Transmission blew up

[Junkrat55]

So, my transmission went out in my '85 KC 4x4. Seeing as 4th is the only gear that will work without the truck clunking and lurching, guessing the bearing on the countershaft fell apart. I've checked around the local junkyards here and none of them have anything from a 720. Even asked about anything out of a 240 or a 300 and no dice. I can only take them at their word since they don't actually let anyone in the yard to look.

 

I may be able to come up with a transmission out of a D21, though. Are those transmissions swappable? Is it just a case of reusing the front half of the case from the 720? I know they're "the same transmission," b VS c model, but I want to make sure it's doable without a bunch of fab work needed. If the D21 will work, that might be the best since it's got better gear ratio for running on the highway I believe.

 

Need to get it fixed ASAP with winter coming as I'm rural and we're not exactly a priority for the plows.

 

Thanks in advance.

[datzenmike]

The D21 Hardbody 2wd has the exact same bolt pattern as your 720 5 speed so it will position itself correctly. Your transmission, (71B) could be 31.5" or 26" long. The Hardbody transmission (71C) difference is the length (slightly shorter at 30"?) and possibly the rear mount lotion.

 

The 71C is a much stronger transmission.

 

Keep and use YOUR release bearing collar on the 71C clutch arm. They should be the same but why worry?

[Junkrat55]

Thanks, Mike. Sounds like I'll probably have to get some drive shaft modification done. I can probably make something work for the rear mounts. Guessing I'm going to need a longer speedo cable? Any guess if the 71C will affect speedo accuracy or if I can fix that? Do you know if it will change the position of the shifter?

 

I'll have time in about two weeks to tear into it. Just trying to think ahead on what I need to be prepared to deal with.

[wayno]

Doesn't the speedo connect to the transfer case, you will need a custom driveline between the transmission and transfer case though.

[datzenmike]

Yes the speedo is on the transfer case so the primary driveshaft can easily be removed and lengthened. They are only a little over a foot long

[bottomwatcher]

I have been running one 15 years in my 720,much stronger. I did have to make a simple adapter out of 1/2 plate to bolt the mount to the crossmember. Then made 2 half inch spacers to go between the Frame and crossmember to drop it back down so the primary driveshaft is at the stock angle. The primary driveshaft also needed to be shortened. Trim the hole for the shifter and you are done.

[datzenmike]

 

33.25"

 

 

You may have had the 240sx 5 speed. Same one as the D21 just longer thus a shorter driveshaft needed. Pretty sure the D21 5 speed is around 30"

[bottomwatcher]

I know mine came from a 1988 d21. You might be right on lengthening the shaft. It was a long time ago, but I do remember the bill was around 100 for driveshaft modification. All in all not a major effort but worth it to get the b series out of there.

[captain720]

I know mine came from a 1988 d21. You might be right on lengthening the shaft. It was a long time ago, but I do remember the bill was around 100 for driveshaft modification. All in all not a major effort but worth it to get the b series out of there.

 Exactly, the 4wd trannies blow all the time. Mine wasted itself at 240 xxx, but I was lucky and Ted had a rebuilt one.

[Junkrat55]

Finally got the chance to tear into my truck this weekend. Found this when I got the rear bracket pulled off.

https://imgur.com/a/4vde4

 

Knew it was going to be ugly, but didn't expect it to punch a hole through the rear housing. Guess that's what happens when you run it 20 miles to get home after it blows. Didn't really have a choice though. Thank God for 4th gear.

[datzenmike]

 

Whats the red cloth stuffed into? or is it?

 

 

 

 

This is the staked lock nut that holds the 5th counter gear. Bottom of trans is to the left, top to right. Main shaft is in the foreground. The counter shaft has three bearings, one ahead of the nut, one in the adapter plate and one at the front. For it to hit the case the counter shaft must be very damaged and unstable.

 

[Junkrat55]

Red cloth is just a shop rag stuffed in the drain hole since it was still trying to dribble oil on me when I was pulling the trans out.

https://imgur.com/a/99CB8

 

Countershaft bearing destructed pretty much like I expected

 

https://imgur.com/a/PjMhk

 

Countershaft is completely toast. 5th gear completely sheared off the shaft. Can't say I've seen something like that happen before.

