65 M21 Muncie Synchronizer asymblies Differences?

[William H.]

Hello I am rebuilding my 65 Muncie M21
I need new Synchronizers ( the whole asymbly Hub Slider etc...)
there are 1-2 and 3-4 parts according to the parts book.
1-2 3924112 superseeded by 357238
3-4 3924113 superceeded by 357239
THey are marked with paint dots yellow on 1-2 and white on 3-4
Can anyone tell me what the differences are between the 1-3 and 3-4 parts. I do not see any and they look interchangeable
but I am sure they are not or there would not be two different part numbers????
Thanks in advance
Bill

[Joe L.]

Re: 65 M21 Muncie Synchronizer asymblies Differences?

Hello I am rebuilding my 65 Muncie M21
I need new Synchronizers ( the whole asymbly Hub Slider etc...)
there are 1-2 and 3-4 parts according to the parts book.
1-2 3924112 superseeded by 357238
3-4 3924113 superceeded by 357239
THey are marked with paint dots yellow on 1-2 and white on 3-4
Can anyone tell me what the differences are between the 1-3 and 3-4 parts. I do not see any and they look interchangeable
but I am sure they are not or there would not be two different part numbers????
Thanks in advance
Bill

Bill------


My vague recollection is that there is a very slight angularity difference between one of the surfaces of the slider. I believe this applies to the 3924112/3924113. I'm not sure if it applies to the 357238/357239.

[Domenic T.]

Re: 65 M21 Muncie Synchronizer asymblies Differences?

Yes,
one slider is not tapered as the other. If put in the wrong place will rub gears.
I found out the hard way back in the 70's.

Dom

[William H.]

Re: 65 M21 Muncie Synchronizer asymblies Differences?

SO are the hubs the same? The sliders look the same except the 3-4 sliders have a groove around the outer diameter?

[Joe L.]

Re: 65 M21 Muncie Synchronizer asymblies Differences?

SO are the hubs the same? The sliders look the same except the 3-4 sliders have a groove around the outer diameter?

William------


I believe the hubs are the same. I believe the difference in the sliders is more than the groove you mention.

[William H.]

Re: 65 M21 Muncie Synchronizer asymblies Differences?

Also Take a look at this output shaft seal. Notice it is made from some fuzzy material. I have never seen an oil seal like this. Is this original?
IF so can they be purchased some where? The appearance is noticeably differend from newer style seals. Both in the seal material and the configuration of the metal housing of the seal.
Thanks again
Bill20200318_164257.jpg20200318_164417.jpg20200318_164250.jpg

[Patrick B.]

Re: 65 M21 Muncie Synchronizer asymblies Differences?

The seals should have rubber lips not fuzzy stuff.

[Joe L.]

Re: 65 M21 Muncie Synchronizer asymblies Differences?

Also Take a look at this output shaft seal. Notice it is made from some fuzzy material. I have never seen an oil seal like this. Is this original?
IF so can they be purchased some where? The appearance is noticeably differend from newer style seals. Both in the seal material and the configuration of the metal housing of the seal.
Thanks again
Bill[ATTACH=CONFIG]97134[/ATTACH][ATTACH=CONFIG]97135[/ATTACH][ATTACH=CONFIG]97136[/ATTACH]

Bill------


This seal looks like it has an elastomer (rubber) lip on the inside and what appears to be a felt seal of some sort on the outside. The original seal for your application was GM #3848353 but it did not look like this so I don't think this one is original to your transmission. In any event, I know of no automotive seals currently being manufactured that are of this configuration.

You should use a seal equivalent to GM #3932255 which is no longer available. No matter; you can get the exact same seal under National/Federal Mogul #9613S which supplied the GM #3932255 to GM for many years. This is a better seal than any that was being used in 1965.

[Domenic T.]

Re: 65 M21 Muncie Synchronizer asymblies Differences?

Not kidding, one (taper) can be used in either position, but the one that does not have the taper WILL rub the gear, and as I remember it was 3/4 that the non taper slider was used.

Dom

[Joe L.]

Re: 65 M21 Muncie Synchronizer asymblies Differences?

There are TWO, repeat, TWO types of hubs: 63-65 and 66-74.
There are TWO, repeat, TWO types of brass synchronizer rings: 63-65 and 66-74.
The STOCK FACTORY sliders are totally, completely, 100% interchangeable 63-74.
As mentioned, many of the early Muncies had a slider (1-2) that had a bevel on one side and the other slider (3-4) was straight, flat, whatever you want to call it.
Also, SOME sliders, had a slight recessed machined area (back cut) on the splines on one side (I have never been able to determine which versions had this).

