bit and I'll need to chase those threads. It's always something (IMG:style_emoticons/default/sad.gif)

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