'66 1500 slipping out of second gear
 

Looking at past threads, I can only assume I have a bad synchro. Just how tragic is this, and what is the difficulty rating between I got this and oh s#%* what have I done. Fairly well experienced with fixin' and rebuildin', just haven't had the mispleasure of having to break one of these trannys open. Any advice, warning, or cautionary tales would be greatly appreciated. Thanks, Dastardly Dave

Re: '66 1500 slipping out of second gear
 

Greetings Dave!

I take it your tranny is a SM318 three speed. This is a very simple transmission, and repairs are pretty straight forward. Anyone with a degree of mechanical acumen can fix one of these transmissions. The SM420 four speed is just as easy to fix except for the fact that it is bulkier, heavier, and more clumsy to work with. And to answer your question, your diagnosis of a bad snychro would be pretty accurate. It's not really tragic, it's just an annoyance of popping out of second gear.

Re: '66 1500 slipping out of second gear
 

if your trans is jumping out of gear on a load, it means the gear is worn not the synchronizer. also shift fork could be worn, not fully engaging gear. synchronizer is used to engage gear, generaly grinds when shifting, or is hard to engage into gear.

Re: '66 1500 slipping out of second gear
 

Isn't the Sm318 (three speed) non-synchro? Like, there's no synchro at all.

Re: '66 1500 slipping out of second gear
 

You would be correct on the worn gear and shift fork diagnosis. I misspoke. While he's at replacing worn parts, it certainly would do no harm to replace the synchronizer as well.

Re: '66 1500 slipping out of second gear
 

Greetings, Fetch!

The SM318 is non synchro in first gear only. Second and third gears are synchronized. While I am right here, Merry Christmas and Happy New Year to you and your Family!

Re: '66 1500 slipping out of second gear
 

Thanks George! And Merry Christmas and Happy New Year to you too!
:rowdybeer::rowdybeer::rowdybeer::rowdybeer:

Re: '66 1500 slipping out of second gear
 

merry christmas

Re: '66 1500 slipping out of second gear
 

I'm rather new to the forum but have spent a lot of time reading posts on here and totally impressed with the level of knowledge here, for most things either so.eone has been through it or knows someone. Not sure on exact numbers on tyranny although it is a 4 speed, or 3 discounting 2nd. It goes I to gear just fine, tends to pop out under load. I agree with the worn gear and changing the synchros while there...thanks for enlightening.

Dave
Medford, Or.

Re: '66 1500 slipping out of second gear
 

Lets get back to first base - is your shifter lever on the steering column or does it come out of the floor? A 3 speed is a whole different animal that a 4 speed when it comes to repairing. So is it a 3 speed column speed or 4 speed floor shift transmission - then we can begin to help you.

Re: '66 1500 slipping out of second gear
 

Greetings Dave!

Since it is a 4 SPEED, (shifter on floor) it would be the SM420.

Re: '66 1500 slipping out of second gear
 

Hi, sorry for any confusion, it is indeed a 4 sp out of the floor. I will have the exact nomenclature off the trans tomorrow. I may just be suffering from cranial rectimitus, but it seems there shouldn't be that much difference between a 3 and 4 sp as there is only one more gear added, however, as I stated earlier, I haven't had the pleasure of rebuilding a transmission yet. And not really lookin forward to it either. Thank you for your time and advice, I greatly appreciate it.

Re: '66 1500 slipping out of second gear
 

Thanks George from all that I have found that would be my guess as well. That's a good starting point..
Dave

Re: '66 1500 slipping out of second gear
 

Greetings Dave!

There is a big difference between the 3 and 4 speed transmissions. For the 3 speed, the transmission can be removed from the truck from underneath the truck without disturbing anything in the cab. Removal of the 4 speed will require the removal of the floor tunnel from the inside of the cab, as well as either removing the shift lever, or the entire top cover/shift fork assembly so that the transmission can be removed from underneath the truck using a transmission jack. The 4 speed is much bulkier and heavier than the 3 speed which can be removed with or without a transmission jack. I have personally removed the SM318 3 speed laying on my back underneath the vehicle more than once. I had a 1963 Chevy Biscayne (car) with the SM318 that jumped out of 3rd (high) gear, so I know that tranny can be removed and reinstalled without using a transmission jack. The SM420 truck 4 speed is an entirely different animal, and will require the use of a transmission jack, unless you are built like Hulk Hogan.

