1981 GMC K20 4wd Driveline Problem/Issue?

[Silvy06]

First I should start by saying I'm not to sure if this is in the right thread because I'm fairly new to this forum.

My question is I just recently bought a 81 GMC K20 and I'm new to the Solid Axle truck game. So far everything seems reasonably good except the U-joints in the Steering Knuckles and I might have to find a new yoke for the front axle. Was wondering about any pointers on what else to look for in these trucks axle wise I don't know what is has under it for axles yet haven't looked for tags.

Another question is that when I put the truck into gear from Park to Reverse or from Reverse to Park and or back it makes a clunk and a slight jolt. I've searched on here and some said high idle and thats not the issue seeing as its pretty low. The U-joints for the rear axle seem pretty solid no up and down or side to side movement. But I had the gf switch it from drive to reverse for me and the rear axle makes a 1/4 to 1/2 turn when it shifts into gear.

I should also mention that its a TH350 Transmission. I'm not to familiar with em so I'm not sure if thats how they shift into gear. I know the previous owner said he replaced the modulator.

If anyone would like to point me into a good direction on what to look at its greatly appreciated.

[79Betty]

The turn is caused by the slop in the rear axle. Mine did it til i went 4 speed. The front axle SHOULD be a corporate 10 bolt. Common axle and pretty strong so long as ya don't get too wicked. New u joints in the knuckles is common. Mine needed em when i bought it. Also welcome to the sight.

[Mr.Chevy4x4]

Welcome! I would suspect the play in the rear differential is from worn spider gears. First thing I would do is pull the rear cover and do a visual inspection. Check the gears for abnormal wear and check the backlash between the pinion gear and ring gear. To do this grab the pinion/drive shaft and turn it back and fourth without causing the ring gear to turn. You should have a few thousandths play there, but not a lot. You can then jack one side up, leaving the other tire on the ground. Turn the raised tire slowly watching the spider gears and pinion gear. This should show you where your "slop" is coming from. A new set of spider gears is probably in order.

Mike

[cadillac_al]

My th350 has a shift kit in it and it always clunks when shifting into reverse and not so much going into drive. It sounds kinda bad if the idle is too high but at normal idle it isn't that bad.

My parents had a couple fords back in the late 70's and both the car and truck had a huge driveline clunk when putting them in gear; so much that I had to investigate, and those vehicles had a lot of rear axle slop even when they were new. I don't think a little rear axle slop is anything to worry about.

[dieseldawg142]

do you have a Detroit in your rear axle? quite common for them to clunk when shifting to reverse

[Silvy06]

Awesome thanks for the info.
Hopefully its just the spider gears and not something too major. I'll have to have a look possibly today sometime. Might attempt ripping the front u-joints out today we'll see what happens.

And dieseldawg142 I'm not to sure, like I said I haven't really gone over the truck to much to figure out what everything is being that I just picked it up a few days ago.

[willett]

There will always be a little bit of slop in your driveshaft because there is supposed to be lash in EVERY differential. If there is no lash you will start shedding teeth within the first few miles of driving.

The clunk is a u joint, I can almost guarantee you that if you do both rear u joints your not going to hear that noise anymore. I've been through the same bs before, couldn't feel any slop in my u joints, figured it was something else, went through everything else, eventually blew up a u joint at cruising speed on the highway,
Replaced my u joints and no more clunk.

If your truck has stock axles its probably a 10 bolt / 14 bolt ff. might be a front 44 but I think they moved to 10 bolts by then.

[Mr.Chevy4x4]

He said the pinion was turning 1/4 to 1/2 a turn when switching gears. Lash in a differential is in reference to the pinion / ring gear clearance, and yes it is required for proper gear setup. A quarter turn of the pinion gear is either BIG problems with the ring and pinion or simply worn out spider gears. My money is on the spider gears.

Mike

[willett]

I missed that part. It was still early in the morning when I read it.

[Silvy06]

Thanks again for the info. If I knew how to upload a video to make it more easier to determine I would. It only does it occasionally and sometimes it shifts in nicely. Like you said I'll have to pull the rear diff off and check and suck it up and just change every U-joint I suppose so then I know there done and over with.