The Stovebolt Site Forums The Shop Area Engine Shop SM 420 Trans. Question

I have a noisy third gear. It howls and rattles. I suspect wear someplace and the synchro.

Any thoughts?

If 1st & 2nd are normal the 3rd speed gear is probably worn on the main shaft. Those arn't very hard to o-haul.

Not hard to o-haul? really? I took the top off mine to inspect it and the gears looked ok. But it looks like a pain to take apart and put together. Is there anything else I should look at other than gear wear?

The best way to tell about a manual trans. is to drive it. If you get excessive whining noise in all gears except high it indicates bad brgs. namely the front drive brg. or the brg. between the shafts. If the gears clash or grind when you shift to any thing but 1st. or reverse it indicates bad syncros. If it jumps out of gear it could be bad gears or thrush brgs. I realize "easy to o-haul" is a relative term. It depends on the mechanics expertise & facilities. Compared to a SM465 these are real easy.

the old chevy manual page has the overhaul instructions on it i found it to be quite simple to work on. the only tricky part
is getting the needle rollers back in the main input shaft
the book listed a spring tool for that but i found that heavy
grease held them long enough to put the shaft back in. philip

Mike, I would hit up Tisslers and get a T5. Then your truck would top out above the 55 your doing now. It sounds like you have a few bearings going.

Scott

Wrenchbender, you hit the nail on the head.It is now whining in two and three, quiet in four and a little grind here and there. So I need bearings and synchros. I just hope the case isn't worn as well.

I had this apart a few years ago when I put the truck back on the road and tearing the trans down was actually fairly simple. I didn't have any new parts then, only a few things from another trans, so I didn't really rebuild or make any improvements. The hard part is pulling it out and putting it back in. That part I really am not looking forward to. There is a Novak kit that has all of these parts.

Scott, as far as a T-5 goes, is this a direct bolt up and is it the same length as the SM 420? I would like to avoid getting the jack shaft re done again.

Does the T-5 require a rear mount? This trans just makes it into place with the original cross member.

Thanks for the advice.

Mike

Mike, with a quick google search, here is a link I found for SM420 rebuild kits
http://www.novak-adapt.com/catalog/sm420_parts.htm
http://cgi.ebay.com/ebaymotors/CHEV...intage_W0QQitemZ8068708421QQcmdZViewItem
http://www.drivetrain.com/munciesm420.html

I am sure that there are many more.

The T5 will bolt up, however is longer than the SM420 and will require a trip to get the shaft shortened. The consensus is out on the rear mount.

Thanks Scott.

I need to get on this, as soon as it stops snowing!

Mike

Mike, if you need to use my shop at my place in Wisconsin, I only ask that you shovel the walk. I can give you directions on firing up the boiler for heat, however the Air Compressor is down here with me.

Scott

Thanks Scott. I'm not sure how I will pull it off yet, but I will keep your offer in mind.

Mike

Heck Mike, your going to be building me a set of shelves in the near future, it is the least I can do, plus I would rather have a local 'bolter use it and clear out the spiders than have it sit vacant.

I have been advised against the T5 for my own 1-tonner. It is a light-duty transmission, best used in a 1/2 ton application. There are several NV 5-speed transmissions which have been suggested as readily available, adaptable and up to the tasks imposed on a 1-ton chassis.

OK, here is my plan.

I have ordered some parts for another SM420 I have and am going to put it together for a direct swap. I am going to change the replacement from a torque tube to open driveshaft configuration. The replacement appears tighter than the one I am using and was blessed as good when I got it.
We will see. I am hoping it is much better than what I have, and keep me on the road. This is my daily driver so I can't let it stay down for long.

Hotshoe, what specifically are these 5 speed you are talking about?

Well, I am not the final authority on transmissions, you understand, but I am an old trucker with some strong opinions on gearboxes. The New Venture 5-speed transmissions I am talking about are OEM in late-model GM and Dodge 3/4T and 1T trucks. There are two different ratio ranges; although both high gears are identical overdrives and both 4th are direct drive, one is a close-ratio in the lower gears and the other has a wider range, beginning with a deeper low gear. I think that is the one used in the GM trucks, but I'm not sure. My tranny guy is George at Bert's Automotive in Surrey, BC. This is their site:
www.bertsauto.com/ and they are my final authority on anything gearbox-related for anything lighter than a 5-tonner, and maybe for heavier stuff as well.

Mike, offer still stands to use my shop. I would recommend bringing a second person because I would not condone working alone.

Scott

Thanks Scott, but I have another place to go that is closer. I considered doing it in the driveway but the bottom is falling out of the temperature and more snow on the way. If all goes well it will be just pull one out and put one back. Should be done fairly quickly. This guy can also do a little welding for me while I am there.

Thanks again

Hotshoe, thanks for the tip.