Sm 420 tranny girnding when downshifting

[GMC1965]

WHen in 2,3,4 and going to downshift it grinds and wont go in that gear, it needs to stop before downshifting. Anyone else ever experience this?

[padresag]

after these years you could have some wear in there causing it, synchros plus bearings..
drain the gear oil and see how many cuttings and such that you collect
ron

[maxwoof]

I have seen that you will have these problems if your clutch doesn't release fully. Check the adjustment.
Posted via Mobile Device

[GMC1965]

is it possible it may just because theres no sycros on the first 2 gears? Mebe sycro rings gone? I did not try to rev match, took it for a drive then took cab off, so adjustment will have to wait. Im only 18 and I just bought it so im not entirly sure what it "should" feel like. Thanks

[padresag]

you don't have syncro between 1st and 2nd but you do between 2 & 3 and 3 & 4. when you going up through the gears it will more than likely shift easier as everything has worn together over the years. but when you are going down in your shifting you have more than likely lifted your foot of the throttle which changes the thrust inside your trans causing the harsher d/shift
new syncros and a brg and gskt kit should put it back into good useable shape. brg and gskts may do it itself. that is why I said to drain the oil and see what comes out in the oil. that will tell you what is wearing. brass for synchros, fine gray sludge is usually brgs and chunks mean breakage.
it is 50 years old. nothing lasts forever without a bit a of work
ron

[GMC1965]

Originally Posted by padresag

you don't have syncro between 1st and 2nd but you do between 2 & 3 and 3 & 4. when you going up through the gears it will more than likely shift easier as everything has worn together over the years. but when you are going down in your shifting you have more than likely lifted your foot of the throttle which changes the thrust inside your trans causing the harsher d/shift
new syncros and a brg and gskt kit should put it back into good useable shape. brg and gskts may do it itself. that is why I said to drain the oil and see what comes out in the oil. that will tell you what is wearing. brass for synchros, fine gray sludge is usually brgs and chunks mean breakage.
it is 50 years old. nothing lasts forever without a bit a of work
ron

Thanks a lot, thats good information.

[GMC1965]

Well.... I have bad news...Very bad I would think. Let me know.. Guess I can stick my fingers in the tranny, doesent sound or look good at all. The seal is completly gone and the inside and outside is coated and baked in 1/2 inch of oil. Thought it was undercoating...

[padresag]

Originally Posted by GMC1965

Well.... I have bad news...Very bad I would think. Let me know.. Guess I can stick my fingers in the tranny, doesent sound or look good at all. The seal is completly gone and the inside and outside is coated and baked in 1/2 inch of oil. Thought it was undercoating...

that is just preservative on the outside. it is when you drain the oil from the trans and you stick your finger inside the drain hole and feel little hard objects, then you mat have a problem.
in the picture your just taking out the torn rubber dust boot
ron

[61K10]

thats the bellhousing and the clutch fork hole not the trans--------------

[GMC1965]

Thats good to hear.

[Shaky]

Here's what it looks like inside. Those little teeth that aren't the big gear teeth are your synchros. They may be worn.

[jocko]

i would agree - it's very likely the synchros that are causing this. You should need to be at a complete stop before downshifting into 1st because 1-2/-21 is not a synchronized shift in this trans (many 3-speeds were this way also), but that is all - the rest of the gear transitions should be smooth. If getting grinding in all gears during downshift (and not during upshift), it is likely the synchros. If you're getting it while upshifting also, then there may be an issue with the clutch not fully disengaging when you press the pedal, as was mentioned above (or could still be synchros in this case also - absence of the problem when upshifting just eliminates the possibility it's the clutch, that's all).

And concur - the pic you posted is nothing to be concerned about, it's just the clutch fork boot. The fact that it's torn and that dirty isn't the end of the world, but might be indicative that the clutch hasn't been changed in eons. Since you're going to have to go to the trouble of pulling the trans to get the synchros fixed, I'd definitely replace the clutch, pressure plate, and the flywheel (or resurface it if no heat cracks visible) while you have things apart.

Hope that helps.

[GMC1965]

Originally Posted by Shaky

Here's what it looks like inside. Those little teeth that aren't the big gear teeth are your synchros. They may be worn.

Thanks for the photo, Is tackeling the tranny and changing them myself to big a job?

Originally Posted by jocko

i would agree - it's very likely the synchros that are causing this. You should need to be at a complete stop before downshifting into 1st because 1-2/-21 is not a synchronized shift in this trans (many 3-speeds were this way also), but that is all - the rest of the gear transitions should be smooth. If getting grinding in all gears during downshift (and not during upshift), it is likely the synchros. If you're getting it while upshifting also, then there may be an issue with the clutch not fully disengaging when you press the pedal, as was mentioned above (or could still be synchros in this case also - absence of the problem when upshifting just eliminates the possibility it's the clutch, that's all).

And concur - the pic you posted is nothing to be concerned about, it's just the clutch fork boot. The fact that it's torn and that dirty isn't the end of the world, but might be indicative that the clutch hasn't been changed in eons. Since you're going to have to go to the trouble of pulling the trans to get the synchros fixed, I'd definitely replace the clutch, pressure plate, and the flywheel (or resurface it if no heat cracks visible) while you have things apart.

Hope that helps.

Thanks for the help, I think il go ahead and change out the synchros, The clutch seems to work great upshifting. How much would the clutch pressure plate and flywheel cost? Im kind of doing this on a low budget. Is there a way to inspect them for wear? That way Il know if it is soon or nessesary? Truck will not get driven 100k, probley 4-7k miles a year.

