1G Double synchro or Single ?

[Adrian97]

Hi!
I bought a build transmission which is Jacks drag build with HD gears, 4 spider e.t.c has not been used since build..
I send it to Jack again for WPC and micropolish after opening the gearbox Jacks says that it has a single synchro and he can do double synchro 1-2 gears for 468$ extra.

I dont know what to do is it worth to have double synchor instead of single ? is there a big difference? or with single synchro will be fine?

 

[randman2011]

I had Jack's do double synchro on my 2g trans back in 2019. I think we already have double synchro first but they can add double synchro for the other forward gears? Double synchro is absolutely not stronger. In most applications double synchros are actually weaker than a larger single but I'm uncertain how that works out in this transmission. But the only singular reason to do double synchro is for improved shift feel. Do I notice a difference in mine? Yes, but that could just be from replacing the old, worn out synchros and maybe it isn't actually better. I can't really say, but I am happy with my transmission. I got asked nearly this exact question via PM last week so I'll paste my response here.

Excerpt from PM conversation:
I've had the transmission since September 2019 but the car has only been running for about a year now. The transmission has been very smooth and the shift feeling is nice. 2nd gear is completely trouble-free, which is a first for me in a DSM. I've never had any problems with clutch engagement or high speed/high RPM shifting, and I tend to be VERY aggressive/quick with my shifts. I have two things to point out that aren't necessarily complaints but things I found odd. This transmission is surprisingly sensitive to play in the shifter. It was a little difficult to get into gear back when I had the worn out original bushings but after buying a full brass bushing kit for the shifter and the transmission it has been very nice. The second thing is first gear. Maybe a quarter of the time I try to shift into first I hit the synchro teeth head-on and it won't go into gear. I have to drop it back into second and then forward into first to get it to realign the shafts and get the splines to engage. It doesn't bother me at all because second gear is so easy and my shifter's throw is so short, but it's not great for other people who drive my car for the first time. [...] If I were going through it again and the cost was the same, I might try a different shop just to see if someone else does first gear differently, but I'm not unhappy with the transmission or what I paid for the work.

According to Fuelly, I've put exactly 4000 miles on this transmission since I got the car running in late July 2020.

My invoice was for $2707, which included a new OEM 1st gear and shipping. MSRP for the double synchro build was $2400 at the time.

 

[Adrian97]

Interesting, thanks
Will se what others thinks

 

[pauleyman]

DSMs do not have a double first synchro. As stated this does nothing for added strength for the transmission. Double synchro adds additional friction material to help shifting because, well there are two synchros. I like it as the shift that sees the most abuse is probably 1 to 2. Does a new single second shift as well? I certainly thought mine did when it was new but I would prefer the double. If a clutch is working properly (BIG IF) then synchros can do their job. If you get a grind and you know FOR SURE the clutch is ok then synchro is probably worn. I would say single would wear faster as there simply isn't as much friction surface.
At this point in my DSM life I also tear open the trans at every clutch drop just because I can. I can check the synchros inside of an hour and if needed spend a little more time toreplace just a synchro ring if I want. Not everybody wants to do that.

 

[XC92]

Not sure if this helps but my '92 Talon TSi AWD has a '94 trans because the dealer replaced the original one with it when I took it in for warranty service on the trans a couple of years in. Unlike the original trans It has a double synchro on 2nd and I haven't had problems with it, at least in shifting from 1st to 2nd. Always smooth.

Looking at the service invoice for that repair job it says that I had trouble shifting from 1st to 2nd, with grinding. But that could have been and was likely due to other issues and not just the fact that it had a single synchro. Just giving you my limited anecdotal experience and you can do with it what you will.

And per Pauley, after having rebuilt my trans this past summer, I'm increasingly leaning towards taking it apart this spring to make sure that everything's ok. I'd never rebuilt a trans before so it might be wise to make sure. I'll probably have to drop it and take it apart anyway to fix a slight leak that appears to be coming from some of the section mating surface RTV seals. I think I botched that.

 

[bastarddsm]

I've broken a lot of double synchros. Tilton twin carbon clutch that shifts like a dream, and trying to get 200ms shifts. It breaks the inner steel piece pretty easily.

I think they seem to help the shifts, but it is by no means tougher, you will break them if you are rough with it.
I think they are a powered metal part, and I think I am going to try and make a chromemoly replacement for it.

 

[Adrian97]

Thanks guys
I decided to stick with single synchro they are all new Jack's brozne synchros so i think it will be okay.

 

[jdxnc]

BTW, "HD" gears are usually just 90 gears.