2nd gear dog rings on 1st

[BRAIDusa]

I've read that one can use 2nd gear dog tooth ring on 1st and a 930 slider on the 915 transmission to improve 2-1 down shifts.

Is this true?

Are the down sides worth it?

Car is used for autocross and rally cross so the occasional 2-1 downshift is necessary and may have contributed to my busted up first gear.

[KTL]

Interesting. I would think if you're going to put 2nd gear dog teeth on 1st, might as well also use the 930 dog teeth and the 930 synchro assembly.

I know for certain this is possible on 3rd & 4th gear for the 915- use 930 3/4 dogs and synchro assy. The 930 synchro is retained with a much more reliable star ring "lock washer" and circlip assembly as opposed to the 915's giant flat circlip.

[Matt Monson]

Hello,

You cannot put 930 1-2 on a 915 gear. The splining is different.

Now for a bit of historical background. Up until the mid-70's BOTH the 930 and 915 used symetrical synchro teeth on 1st gear. It was the same as 2nd gear. Then both gearboxes went to the asymetrical 1st gear. 1977 or 1978 IIRC.

As for whether it is better on the downshift? That's been debated for years. Some people think so. Others say it makes no difference if you rev match properly. I don't really have an educated opinion on it because most of the cars I drive have been regeared with my gears and aren't a fair sample set. On my street driven 1986 Carrera I do NOT shift into 1st on the fly.

Lastly, the comment about a 930 1-2 slider isn't really accurate based on what I said above. 930 and 915 1-2 sliders are the same part. It's just a question of early symetrical slider or later asymetrical slider. If you have any trouble finding an early one, I stock them.

Regards,

Matt

[john walker's workshop]

you would also have to back date the brake band, stop dog and anchor block inside the syncro ring. can't say i ever noticed much difference between the two designs.

[BRAIDusa]

Great. Thanks guys.

So it's probably a "no" then.

Ordering parts today

[KTL]

I think getting into 1st on the fly is always a bear because of the gear ratio being so quick, which requires a lot of work for the synchro to spin-up the speed of the loose/free 1st gear assembly? I do it in my racecar rolling along (at slow speeds like when i'm coming off track into pit lane) by double de-clutching it. W/out double clutching there's no way it'll go into gear on the move.

If there's nothing to be gained by the swap, and your assymmetric 1st gear dog teeth are in OK condition, might as well stick with what you've got and just get a new 1-2 assymmetric 915 sleeve and synchro ring.

Be sure to check the installed dimension of your synchro. One thing that is often overlooked by those of us who service these 915s once in a blue moon, is that the synchro ring rotates back and forth on the inside of the dog teeth. The friction material of the synchro bears on the teeth as this rotation happens and can wear down the back of the teeth. Point being is that your dog ring wear is a twofold issue to look at- actual teeth condition where they engage the sleeve AND how the synchro fits into it

If you need new teeth, check with Matt. The aftermarket dog teeth maker with the initials R&S doesn't do the originally spec'd 9 degree backcut on the teeth and that's crappy.

[Matt Monson]

Kevin,
I don't do them on replacement teeth either. Unfortunately the jig we have for making the cut is set up to only work on installed teeth.

Jerry Austin has a set up to do the backcut for people.

[KTL]

Good to know. Thanks for clarifying that.