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SM465 - second gear synchro

JoeliusZ28

1/2 ton status
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Utica MI (Metro Detroit)
Hey all,

I'm thinking my 465 might be due for a rebuild but figured id run the symptoms past the forum before I jump to conclusions. When i bought my truck 2 years ago, everything seemed to work well but I had a different clutch at the time. I recall it being a little difficult to get the truck into second gear at speed which I understand is normal for these trucks, and I learned to compensate for that by double clutching.

However things changed last year when I added a new clutch. In another thread on this site we had theorized that the throwout bearing might be sized incorrectly as I have to have the clutch pedal all the way to the floor to fully disengage the clutch and this is with the best possible adjustment of the linkage. Its been my intent to take it back apart and remedy this but I just haven't gotten around to doing it yet. Since then I've been driving it and just putting the pedal all the way in for shifting however over the last year I'm noticing some worsening symptoms. It seems to be increasingly difficult to get the truck in second gear, even at a complete stop. For that reason, I typically start out in whatever gear the truck lets me into first but the second gear synchro is always reluctant to let me in unless im doing all of the rev match work for it. With 3.08 gears starting in low has never seemed impractical, and now im wondering if ive finished off an aged 2nd gear synchro by taking off in low fairly often on the street. When I go to shift from low to second, the shifter has started to occasionally grind/shake going into second if I dont get the RPMs just right which doesnt seem normal for a synchronized 2nd gear. It is by far the worst when the truck is cold. Yesterday before leaving my driveway the transmission didnt even want to spin down at all (clutch all a the way in) and thus creating a situation where i basically have to force it into gear. Yet once its warm it wont do this and it'll spin down in a few seconds. This seems backwards to me as I would expect if the clutch was dragging, cold fluid in the trans would resist spinning. On top of this, when its cold it will occasionally get stuck in first/low for no apparent reason. 3rd and 4th gear are issue free. It will shift into those gears like butter at just about any speed and temp even without fully pressing the clutch pedal.

Basically want to confirm my suspicions are correct that it is time for a rebuild or if there is anything else I should be considering here. Theres about 113k miles on this trans and its an old man truck that probably never opened the 4-barrel secondaries before i bought it. Any feedback is appreciated!
 
I'd suspect the pressure plate or release bearing may be at fault ,but unfortunately the only real way to tell would be to replace the clutch and bearing and see if anything improves..
The "sticking in first gear" symptom could be internal transmission woes though too though..it may well be the syncho's and other parts have enough wear with over 100K to cause these problems..
 
I think it has definitely played a role in the development of these problems. On the contrary, if the clutch was dragging shouldnt i also have issues shifting into 3rd and 4th? It drops into those gears with no effort on the shifter whatsoever.
 
I am having the same issue with my sm465. I swapped to a manual in my blazer using the factory hydraulic setup. My trans was used. I inspected it thoroughly and everything seemed decent. Synchros looked good, bit of wear on the edge of the reverse gear. Pretty normal looking overall. I decided to install it and see how it worked. When I start off in low and then shift to 1st it grinds. Shifting to 2nd and 3rd are fine. Coming back down, 3rd to 2nd is fine but 2nd to 1st grinds. Obviously 1st to L will grind. Not synchronized. I am about to pull the trans and rebuild but your issues make me concerned about the clutch parts being correct also. How do you tell if a synchro is bad if the teeth still look decent?
 
If your clutch is disengaging well before the clutch pedal hits the floor I think you can rule that out. I think in my situation there are times where the clutch is only 98% released which is causing extra work for the synchros.

As for synchros im not the one to ask. Debating trying to rebuild this myself to learn though.
 
If your clutch is disengaging well before the clutch pedal hits the floor I think you can rule that out. I think in my situation there are times where the clutch is only 98% released which is causing extra work for the synchros.

As for synchros im not the one to ask. Debating trying to rebuild this myself to learn though.
Check out the video from GebBuilt on youtube. Just search sm465 rebuild, it shows all the steps and the stuff that is difficult or unexpected. I am going to do it to mine.
 
I watched that video about a year ago and thats whats giving me the confidence to try it. I need to give a re-watch.

Ironically the truck seems to know I'm making plans for it because now that I posted this the slow drip oil leak has now evolved into the more advanced "lets leave a small puddle everywhere we go" scenario. Apparently I'm terrible at replacing seals because the rear main and trans input seal both started leaking about 4 months after I replaced them. This thing is a needy girlfriend sometimes:rolleyes:
 
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