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Minitruckmarauder

Transmission surgery - diesel kingcab 720

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I'm seeking expertise and parts to save my '81 720 diesel. Second gear has staying grinding very badly and fifth will not engage at all. I'm assuming this will require a new transmission, which has been hard to locate for a diesel. Anyone have any leads? Or experience with a project of this magnitude?

Located in Vancouver.

Thanks!

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Just get any 71B 5 speed and swap your front (diesel) case onto it. All you need is a 12mm socket, extension and a ratchet. Tube of RTV and snap ring pliers, although a thin screwdriver works just as well. Done. It's the front case that makes it fit a diesel bolt pattern and starter location.

 

FS5W71B donor vehicles...

'77-'78 280z

'79 280zx. The '80-'83 zx (non turbo) 5 speed will also work but it has an insane over drive ratio and the diesel might struggle with it.

'77-'79 620 truck

'80-'86 720 truck*

'78-'80 810 car

'81-'84 Maxima

'80-'83 S110 200sx (second generation)

'84-'86 S12 200sx (third generation)*

 

from '85 on these will have a larger 62mm counter bearing that won't fit the smaller 56mm counter bearing opening in your transmission front case. Just swap your smaller bearing on. Actually a better idea is to replace this bearing with a new one.

 

Buy far... the hardest thing will be getting the transmission out. I've pulled the front case off of one of these in under 10 min. It's a simple job.

 

 

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I know you can do this. The 71Bs are all pretty much the same, the front cases were the difference.

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Many have found it is quicker to just pull the engine/transmission than trying to drop the transmission. Far quicker actually

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I can drop and work on and then replace a 5spd transmission in my 521 work truck/chassis in 5 hours and it doesn't have a removable cross member like the 720, no way I could do that removing/replacing the diesel engine and the transmission together even in a 720, but I have done that in the 521 work truck so many times when it had them POS dogleg 5spds that I got good at it.

As mentioned, find a longshaft 5 speed out of a 720 if you can, or the Z car 5spd mentioned will work also if you cannot find anything else, then as mentioned put your front diesel case on that transmission and re-install it, make sure you use the arm/throw out bearing/collar that is on your transmission now also, also check your pilot bushing.

Do you have reverse the way it sits now?

Did it make noise while idling before it self destructed?

I wrote an article on how to make a short shaft 5 speed for a 521 a long time ago, it is in the how to section, it shows you all the steps on how to take your front case off, it is not that hard to do.

Here is a link to that thread, it's been 4 years, I actually made a short shaft 5spd for my 521 turbo diesel over 8 years ago, do not throw your front diesel case away even if you think it is ruined, you can likely save it, they are very hard to find anymore.

https://ratsun.net/topic/28473-how-to-put-together-a-shortshaft-5-speed-for-a-l20b/

 

 

 

Edited by wayno

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Thank you kindly Wayno; I do still have reverse and no it did not make a noise while idling before it self destructed. Fifth is totally unusable and second is very grindy and often spits itself out....

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What do you mean by 5th is unusable, nothing there(like it is in neutral)?

If there is nothing there then you likely destroyed the cluster gear shaft, meaning it is broken, although you could have lost the rear bearing on that shaft(all the bearing are gone) and now 5th gear is too far away from the gear it meshes with, but that should have made noise when idling in neutral.

I would pull the transmission and pull the front case off so you can see what the issue is, maybe it can be rebuilt.

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9 hours ago, Minitruckmarauder said:

Thank you kindly Wayno; I do still have reverse and no it did not make a noise while idling before it self destructed. Fifth is totally unusable and second is very grindy and often spits itself out....

 

I had the exact same thing happen with my '78 620 5 speed. Loss of 5th and within days second would not stay in gear, you could hold it in, but would jump out. No odd noises. This was before I knew much about them. Wish I still had it to know what it was. Early 5 speeds the mainshaft nut could come loose and back off destabilizing 5th. Reverse was fine it was caught between the shift fork and the adapter plats and can't go anywhere. The next gear on the mainshaft after reverse, on the front side of the adapter plate is first/second. So.... ???

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