 

Ps. Stupid question, but how do you link pictures instead of hotlinks?

[datzenmike]

 
 
YES! Pulled a transmission once. Ball bearing came out with the oil. Front counter bearing failed and without front support the counter shaft bends under load and sheared off right at the adapter plate like yours.
 

 

.

[G-Duax]

Guess that's what happens when you run it 20 miles to get home after it blows. Didn't really have a choice though.

 

What was more expensive, the damage caused after you decided to drive it home, or a tow bill for 20 miles?

[wayno]

What likely happened is a bearing got between the gears and it pushed the counter shaft to far down and it broke, and when it does this the whole truck shakes when it breaks, I know this because I just had to get the 5 blocks home also and it destroyed the transmission, but I had another one to replace it, the next day I was on the road.

What does your front case look like(bearing holes in the front case), did it screw them up also, front bearing cover?

[datzenmike]

 

Yeah this is most likely. The front counter bearing goes and the support is lost. Side loading spreads the counter from the main above it and the counter tries to flex at the adapter plate, eventually the metal fatigues and fractures or takes out the bearing in the adapter plate or both.

 

Might be me but the bearing cage on the mainshaft looks like it's come apart also.

 

 

 

 

These don't 'let go' without prior warning. Usually a growling in every gear except 4th. It's fixable but if left it only progresses. Visually checking for leaks a couple of times a year or just watching for drips on the ground, (specially the rear seal) checking the oil level once a year and change it every 30K is good insurance. The trouble with old trucks is all the owners before you.   

[wayno]

In that photo it appears to be in two different gears at the same time.

I suppose one could cut the bottom of the case off and drive pretty far in 4th gear without gear oil, who needs gear oil when it is basically a drive shaft inside, don't even really need a cluster gear shaft, but the bearings will eventually go dry and start making noise on the main shaft.

Usually this transmissions start making noise before they self destruct, I figured I could make it 5 blocks, and I did make it, but the transmission didn't.

[Junkrat55]

What was more expensive, the damage caused after you decided to drive it home, or a tow bill for 20 miles?

You mean after the tow would have needed to drive 50+ miles to get to me? I live remote and I wasn't anywhere where I could call a tow. I was a few miles from even having cell service or other another person, and I wasn't going to walk an hour or more in the dark hoping someone might drive by or get to someone's house. If it drove, I was going to drive it as far as I could. It would have needed a rebuild regardless, which between a tow and the shop rate, it'd be easiest to put a new transmission in. Now I have an opportunity to put a better, stronger transmission in.

 

It was worth more to get home rather than turn it into a 4 hour ordeal in the middle of the night.

[Junkrat55]

 

Might be me but the bearing cage on the mainshaft looks like it's come apart also.

 

 

 

 

These don't 'let go' without prior warning. Usually a growling in every gear except 4th. It's fixable but if left it only progresses. Visually checking for leaks a couple of times a year or just watching for drips on the ground, (specially the rear seal) checking the oil level once a year and change it every 30K is good insurance. The trouble with old trucks is all the owners before you.

 

I think it's just the picture. Bearing on the primary was still "good"

 

I wish I would have had more warning than I got. It started growling only a few miles before it actually went with a bang and a shudder. Just long enough to know what I was hearing.

 

In that photo it appears to be in two different gears at the same time.

I suppose one could cut the bottom of the case off and drive pretty far in 4th gear without gear oil, who needs gear oil when it is basically a drive shaft inside, don't even really need a cluster gear shaft, but the bearings will eventually go dry and start making noise on the main shaft.

Usually this transmissions start making noise before they self destruct, I figured I could make it 5 blocks, and I did make it, but the transmission didn't.

Yeah, thank God 4th is direct drive. Glad to hear you got your truck home and fixed so quickly. Wish I had a spare transmission laying around.

[datzenmike]

Rebuild? 

 

[bottomwatcher]

Don't rebuild. Get the c and Don't look back. A lot more plentiful anyways.