whats the difference between eary and late np420 trans

[davek]

i wanted to know the difference between the eary and late model np420 trans used in the m37 and m37b1. thanks dave

[w30bob]

Dave,

I copied this reply from a post to a similar question I found on this forum from a few years back. This response is from Charles, I hope he doesn't mind me reprinting it here.

regards,
bob


Easy way to tell if it is the early unit or the later NP420 is this. The gear shift stub lever on the early one can not be removed from outside the tower on the top cover. The 420 has a retainer that can be removed allowing the stub and ball assembly to come out from outside. The early will have the ball retainer pin visible from the outside of the tower on the right side only; the 420 will have the pins on both sides of the tower.

The rear main shaft bearing retainer on the early series has a handle like extension that is part of the casting that protrudes toward the left side; its purpose is to keep the reverse fork pin from backing out of the case. The 420 does not have the protruding handle or the reverse fork pin.

The oil fill / check plug is on the left side near the rear on the early case. The 420 fill plug is on the right side of the case near the front. As was mentioned, the early could possibly have the fill plug in a high or low position on the case. Most that we see are the lower configuration, which has a demple in the case directly above the fill plug. The earliest of the early series has the plug higher on the case, on those the demple will be directly below the plug.

Charles Talbert
www.mseriesrebuild.comMSeriesRebuild

[jimmy_stikx]

Is there a difference in bolt pattern though? Also, though not to try and hijack the OP's post, is there much difference between the earlies and the SM420?

[w30bob]

Errrrr...ummm.....uhhh......Jeeez Dave....I don't know. The early tranny was a New Process 88950.....but as for bolt pattern differences......I'd suggest that if Charles doesn't jump in here you PM him. Seems he's a storehouse of knowledge about this stuff.....unlike me.

regards,
bob

[Frank Irons]

Bolt pattern is the same. The later transmission is supposed to be better. Shift fork configurement and synchros are different between the two.

Frank

[w30bob]

Frank,

I read that the NP420 was synchronized in 3rd and 4th gears. Does that mean you only have to double clutch in 1st and 2nd gear?

thanks,
bob

[jim lee]

Mine is syncroed in 3rd & 4th, not 1st or 2nd.

-jim lee

[Frank Irons]

Frank,

I read that the NP420 was synchronized in 3rd and 4th gears. Does that mean you only have to double clutch in 1st and 2nd gear?

thanks,
bob

Correct.

[jimmy_stikx]

I have an early ( well assuming so) and have never had to double clutch through 1st- 3rd.

[wvcharlie]

To answer your question about bolt patterns, here is my experience. When I was planning on swapping in a 318 V8 into mine, I researched and all the sources said that the stock NP420 would bolt right up to the small block V8 housing. I talked personally to guys that had done this and saw it with my own eyes. BUT, when I went to bolt mine up, it did not match. After much research and many small block bell housing purchases of one kind or another, I came to realize that my transmission was not going to bolt up to the small block bellhousing. What has this got to do with early and late NP420 transmissions you ask? I found that there are in fact two different bolt patterns that also have matching bellhousings. Both bellhousings will bolt up to the stock six cylinder, but the trick is you have to swap transmission and bells as a unit. I have a 65 M37 but at some point in time the original transmission was replaced with an older model transmission and bellhousing. As for my swap, I swapped in an NP435 transmission that was commonly used behind small block V8s in trucks and that worked out fine. Digging up this information delayed my engine swap about a year. I hope it helps somone else.

[MSeriesRebuild]

The 420 is the late trans. Used only in the B1 trucks. The early trans, used prior to the B1 series is quite different internally. It and the 420 will interchange readily as a complete unit, but most internal parts are not interchangable. Both have synchronized 3rd and 4th gearing.

Major differences are in the top cover that contains the shift rails. The early trans has only 2 shift forks, the 420 has 3; the 3rd being for reverse. The reverse fork on the early unit works on a shaft down low in the case, so it doesn't pull out when the cover is removed as does the later style. A shaft must be removed from the case to get the fork out of the early unit. The shift lever stub shaft must be removed internally from the tower. The 420 shifter can be removed from the outside. The stub shaft ball in the 420 tower has 2 grooves that engages tower guide pins on both sides of the tower. This promotes a much tighter and longer lasting shift mechanism. The early stub shaft has the pin and groove on 1 side of the ball only. They become worn and sloppy rather quickly. This is why so many of the early trucks have a long shifter swing to the right in order to engage reverse gear. Most synchronizer components will interchange between early and later styles. The only different component as far as syncho's are concerned is the ring the 3rd and 4th fork engages. The early has a groove around the ring that the fork ear engages; the 420 has the groove in the fork ears and engage a tongue on the ring. General operation of the synchro is the same. The early has the infamous snap ring problem I have spoken of many times. Only 1 snap ring was used on the front end of the mainshaft to hold everything from 2nd gear forward on the top shaft. This causes lots of stress on the single ring; especially after things start to become worn. They tend to either break the ring or the groove on the shaft that the ring fits into. This lets everything start to drift freely fore and aft on the mainshaft. Early signs this has happened is gear clashing when shifting into 3rd. As it progresses, it starts to jump out of 3rd gear. If left unchecked, this issue will totally destroy the entire trans internals over time. The 420 has the same ring at the end, but also has a separate heavier ring that holds 2nd gear in place on the shaft. This relieves a bunch of stress from being applied against the front ring. While it is very common to see this issue in the early trans, it also happens sometimes in the 420. I'd say we see it in 85% of the early units we go into; and maybe 10% of the 420's.

Because of the improved features of the 420, it is surely the better, longer lasting unit of the 2. A totally rebuilt 420 that is built correctly will last a long time. There is no substitute for a precisely built gear box. And remember, lubricants have changed greatly since the lube order that spec'ed 90 weight in the early 50's was written. 50 weight synthetic transmission oil is the way to go. Your box will last longer, run cooler, and shift much better in the winter when using the 50 weight lube.

[Larry S]

Charles any truth to the synthetic oils breaking down the synchronize-rs

[MSeriesRebuild]

Charles any truth to the synthetic oils breaking down the synchronize-rs

None whatever. It is yellow brass that has the breakdown issue. Most high quality oil producers have ceased to use ingredients that attack the brass.

Getting back on subject, the NP transmissions have no brass in the synchros unless you have a really old one that has never been gone through. Most at rebuild time got aluminum parts installed. The early series has a bronze bushing that 3rd gear rides on; that would be the only brass related part. The NP420 does not have the bronze bushing at all.

If you pay attention to oil specs; most producers that are putting out a decent quality product will actually list whether their product is harmful to brass or not. Do your homework, there is cheap junk out there that has a synthetic title also. Higher end products like Amsoil, Royal Purple, Synergyn, and Hydrotex are the better oils.