915 1st gear synchro upgrade parts list (long and boring unless you need the info)

[Jim Sims]

In the process of rebuilding my son's 1973.5 T 5-speed 915 transmission one of the most confusing and frustrating issues was that of the 1st gear synchronizer upgrade and the required parts. I spent a fair amount of time on this issue and the below listed information is what I learned; please inform me of any errors or required clarifications. I don't have Porsche part numbers for the 1977 and later style 2nd gear synchro hub (dogteeth) nor the asymmetrical 1st gear synchro hub( dogteeth); if you have these part numbers I would like the know them. Thanks.

There are apparently 4 synchronizer options, although option 3 probably only makes sense in a very limited circumstance. Before I go into detail about the options, one should note it is generally recommended that when rebuilding a 915 that all the synchronizer hubs (dogteeth), all synchronizer rings, all brake bands, all anchor and thrust blocks, and all sliders (operating sleeves) be replaced during an overhaul. In addition, one should replace the seals and gaskets and the factory shop manual specifies replacing the lock washers for the shaft retaining plates and the flange nut on the pinion shaft. The expense of replacing all these parts as a matter of course will likely easily reach US$800 to 1000 even with some discounts: therefore some judgment and weighing of part wear and risk of reusing old parts has to be applied. After receiving a new 2nd gear symmetrical synchro hub (dogteeth) we compared its condition to all the other synchro hubs and elected to reuse the 1st, 3rd, 4th and 5th synchro hubs. We did decide to replace all 5 synchronizer rings, the 1st/2nd and 3rd/4th sliders and all the brake bands given these parts receive most of the sliding wear. We reused all the thrust (energizer) and anchor blocks. We installed new shaft retaining flange lock washers and replaced both flange nuts (pinion and input shaft) and of course new seals/gaskets everywhere. In addition, we replaced all the thrust washers as all the original thrust washers exhibited an irregular crack running around the edge (heat treatment or material quality problem?). The transmission shifted reasonably well before the overhaul with only an occasional balky/grinding shift into 2nd (the 2nd gear synchro hub was worn with hooked and blunted teeth) so we are fairly comfortable with our decisions regarding which parts we replaced but only time will tell if made the right choices.

Advantages/disadvantages:

Option 1 (original 1973 configuration): shifts from neutral to 2nd easily but harder to down shift from 2nd into first without a clash while car is moving; requires fewest parts to be changed but only feasible if 1st gear synchronizer hub (dogteeth) are in good condition as the original (two slots in hub/single brake band) symmetrical style 1st gear synchronizer hubs are truly no longer available. If someone says they have them, what they are likely selling you is a 2nd gear symmetrical synchronizer hub; this is not the same but will work (this is option 2).


Option 2: shifts into first when car is stopped are easier than original configuration (Option 1) but downshift from 2nd to 1st while moving is more difficult; requires few parts to be changed (new anchor and thrust blocks required for 1st gear along with one more brake band).

Option 3: in between Option 2 and 4 as far as parts requiring changing; the only reason to use this option is to upgrade the 2nd gear synchronizer to the 1977 and later style while retaining a symmetrical 1st/2nd slider. I'm not sure if any one uses this option but it appears the parts will assemble and work; its shifting behavior is likely between Option 2 and 3. I really do not recommend using this option.


Option 4 (this is the usual upgrade that is done): shifts more readily from 2nd into 1st while car is moving but shifts up into 2nd are sometimes more "balky". I note that in 1977 when this change to the asymmetrical 1st gear synchro hub and slider was first introduced that there was mid-year change to the anchor block so perhaps this anchor block change helped alleviate the " balky" up shift into 2nd. Requires the most parts to be changed. If this upgrade is made, ensure the 1st/2nd slider is installed with the asymmetrical teeth towards the 1st gear synchro hub which also has the asymmetrical teeth. The asymmetrical teeth have a big and a small facet forming the point while the symmetrical teeth have equal sized facets forming the teeth.



Option 1, 1st/2nd Internal Transmission Wear Parts; retains 1973 configuration,
i.e. one brake band on 1st gear synchronizer. This option allows the reuse of original 1st gear
thrust (energizer) and anchor blocks.

Part Name Porsche Part No. Qty

Slider for 1st/2nd gears 915.302.411.00 1
(symmetrical)

Synchronizer ring for 1st & 2nd gears 915.302.301.04 2

Brake bands for 1st & 2nd gears 915.302.316.01 3

Synchro hub (dog teeth) for 1st gear xx.xxxx.xxx? 1
(symmetrical; no longer made)

Synchro hub (dog teeth) for 2nd gear 10 1550 316 1
(915.302.242.00)


Option 2, 1st/2nd Internal Transmission Wear Parts; retains 1973 2nd gear configuration but uses a
2nd gear synchro hub, brake bands (2), 2nd gear earlier style anchor and thrust (energizer) blocks for 1stgear.
Note also, that this retains the use of a 1st/2nd symmetrical slider.

