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.
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[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

[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.

[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]

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.

[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

[factorystock]

If you never repaired a manual trans before, I suggest having it professionally rebuilt or look out for a good used one, ( many granny 4 spds have been swapped out for car type 4 speeds and automatics and are still found cheap in classified ads. These are very heavy transmissions, use caution when dropping them. Also, these 4 speeds were designed for slow speed work trucks, in fact the SM 420 came out in 1947, way before the interstate highway system and V8 engines. 3rd gear is spaced very wide in relation to 4 th gear, which means downshifting on a freeway will send your rpm's into orbit.

[padresag]

Originally Posted by factorystock

If you never repaired a manual trans before, I suggest having it professionally rebuilt or look out for a good used one, ( many granny 4 spds have been swapped out for car type 4 speeds and automatics and are still found cheap in classified ads. These are very heavy transmissions, use caution when dropping them. Also, these 4 speeds were designed for slow speed work trucks, in fact the SM 420 came out in 1947, way before the interstate highway system and V8 engines. 3rd gear is spaced very wide in relation to 4 th gear, which means downshifting on a freeway will send your rpm's into orbit.

\and sometimes there is only one way to learn. there is always a first time and he says that he has help. if he buggers it up then he should be able to find another reasonable and then has the new parts for it. how much can he lose?
v8s have been around for a long time
ron

[GMC1965]

Originally Posted by factorystock

If you never repaired a manual trans before, I suggest having it professionally rebuilt or look out for a good used one, ( many granny 4 spds have been swapped out for car type 4 speeds and automatics and are still found cheap in classified ads. These are very heavy transmissions, use caution when dropping them. Also, these 4 speeds were designed for slow speed work trucks, in fact the SM 420 came out in 1947, way before the interstate highway system and V8 engines. 3rd gear is spaced very wide in relation to 4 th gear, which means downshifting on a freeway will send your rpm's into orbit.

I agree downshifting to 3rd on the freeway is kinda not a good idea. I was making a point. I don't plan on takin it on the freeway much. 65mph is fast enough for me. Id just like to gasser through the first two gears and listen to the engine. Mainly will be used on 80km/h roads(like 52mph). Is this a decent tranny for that? I will attempt, even those "pros" that you think I should take it to once had to open it up and do it before. As long as its not a 10/10 on the difficult scale.

[64shortbox]

GMC1965,
I guess I should have been more specific about the NEVER, NEVER use your transmissin as a brake. In an emergency situation I would definately use my transmission as a brake. Personally I USED to down shift all the time to brake and NEVER had a problem. I read a post by a Ford engineer never to do it and he went into a very detailed explination why, I tried to find the particular post (off my Pontiac site) but was unable to locate it. Needless to say I no longer down shift to slow down. When I down shift it's to shove my foot further into the needed RPM band. I would also say Never throw an automatic transmission into reverse at 35 MPH, I did it several times and never blew up the transmission. Never do an emergrncy brake U turn at 85 MPH, I did that too and scared the holy snot out of myself, I'll definately never do that again at 85MPH, 50MPH is a kick but it sure tears up the rear control arm bushings when you do it almost daily. Never force a standard trans in to reverse while going down the freeway-it will send the reverse idler through the side of the case. But most importantly NEVER and I mean NEVER quip back to a cop when he asks "Your eyes are red what are you on?" NEVER tell him "Your eyes look glazed, have you been eating donuts" My eyes were red because I have allergies, but that little remark got me arrested for being under the influence of a controlled substance, my car towed & me spending the night in jail. My B.A.L was 0.0 and my urine test was 100% clean, all charges were dropped but it still cost me $360.00 to get my car out of hawk. Just food for thought is all.

[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.

[GMC1965]

Originally Posted by 64shortbox

GMC1965,
I guess I should have been more specific about the NEVER, NEVER use your transmissin as a brake. In an emergency situation I would definately use my transmission as a brake. Personally I USED to down shift all the time to brake and NEVER had a problem. I read a post by a Ford engineer never to do it and he went into a very detailed explination why, I tried to find the particular post (off my Pontiac site) but was unable to locate it. Needless to say I no longer down shift to slow down. When I down shift it's to shove my foot further into the needed RPM band. I would also say Never throw an automatic transmission into reverse at 35 MPH, I did it several times and never blew up the transmission. Never do an emergrncy brake U turn at 85 MPH, I did that too and scared the holy snot out of myself, I'll definately never do that again at 85MPH, 50MPH is a kick but it sure tears up the rear control arm bushings when you do it almost daily. Never force a standard trans in to reverse while going down the freeway-it will send the reverse idler through the side of the case. But most importantly NEVER and I mean NEVER quip back to a cop when he asks "Your eyes are red what are you on?" NEVER tell him "Your eyes look glazed, have you been eating donuts" My eyes were red because I have allergies, but that little remark got me arrested for being under the influence of a controlled substance, my car towed & me spending the night in jail. My B.A.L was 0.0 and my urine test was 100% clean, all charges were dropped but it still cost me $360.00 to get my car out of hawk. Just food for thought is all.

Aha, good story, Never knew some of those things were possible. See them on movies but never real life. All look more into the downshifting for sure. Is this new and old or just older trannys?

[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