Re: '66 1500 slipping out of second gear
 

Merry Christmas folks, thank you for the I formation, I even know what a sm318 is now lol. I do believe it is a worn gear. It goes into gear just fine with no grinding, and if I baby it, it may stay in second but will feel the spot that is worn. Under load it has no chance of staying in gear. Brings me to my next question...where might I find a second gear for a sm420 4 sp, and any idea what it will set me back? So far I've been blessed and very fortunate in costs for parts. While not original the 350 v8 previous owner put in is in great shape. The only issue needing immediate attention other than tyranny is the heater fan screams like a banshee when first turned on. Took it out cleaned and oiled/wd40'ed and put back in...maybe time for a new one... thanks again for I fo and help.
Dave

Re: '66 1500 slipping out of second gear
 

Greetings, Dave!

You may very well be able to obtain the part(s) that you need from The Filling Station, LLC
E-mail: fssales@fillingstation.com They specialize in many older model GM Parts. Their phone numbers are;

800-841-6622 (direct line)
and
541-258-2114 (tech line)

Give them a call, and keep us posted. Don't forget to tell them that you need parts for a SM420 transmission!!

Re: '66 1500 slipping out of second gear
 

[Due to holidays and other pressing matters it's been a minute since I've been able to get back to my 1500. I'm having trouble finding a 2nd gear for the sm420 that is in it. Anyone have any suggestions? I guess I made the mistake of starting a new thread to ask the ? since the thread was deleted...oops! Thanks for the help I have recieved here. There is an unlimited amount of knowledge here.

Re: '66 1500 slipping out of second gear
 

Well I guess it would help if I checked up on posts huh? Thanks George for the info I'm on it and will let ya know...

Re: '66 1500 slipping out of second gear
 

Good morning, I think, folks. The filling station was no help on this one George, however, they can help on other needs. Having searched searched far and wide and the internet too, I have learned a couple of things. 1 they no longer make parts for the sm420, I know big surprise there and 2 if ya see the part ya better get it quick. I ha e found a couple of used 2nd gears which I was trying to stay away from but have no choice but to get one. My question is this, there is some wear on some of the teeth is it possible to do so e tack welding and grind back to shape? Without causing any weakness at this spots? I've got the skills to do it just can't remember if it will cause a weak spot if I do that. Anybody else try this?

Re: '66 1500 slipping out of second gear
 

Could u have one cut at a machine shop?

Re: '66 1500 slipping out of second gear
 

i would suggest finding an older junkyard to get a used trans for parts. there are still a few mom and pops style junk yards out there. those trans are still out there, but it will take some looking. i have a used one i would sell, but i am quite a distance from you.

Re: '66 1500 slipping out of second gear
 

Knotts, I don't know what the expense would be of having one cut at a shop. I was a machinist in a former life lol I know it could be done especially with today's cnc's, I used the good old manual lathes and mills. Ilvracn, that is an option but depending how far away ya are the shipping costs would probably negate the feasibility. What I've found isn't bad and a couple of light tack welds and reshaping would be a breeze. Since I wouldn't ha e to do any heavy welds it shouldn't heat up enough to cause a weak spot, thinking out loud here, which is painful at times lol...

Re: '66 1500 slipping out of second gear
 

i agree on shipping costs. i'm from atlantic iowa, south west corner, between omaha and des moines. i see you are from oregon, trans would have to be shipped on a freight truck.

Re: '66 1500 slipping out of second gear
 

OK. Let's try this again. I have located a complete SM420 transmission that is marked as being a good transmission. Unfortunately, it is located a long way away from you. Used internal parts (as well as new ones) are next to impossible to come by. The transmission is located in New London Wisconsin at Highway Truck Parts, E9517 Crain Road, New London WI 54961-8520. Phone number is 920-982-3960, ask for Blair. The price of the complete Transmission is $300.00. I don't know how much you are willing to spend, but this is another source of parts for GMC Trucks. Now, another thing that is worth noting is that Highway Truck Parts has a selection of used parts for the GMC Big Block V-6 engines ranging from manifolds, crankshafts, cylinder heads, and other parts, but for anyone needing used engine parts, or other parts, I would suggest giving Blair a call at 920-982-3960. Highway Truck Parts is another source for Big Block parts that are getting increasingly harder to find. I hope this information helps you, or anyone else interested in finding parts that they may be looking for, and have been unable to find elsewhere.

Re: '66 1500 slipping out of second gear
 

Thanks George for the heads up. Sounds like a decent deal if I have to go that way. I haven't given up finding one locally yet. I just haven't turned over the right rock...

Re: '66 1500 slipping out of second gear
 

The only drawback to obtaining parts so far away from where you live is the dad gum shipping charges! The price of the Transmission is reasonable, but the shipping charges are killers! Still, Highway Truck Parts is another source of hard to find Big Block V-6 parts. Good luck, and let us know what you find, and where you found it. We need all of these sources to keep those GMCs trucking!