The Stovebolt Site Forums The Shop Area Driveline SM420 Problem?

Hi all,

I have a 56 Chevy pickup with a small block and an SM420 Muncie. When I let out the clutch to start in 2nd gear, it lurches severely unless I am extremely gentle with it. The pickup got a new clutch and flywheel when I put the V8 in last year, but it acted the same with the previous 235 and previous clutch. I put new synchros in about 10 years ago but have only driven 2K miles since. Is there any reason that synthetic transmission fluid would cause this? Any other thoughts? The motor mounts are good/new. Thanks.

I use synthetic, but I don't use the clutch to shift. Synchronizers need friction to work and from what I hear, synthetic is too slick to work. If you depend on synchronizers, 90W mineral oil would probably work better. Stay away from GL 5 because it will attack your brass synchronizers.

I'm not sure what you mean by lurching, but I would think the clutch is suddenly grabbing and the synchronizers have nothing to do with this. Either you have messed up clutch linkage or a rough main drive gear (input shaft).

I would take the flywheel cover off and get under it and watch with a flashight to see what your throwout bearing does when someone steps on and releases the clutch. Make sure the motor is turned off and the wheels blocked when you do this.

Hi,

I think I will drain the synthetic and replace with regular 90 wt oil first. If that doesn't help it sounds like the transmission will need to be pulled.

I'm with crenwelge on this one. The lurching doesn't sound like a lubrication problem in the transmission. The lurching could be related to the clutch disk or something wrong with clutch disk surface. Like oil or missing clutch facing/lining and/or broken pressure plate springs, etc.

If diagnosing the throwout bearing and/or clutch linkage doesn't turn up anything, my bet is with something snafu'd in the clutch assembly.

SimS

Yep, got'em clutch problem Barry, no reason the trans would
have anything to do with that.

And on the synthetic, what Ken talks about is the same result
I encountered with my SM420 after I rebuilt it. Used synthetic
thinking newer chemistry was better, and the Synchro's
couldn't grab and hold the large massive gears well enough to
mesh properly. Drained it and refilled with the good old
meneral oil and she shifts good again.
Apparently synthetic works well in the later smaller 4, 5 and
6 speed transmissions because the gears that have to be move
are much less massive.
Contrary to that, Ken says he uses synthetic in his big rigs
so my only conclusion is that the synchronizers in the newer
transmissions in his trucks are designed to operate with the
newer chemistry.

Denny Graham
Sandwich, IL

He also says he doesn't use the clutch, so whether or not the transmission even has synchronizers is a moot point. Unless the clutch is disengaged, the synchronizer can't work at all.
Jerry

Sounds like a consensus. Puzzling though. The clutch, throwout bearing, and flywheel were all new last summer and looked fine when I pulled the engine during winter. I will still change out the synthetic and check the linkage and throw out bearing.

Happy Independence Day to all af you!

Sounds like the flywheel might have some rust on the surface.

I checked the clutch linkage and throw out bearing while my dear wife moved the clutch in and out a bunch of times, fast and slow. It works smoothly, no binding or anything. I put a bit of grease on the throw out bearing per the service manual. Changed out the synthetic lube for conventional. Checked the clutch pedal adjustment and it was still in spec. Took it for a drive and the jerkiness didn't completely go away but seems to be better. May also be that I am getting used to it. The jerkiness only happens when starting out from a dead stop. Also checked the u-joints. The front one is the tiniest bit loose, but no clanging or vibration. Could that be the problem?

Most of the time when I've run into clutch chatter it is the motor mounts. If two short pieces of chain and two turnbuckles hooked high on the engine and to the frame, one on each side, stops the chatter it is the mounts. Flathead Fords up through 1948 had anti chatter rods from the engine to the frame which if left unhooked would chatter enough to shake things loose inside the car. Some genius decided to mount the engine at the very front of the engine and very back of the transmission which is still done by some hot rod builders---a real no-no.





































































to the frame stops the chatter

Hi Coilover,

I put new side mounts in when the 350 was installed. Also, it had the same chatter with the old 235 and previous clutch. That's what led me to think about the trans instead of the clutch in the beginning.

You mentioned u-joint, how about broken leaf springs or loose axle to spring mounts?

Hi Don,

I pulled the front springs recently, cleaned, inspected and added Teflon liners. The chattering/jerking in second gear was present before and after the spring work. When I test drove it today, I tried starting out in granny 1st gear, 2nd gear, and 3rd gear. It only was jerky in 2nd gear.

We had 5 Chevy and GMC trucks on the farm back in the 70's with SM420 transmissions including my 56, and none of them had this sort of chatter at the time. I drove the 56 to high school and used to haul hay in the summer for extra money. Kinda stuck on what to do to fix it. Thanks.

I'm thinking rear springs

Hi Don,

The rear springs are new from Speedway. What should I look for?