5 speed shift forks

[tirediron]

can someone learn me what the common failure with shift forks/rails are in a new 5 gear? all mine are '90+ transmissions. I have 2 race cars with what I believe are shift fork issues.

1st car has no second gear. first is fine. will not go into second. all other gears are fine. seems to partially engine sometimes, but requires lots of shifter effort and im not one to force things (see second car issue)

2nd car has no first or second gear. kid that had it liked forcing things, and breaking all my shifters. when I had the car it shifted fine. he said it initially lost first gear and was hard going into second. now that I have the car back, it will not go into first or second. popped the side cover off and the selector moves over fine, but the shift fork will not budge forward or back.

I have oh, 7 or 8 transmissions that may or may not be good. what causes this type of failure? I can fix it, would just like to know what parts i'm after before I take them apart.

[Jordan]

A couple of issue come up.

But the later synchros (91+) are larger in diameter which mean the forks extend farther away and have more leverage put on them. Some of them are are also molybendium coated which can flake off and create synchronization problems. It is better to use the all-brass for racing applications I think. My experience with bent shift forks has usually been where the selector finger engages the fork. The forks are relatively soft so it is easy to bend them downward and which opens up the fork opening. I've seen the forks reinforced by adding some brazing material to the bottom OR replacing the whole upper piece with a steel sleeve and machined billet.

[tirediron]

would a bent fork cause one gear to be engaged but not the other? or if they are bent outward cause it to jam and not slide at all?

stock parts are probably fine in my application because almost zero shifting is required when under power. the transmission in my oldest race car (driven only by me) has never had a problem. it's only the wrecks I have inherited from other people that have had issues.

[Jordan]

It could cause problems getting one gear and not the other, but usually that is a problem between the hub and the muff where burrs build up and end up blocking the muff from engaging fully.

[s900t8v]

I'm confused by Jordan's post. 1991 and later trans have narrow steel shift forks which are far less prone to bending or wear an were implemented to allow or wider gears than 89 series trans. 3/4 fork is silver dipped as is 2/3 synchros with moly. The synchros are narrower and a harder composition brass which was designed to make them last longer but the synchros and mucfs suffer if abused due to profile and composition. The diameter is the same but width and tooth taper and spacing is different . The 1991 boxes had lots of synchro problems which was resolved by changing them over with 1992 and later synchro muff sets IIRC. Specifics are in service supplements. .

1990 boxes were a mess of 89 style and future 91 style parts so youd have to look to know whether you have the brass shift forks of 89 style or the steel forks of 91 style.

Anyway I'm not so convinced your problem is with the fork (although as jordan mentions muffs are a possibility in a hard worked trans... you should check for destroyed engine mount that sags. If it has failed it prevents you getting enough rotation on the selector shaft to enter 1 particular shift fork gate. This results in you trying to select 1st and 3rd or 2nd and 4th at the same time which obviously is prevented by the lockout pin aka you get no gear. A rotten selector shaft coupling can also cause these gate problems.

The easy way to isolate shifter/coupler slop or bad engine mounts as the culprit is to remove the coupler and use vice grips to move the selector manually. If you can get said gears problem is external. If not problem is internal.

[tirediron]

thanks for the info. while the motor mounts have all been destroyed and recreated, there is no shifter on it. just shifting it by grabbing the shaft with pliers. I took the cover off and can see the rod engage the fork but the fork does not move like it is jammed in place. this is the car with no first or second gear. the one with no second only I haven't gotten into yet.

[Jordan]

I've never seen a steel shift fork in a 900 trans, I have a narrow one out and double checked it with a magnet, and it is definitely not ferrous. In fact the last transmission I had apart I had the fork bend downward towards the gear. I didn't feel like trying to reinforce it at the time so I just put it in a vice and gave it a little tweak back. This was a '92 box. The sychronizer rings (syncro mesh) do have a larger diameter, but you are correct, the actual diameter of the muff is the same, they are just bored out a bit and have less meat on them.

My guess is still that your hub is jammed behind some built up burrs on the muff which is the part that the fork is trying to move. 1/2 gears share a hub as do 3/4.

I hope I clarified a little, transmissions are a lot easier to figure out once you have them apart then trying to explain the moving parts on the internet (for me at least anyway). Let me know if you need some pics, I've got lots of stuff out on my work bench.

[s900t8v]

Hmm yeah you're definitely right Jordan I looked at the 91+ series forks :s must have had a stroke haha. Sorry for the misinfo OP.

You have compared the series more closely than I have. I always thought the rings were same diameter but different taper angle and tooth spacing. Admittedly I've only rebuilt pre 88 boxes due to parts availability etc but I do have a couple of 90 series boxes in parts for storage ease that I compared a whole back .

Can't think what made me think the forks were steel. :p

[Jordan]

I wanted to do a post some time ago while I had a 87,89 and 91 box apart to show all the differences I could find. But that hasn't happened yet. I'm certainly not a transmission expert, but they really aren't all that complicated despite the mystique they are given. Hardest part it remembering the differences...and soon I'll be adding a 4-speed into the mix. I also am guilty of mis-remembering things, Haha.

[Luke]

I've noticed that the '91 boxes have syncros that tend to jam up. They are lacking a radius on the front edge and burrs form and eventually jam them up, in 92 and 93 a chamfer was added. Taking them apart and carefully chamfering them with a very fine stone on a die grinder will solve the issue.

In the pic below, under where I put the red line - notice how there is a little darker triangle, that's a shadow caused by the slight chamfer because this is a '93 muff and already had the chamfer, in a '91 box the inner surface would just be flat right out to the tip... I hope this makes senses, its sort of hard to explain on the internet, but easy to see in person...

[tirediron]

makes sense to me. quite familiar with transmissions, just not THESE transmissions.
come to think of it this car may be a '91 and not a '92 like my others....
wouldn't that make it more likely to jam in gear instead of in the neutral spot of the 1-2 fork?
ill figure it out when it comes apart. good info, thanks a bunch.

[Jordan]

They can jam in all sorts of ways