915 Transmission-1st & 2nd Gear Problems

[mbrown]

G'day Team...

I have a 915 transmission that I just replaced all of the synchro rings and all linkage bushes/bearings. It was 'crunchy' in 1st & 2nd.

Now that it is back in the car, it is difficult to shift into 1st & 2nd. For example, if I start off in 1st gear and then change to 2nd, I have to push very hard on the gear lever to actually get it to engage 2nd. There is no crunching/grinding associated with the gear change, it is just takes a lot of force to select 2nd gear.

It is the same for 1st if I am stopping at the lights. For example if I am in second, put the clutch in and stop at the lights and try to select 1st, it is very difficult and takes considerable force.

3rd, 4th & 5th gears seem OK.

Any suggestions?


Regards

Mike

[84toy]

Did you replace the shift sleeve for 1-2 as well as the dog teeth?

[Jim Sims]

Year and model?

Did you replace brake bands?

Did it "bench shift" okay?

How many shifts have you made since the overhaul? It takes some time to get things worn in to each other.

[mbrown]

Gentlemen...

I only replaced the synchro rings, nothing else. Should I have replaced other parts, or at least measured other parts?

The car is a 1980 911 SC. Don't know what the number is on the transmission but will find out tomorrow.

It bench shifted OK.

My guess is that I have shifted approximately 200 times.

My engine will be coming out within two weeks, so now is a good time to replace some parts if necessary.


Regards

Mike

[mbrown]

The transmission is a 915/61 type.


Regards

Mike

[MBEngineering]

HI Mike
where the sincro bands aftermarket or Porsche??, I have found with the miss match of Porsche/aftermarket makes for a tight shift+ the old slider is worn also, and as Jim Sims has said it will take some time the bed them in to each other, if you leave the box as it is.

regards mike

[tcar]

Try re-adjusting the shift linkage first.

Clutch adjustment also.

[356RS]

Mike, The teeth on the slider engage with the dog teeth on the gear and these are the 2 parts that really engage the gear. The synchro ring slows, stops the gear before it gets engaged. Before the rebuild, all the grinding noise when you were shifting was the slider and dog teeth trying to engage a gear when it was still rotating. All the teeth are worn because of that grinding. Sooo, the slider, and dog teeth on the gears should have been replaced, not just the synchro rings. 84toy is right on.

[boyt911sc]

Originally Posted by tcar

Try re-adjusting the shift linkage first.

Clutch adjustment also.

Mike,

Consider doing TCar's suggestion before removing the motor/trans and if that doesn't help improve the gear shifting, then go for the brake band, dog teeth, and slider remedy.

Tony

[84toy]

Clutch adjustment test......Press down clutch, count to 2 and engage reverse. If it grinds, adjust the clutch. The fact that it was grinding before rebuild would indicate the need for dog teeth/slider replacement. Rarely does anyone rebuild until those parts are beyond re-usable. And yes, they do shift rather stiff when new.

I posted a break-in sequence to quickly get the 915 to shift decent.
try this.....
915 break in

[356RS]

Paul, I love the 915 break-in procedure.

[mbrown]

Gentlemen...

Thank you for the advice.
With regards to the stiff gear selection of 1st & 2nd being a break-in thing, wouldn't this be the same for all gears since I replaced all synchros?

I bought the synchro rings from Pelican Parts, OEM.

I'll do some 'testing' this weekend.


Regards

Mike

[Jim Sims]

There is a possibility the 1st and 2nd gear synchro assemblies were assembled incorrectly - in your model of 915 the 1st gear brake band is possible to get flipped into the incorrect oreintation and still partially work. Also is everything correct with the detents? However, I suspect the 1/2 slider (operating sleeve) and 1st and 2nd gear synchro hubs (dog teeth) may just be more worn than the other gears - 1st and 2nd get more wear and if the car was "drag raced" in the past, abuse. That being said, I did one "economy 915" rebuild where only synchro rings and brake bands were replaced and the report back was the transmission shifted just fine. First and 2nd dog teeth and the 1/2 slider will deplete your wallet significantly so I would try to get another hundred more 1-2 shifts in before you drop the engine just to make sure. What gear oil are you using in the transmission?

[mbrown]

G'day Jim...

Transmission oil is Swepco 201.
If I have no luck this weekend with the testing, I'll disassemble the transmission and post some photos of the 'offending' parts. I'll also ask for dimensions and wear limits...


Regards

Mike

[mbrown]

Gentlemen...

I tried more shifting with no improvement. Clutch seems fine, no grinding when selecting reverse. I now have the gearbox apart.

I'll post some pictures of the parts. Not sure if this is useful though.

The synchros are within spec. for diameter (should be 'cause they are new). All the dog teeth on the gears look OK too.

Not sure how to screw up the assembly of the brake band, please elaborate.

The detents seem OK. 3rd/4th detent seems to be a little stiffer than 1st/2nd.

See photos.


Regards

Mike

[MBEngineering]

HI Mike
the dog's on first are cream cracked and are no use to man nor beast, 2nd is no better, the slider looks a little burnt/scuffed , did you replace the 1st/2nd slider with the sincro bands??, can you flip the 1st/2nd slider round the show the other side for 1st gear, please

regards mike

[ischmitz]

hm, even with the dog teeth rounded off a little bit that should not increase the force required to get it into gear. The scuffed slider is a different story. If the inside does not have the nice hollow profile or the teeth are ever so slightly deformed that would explain things. See here for an extreme case of damaged slider. It wouldn't go into gear at all.

Due to excessive force and not pressing the clutch the teeth on the slider got heated up so bad that they softened and got squished flat. Ouch.....

[mbrown]

Gentlemen...

I only replaced the synchros. Perhaps I need a new slider...
See pictures below. These are the only pictures I have for now. Does anyone have a photo of a new one showing new teeth?


Regards

Mike

[john walker's workshop]

the slider looks ok, but the 1st and 2nd teeth are trash. you don't see the tips all beat to hell? whenever you incompletely rebuild a syncro assembly, you stand the chance of it functioning less than 100%. new syncros, brake bands, teeth, slider, dogs, and they tend to work well. reusing old parts is not a good idea. on the other hand, you may improve shifting by working the offending gear repetedly, like 200 times or more, by getting up to say 40mph, pushing in the clutch, coasting and shifting back and forth, forcefully if needed, between neutral and the offending gear, or between the next gear and the offending gear.

[mbrown]

Gentlemen...

I don't fully know how this system works, but I don't understand how worn dog teeth would cause 1st & 2nd gear to require more force than usual to engage.

Since fitting the new synchros I have no grinding. This suggests to me that the slider is squashing the synchro against the brake band and stopping the gear so the slider and gear can mesh without grinding. Assuming I'm properly understanding how this system works.

If 1st & 2nd require more force to engage wouldn't that suggest detent problems, selector misalignment or transmission linkage problems?

It seems a little coincidental to me that both 1st and 2nd gear would have the same problem at the same time assuming that the problem is the dog teeth on 1st and 2nd gears.

Let me know your thoughts (with full explanations) so I can learn something. Thank you very much.


Regards

Mike