The Stovebolt Site Forums The Shop Area Engine Shop 4 speed problems

There is something wrong with my 4 speed transmission, but I don’t know how to figure out what to do.
When I shift from first into second, second into third, or third into fourth nothing is wrong and everything works as it should, same with downshifting except for third down to second. I have to double clutch to get it into gear. If it’s a synchro problem wouldn’t it also have trouble going from first into second? If I take the top cover off the tranny can I visually inspect the synchros, and how do I know if they are worn? What should I look for?
Is there an easy way to replace the synchro and where might I find the parts I need.
I just replaced the fluid with 80w90 hypoid gear lube, and no difference at all.

If it grinds when you try to downshift to 2nd it is a problem with the syncro. If it just won,t go in gear that is normal for those tranys. There is such a split between ratios the syncro cant spin the clutch disc. You have to slow way down or double clutch it.

okay, so it must be the synchro than. Is there a way to visualy inspect it, or even a way to fix it? Any and all suggestions are welcome!

You can,t tell much by looking inside the top. They are pretty easy to work on. Get a book & follow step by step. You wont need to pull the cluster or reverse gears if the bearings are O.K.I.m not shure about the parts but someone would have some salvage.

I'm mostly interested in the visual inspection because I have 2 other sm420's that I can swap into the truck, but I would hate to do the swap just to find out that the new tranny also has syncro problems.

Gecko,

I was having the same problems plus not being able to up shift without grinding gears before waiting for RPMs to die down. I pulled apart my trany and here's what I found.

Clutch synchro's teeth were worn almost flat (should be triangle mountain topish)

3rd speed gear synchro's teeth are fine and I didn't think I'd have to replace it until I took it out of the 3rd/4th clutch hub and found that it's rim walls where just about completely worn out.

2nd speed synchro cone had somehow shorn off one of it's energizer lugs (beats me how) (not supplied in rebuild kits ( )

Both the worn teeth and shown energizer lug can easily be inspected visually, by rotating the shafts.

However the worn rim walls on the synchro couldn't be unless you are able to move the 3rd&4th clutch sleeve off of the 3rd/4th hub and spin the hub to view the ring through the clutch key slots.

I discovered this while in the process of trying to yank the main shaft out and things were loosening up.

If you don't have a breakdown diagram here's a link to a site with one. http://members.cox.net/cglabe1/SM420/sm420.html

Also if you have 3 tranys you may be able to parts two to get one up and running (you might identify each trany as there were 3 different versions of the SM420. But some parts may still be interchangeable)

You can also get a complete kit (except for 2nd speed synchro) from Novak for about $200

Hope this helps.

Jacob