The first picture compares a 63-65 and 66-74 hubs. Notice the 63-65 hub is thicker. This is because the 63-65 brass syncro ring (2nd picture) does not have the added shoulder that is on the 66-74 brass ring.
THE 63-65 HUBS AND BRASS RINGS MUST BE USED TOGETHER!!! SAME FOR 66-74 HUBS AND BRASS RINGS!!!
A 63-65 Muncie can be upgraded during a rebuild by replacing the thicker hubs with the thinner 66-74 hubs, and the no-shoulder 63-65 brass rings can be replaced with 66-74 style brass rings with the shoulder.
The third picture compares a slider with the taper on one side with a slider which is thicker and flat in the same area. The slider with the thicker, flat side was on the 3-4 side. As I mentioned, NOT ALL Muncies had 3-4 sliders with this flat side. I have NEVER, repeat, NEVER seen a replacement slider with the flat side. They have all been tapered on that side.
The last picture shows the recessed machined areas (back cuts as I call them) on one side of the splines. As I have observed over the years, this was ONLY on the 3rd gear side of the slider. I have never seen this back cut on a 1-2 slider. This back cut area is very similar to the back cut that is on aftermarket replacement Tork Loc sliders----------------which I install in MOSt Muncie rebuilds. The purpose of that back cut on Tork Loc sliders to to pull the slider tighter onto the engagement teeth of the gear.

Tom------


The 1963-65 synchro assemblies, GM #3831733 (1-2) and 3831737 (3-4) were discontinued in, respectively, July, 1966 and December 1965. They were BOTH replaced for SERVICE by synchronizer assembly GM #3887887. GM explicitly stated that this synchronizer could be used for either 1-2 or 3-4. This must not have worked out too well as the 3887887 was discontinued without supercession in December, 1967. However, supposedly, the 3887887 was used for 1966 Muncies and, possibly, early 1967 Muncies.

In 1967 synchronizer assemblies GM #3914115 (1-2) and 3914116 (3-4) were released for SERVICE for all 1966+ Muncies as well as SERVICE for 1963-65 Muncies.

The GM #3914115 was discontinued from SERVICE in January 1970 and replaced by GM #3924112. The GM #3914116 was discontinued from SERVICE and replaced by GM #3924113. Most likely, the 3924112 and 3924113 were used in PRODUCTION for 1968 or 1969 through 1974 Muncies.

The GM #3924112 was discontinued in September, 1974 and replaced by GM #357238. The latter was a SERVICE-only assembly and was discontinued without supercession in May, 1995. Likewise, the GM #3924113 was discontinued in February, 1975 and replaced by SERVICE-only assembly GM #357239. It was discontinued without supercession in August, 1995.

The 3914115, 3914116, 3924112, 3924113, 357238, and 357239 synchronizer ASSEMBLIES will SERVICE all 1963-74 Muncie 4 speed transmissions. All of these assemblies use the 1966-74 blocking rings. However, if original 1963-65 synchronizers require blocking ring replacement, the 1963-65 blocking rings must be used.

[Tom P.]

Re: 65 M21 Muncie Synchronizer asymblies Differences?

That's correct.

[Tom P.]

Re: 65 M21 Muncie Synchronizer asymblies Differences?

Sorry I mentioned it.

[Joe L.]

Re: 65 M21 Muncie Synchronizer asymblies Differences?

Sorry I mentioned it.

Tom------


Why do you say that? I was simply providing some additional information and certainly not correcting you in any way. I considered that the information I provided would be useful to folks looking for NOS synchronizers as well as just general information.

[William H.]

Re: 65 M21 Muncie Synchronizer asymblies Differences?

Hi Tom
DId you delete your post?
THe pictures and information were very helpfull!!
I copied the pictures to my PC but would like to have the post too? I can no longer see it?

[Ray K.]

Re: 65 M21 Muncie Synchronizer asymblies Differences?

The brass synchro rings #3831742 that were used in the 1963 - 65 Muncie transmission were prone to cracks and breakage in the slots cut for the shifter keys. As mentioned above the 1966 design syncro ring #3880850 had shoulder which generally prevented this condition. If a new synchro assembly was purchased ( except 3831733 & 737 ) it contained the later design brass ring which was a good upgrade for the earlier transmissions.

Ray