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dead 620 tranny


scooter

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Hey guys, I think I nuked the tranny in my truck, it randomly does this howling/grinding/vibrating thing at random and at load. It seems like if I ram it back and fourth between gears it goes away. Anyways I'm going to put the dogleg from my 200 sx in it as I hhave a 71c for it. I'm 99 percent sure it's a bolt in but wanted to see from you guys if I have anything to watch out for. It allready has the motor and clutch in it from the 200 lol

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Transmissions don't usually vibrate when in trouble. Check the oil but I think it more likely the vibration is from the two piece driveshaft. Where they join in the middle is a rubber doughnut that will rot away and the driveshaft will be unsupported and flop around and vibrate under load.

 

620driveshaft003Large.jpg

 

Another cause of random vibration and noise are the U joints. There are three of them to check. Back of the transmission, just to the rear of the rubber ring and at the front of the differential.  ANY looseness (other than just turning) will allow the spinning driveshaft to move off center. It's heavy and precisely balanced and this will throw it off.

 

These two are much easier to check and then fix than replacing the transmission... which may turn out to not be the cause.

 

 

 

The S10 200zx and the 620 transmission up to Sept. '73 were the same length at 26". Yes it will fit without having to worry about the driveshaft length. It will bolt to the engine for sure and the clutch is not a problem either but the rear transmission mount is. This same swap on a 521 truck with the same 4 to 5 speed needs the transmission mounting point to the crossmember to be moved to the rear slightly.

 

Keep in mind this mount is nothing like the 620 and is only to roughly show that it has to be moved to the rear slightly.

 

Rear of truck.......................................................................................................................front of truck

F4W63toFS5W71Btransmount.jpg

 

 

Shortened

 

Front.................................................................................................................rear

doglegmountfor521001.jpg

 

You may be able to make a flat plate with bolt holes further back for the transmission and bolt it to the crossmember using the original bolt holes.  It is important that the transmission output to the driveshaft sit at the same height as it is now.  

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  • 2 weeks later...

That you are describing sounds like your input shaft bearing is going bad for that matter all your bearing. I just rebuilt one with the same symptoms I will try to post pictures of them latter.

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Well it turned out the output shaft bushing must be nuked. I pulled the yoke out and the yoke was purple! And it was all chewed up where it rides on the bushing. I have a gently used yoke to put in and ive changed all the u joints as well as they were kinda sad too. I'm looking into trying to get a new tailshaft bushing so all is well in the world when I put the new yoke in. Its allready better with new u joints but it still does random shit as the old yokes still in it.

 

This happened originally from returning to bc from calgary at 80+mph speeds. That rear bushing must have dried up.

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The stock 620 4 speed through '73 and the '74 and on 71B has a replaceable bushing in the tail but later 71B 5 speeds do not.... tails are just machined to fit the mainshaft. 

 

The part is 32132-B3000 about $10 but if that burned, probably best to get a later 71B 4 or 5 speed.

 

Never seen one failed. Bad U joint would do it, rear seal would fail next and trans would leak oil and run the bushing dry.

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