[Shaky]

Originally Posted by GMC1965

Thanks for the photo, Is tackeling the tranny and changing them myself to big a job?

Well, I plan on re-doing mine when I have plenty of time to do it. The Chassis Overhaul Manual will give you a pretty good idea of what is involved. I wouldn't call it a beginner project.

These guys have parts and info. I've never dealt with them, but they've been around for awhile.

http://www.novak-adapt.com/catalog/sm420_parts.htm

[64shortbox]

GMC1965,
Since you're only 18 I thought I would pass along some info I wish someone told me when I was your age Never, I mean Never down shift to slow your truck down. Your transmission is NOT a Brake. Down shifting to slow the truck loads the rods Not a good thing. Down shifting to keep the truck up in the RPM band is fine, like down shifting after making a turn etc. to get your speed back up under power is OK but down shifting to slow your vehicle down is definately a bad thing. Not to mention it's a LOT CHEAPER and easier to replace brake shoes or pads than it is to replace a clutch, pressure plate, throw out bearing and have your flywheel resurfaced.
Another thing I learned while replacing the trans in our company dump truck C50? I went through the passenger door with the engine hoist and pulled & installed the trans that way. I used my personal Harbor Freaight cherry picker to do it and it worked great. Just make sure you have enough chain to let the trans to the ground. I made a plate that bolts to the shifter hole and used an "Eye Bolt" dead center of the plate to raise & lower the trans into place. I still have the plate if anyone wants it for FREE just pay shipping. It might be a Little heavy though as I made it out of stainless steel.

[GMC1965]

Originally Posted by 64shortbox

GMC1965,
Since you're only 18 I thought I would pass along some info I wish someone told me when I was your age Never, I mean Never down shift to slow your truck down. Your transmission is NOT a Brake. Down shifting to slow the truck loads the rods Not a good thing. Down shifting to keep the truck up in the RPM band is fine, like down shifting after making a turn etc. to get your speed back up under power is OK but down shifting to slow your vehicle down is definately a bad thing. Not to mention it's a LOT CHEAPER and easier to replace brake shoes or pads than it is to replace a clutch, pressure plate, throw out bearing and have your flywheel resurfaced.
Another thing I learned while replacing the trans in our company dump truck C50? I went through the passenger door with the engine hoist and pulled & installed the trans that way. I used my personal Harbor Freaight cherry picker to do it and it worked great. Just make sure you have enough chain to let the trans to the ground. I made a plate that bolts to the shifter hole and used an "Eye Bolt" dead center of the plate to raise & lower the trans into place. I still have the plate if anyone wants it for FREE just pay shipping. It might be a Little heavy though as I made it out of stainless steel.

Iv been driving a std for years, only new ones though, Myself I bought a mazda3 hatch sport. I was told by everyone including people that race to downshift and use engines compression, not rely on it but use it to assist in slowing down, while it does raise rpms it also increases the compression. Mebe the issue of rods only happons in older trucks but my father has never owned a auto his whole life and never had a lick of trouble. He put on about 500k on about 10 diff trucks and cars. So your saying with this truck when slowing down on the highway if an accident occurs, I should only use brakes and no compression to slow it down? Im hoping not because these pump brakes sure don't stop like the mazda's.

Thanks

[jocko]

clutch kit (disk+pp+release bearing) will run around $120-$150, depending on your app. Here's the oreily's page for a 66 C10 283 - (not sure what your truck is) - you'll need to know hub diameter and overall disk diameter (10.5/11/12", etc). http://www.oreillyauto.com/site/c/se...kup&vi=1321872

Fly re-surf is approx $25-$40 bucks at your local machine shop.

[GMC1965]

Oh alright doesn't seem that bad. I was thinking it would cost me 500 or more. I asked a friend and he said its not a big job to change synchros so I guess il be doing that with him. Il get him to look to wear on anything else while im at it.

[Dino Driver]

My 1960 had an granny gear 4 speed. When I pulled it out of the truck I took the tranny hump off and dropped the tranny to ground from inside. It was way too heavy to lift so I tied a rope to it and drug it out from under the truck. I bought a full rebuild kit for it but change my mind on the tranny. I still have the kit if you are interested in rebuilding yours, it's listed in the classifieds on this board.

[powerdriver 1958]

I'm wondering how much free play you have . That is the clutch pedal travel that that doesn't do anything . If you push the clutch pedal down with your hand you can feel the free play . More than 2'' is too much .

And isn't that piece on the clutch rod on the wrong side

[GMC1965]

Originally Posted by powerdriver 1958

I'm wondering how much free play you have . That is the clutch pedal travel that that doesn't do anything . If you push the clutch pedal down with your hand you can feel the free play . More than 2'' is too much .

And isn't that piece on the clutch rod on the wrong side

I had to lol on the wrong side comment. It fell when removing the pedle and I found I laying on the ground so I put it were I thought it should go but were was no thread so I just left it. Thanks Now I know how it goes. I will check for play. Pedel is off can I still check the play? I am removing cab tomorrow. If there is play what would this affect? just the clutch pedal needing adjustment or something internal?

[powerdriver 1958]

Pedal has to be in place to check it . Adjusting the free travel is the clutch adjustment . with out it being right it would be tough to get in gear at a standing start and make downshifting tough