Part Name Porsche Part No. Qty

Slider for 1st/2nd gears 915.302.411.00 1
(symmetrical)

Block, thrust (energizer) for 1st gear 915.302.315.04 1
(note: this is normally an earlier style 2nd gear thrust block)

Block, anchor for 1st gear 915.302.321.00 1
(note: this is an earlier style 2nd gear anchor block)

Synchronizer ring for 1st & 2nd gears 915.302.301.04 2

Brake bands for 1st & 2nd gears 915.302.316.01 4

Synchro hub (dog teeth) for 1st gear 10 1550 316 1
(note: this a 2nd gear synchro hub) (915.302.242.00)

Synchro hub (dog teeth) for 2nd gear 10 1550 316 1
(915.302.242.00)


Option 3, 1st/2nd Internal Transmission Wear Parts; this option is the upgrade to the 1977 and later 2nd gear synchronizer; but uses the earlier 2nd gear synchronizer hub, 2 brake bands and the earlier style 2nd gear thrust and anchor blocks for 1st gear. Note also, that this retains the use a 1st/2nd symmetrical slider.


Part Name Porsche Part No. Qty

Slider for 1st/2nd gears 915.302.411.00 1
(symmetrical)

Synchronizer ring for 1st & 2nd gear 915.302.301.04 2

Brake bands for 1st gear 915.302.316.01 2

Brake band, double for 2nd gear 928.302.318.02 1

Block, thrust (energizer) for 1st gear 915.302.315.01 1
(note this is an earlier style 2nd gear thrust block)

Block, anchor for 1st gear 915.302.321.00 1
(note this is an earlier style 2nd gear anchor block)

Block, anchor for 2nd gear 928.302.322.00 1

Block, thrust (energizer) for 2nd gear 915.302.315.01 1

Synchro hub (dog teeth) for 1st gear 10 1550 316 1
(note this is a 2nd gear synchro hub) (915.302.242.00)

Synchro hub (dog teeth) for 2nd gear xxx.302.242.03? 1
(1977 and later style)



Option 4, 1st/2nd Internal Transmission Wear Parts; this option is the upgrade to an asymmetrical first gear synchronizer and a 1977 and later 2nd gear synchronizer. Note also, that this requires the use a 1st/2nd asymmetrical slider.


Part Name Porsche Part No. Qty

Slider for 1st/2nd gears 930.302.411.01? 1
(asymmetrical) (or 930.302.411.02?)

Synchronizer ring for 1st gear 915.302.301.06 1

Synchronizer ring for 2nd gear 915.302.301.04^ 1

Brake band, double for 1st gear 928.302.319.08 1

Block, anchor for 1st gear 928.302.321.00 1

Block, thrust (energizer) for 1st gear 915.302.315.01 1

Brake band, double for 2nd gear 928.302.318.02 1

Block, anchor for 2nd gear 928.302.322.00 1

Block, thrust (energizer) for 2nd gear 915.302.315.01 1

Synchro hub (dog teeth) for 1st gear xxx.302.242.014? 1
(asymmetrical)

Synchro hub (dog teeth) for 2nd gear xxx.302.242.03? 1
(1977 and later style)



Some other useful part nos.:

Gasket and seal kit (also contained o-ring and input shaft seal for removable guide tube type bell housings and seal for transmission oil pump flange) 915.300.911.01

Pinion shaft flange nut 915.302.282.00

Shaft retaining plate lock washer (10 req'd) 900.142.002.01

Input shaft seal (late 1973) 999.113.327.40
or Elring 750.859

Final drive seal (at CV joint flange shaft), 45x68x10 999.113.183.40
Or Elring 089.567


Whew! Jim

[orbmedia]

I wish I knew what the heck you were talking about.

[Doug Zielke]

Jim,
L-o-n-g, but definitely not boring.
I've printed it out for my "scrapbook" of useful info.
Thanks for taking the time to post!

[Peterfrans]

nice when people take the time to post such a complete reference. 13 years (!!) later it is still very helpful.

[Tremelune]

I had no idea.

Is this information still current?

I don't ever want to take this thing apart...

[Peterfrans]

Symmetrical synchro hub is available again, but non oem. It distorted the bore of the first gear after being pressed on, so imho the options presented are still valid

[kevingross]

Nice post, thanks for the info!