‘58 B-61 Duplex 1st gear hanging

[pdgx]

I have a ‘58 B-61 with a duplex transmission that wants to hang up in 1st gear.

I suspect it’s a worn detent/ball. 

Does anyone have a parts manual with an exploded view showing the 1st detent area ? 

Would appreciate a pic of the page(s). 

Thx

[mowerman]

You might just have dried out fork slide bar  I’m sure you’ll get all kinds of reply’s good luck

[Freightrain]

This is all the old Mack manuals show.

No other details, just specs for bearings and such.

I rebuilt my Triplex using similar picture and the instructions in the book.  I made three videos showing the process.

I didn't mess with the covers as they were not an issue.

 

[pdgx]

Thanks for that. Watched the video. 
What could be holding the first from disengaging 1st ? Didn’t see anything in the disassembly that would point to it resisting. 
When it does stick, it takes a couple of loaded clutch activations to let it release. 
Of course it always gets you when pulling into an intersection……

[Freightrain]

Good question.  It doesn't do it every time?  Only under a "heavy" load of acceleration or even if you just take off easy?

Could be a detent ball on the shift rail or maybe the sliding clutch on the main shaft.

[Mark T]

Sliding clutch, or moving the gear with it a little.  Not an easy quick fix for that sort of thing.

[mowerman]

I was thinking maybe clutch not disengaging all the way 

[Freightrain]

4 hours ago, Mark T said:

Sliding clutch, or moving the gear with it a little.  Not an easy quick fix for that sort of thing.

Oh ya, that is an entire tear down deal.

Have you had the lid off?

[pdgx]

I wish I could say it’s consistent, but it’s not.  It just gets you when you don’t expect/want it. 
On level or a down grade start, it will ‘usually’ stick..(except on the second Tuesday of the week). Aux low or high makes no difference..

If I put the aux trans in neutral first, it will release. But then you can get stuck in double neutral on the way to 2L. (1H to Aux L will not let you out of 1 to get to 2L)

On a slight UPHILL start, it will usually release.  ??

At very low speed <5mph, it will release.

Repeated attempts to replicate with F/R, will ‘usually’ allow it to release in subsequent starts - hi or low. (Yes the trans oil is at full level, and you can travel long distance and still hang w next start, so lube should not be a problem)

Makes no difference bobtail, or loaded towing. (Obviously don’t need 1st when bobtail, but it still sticks w no load)

Just a PITA when towing loaded. 

[pdgx]

3 hours ago, Freightrain said:

Oh ya, that is an entire tear down deal.

Have you had the lid off?

No. Have not pulled or opened it as it’s my only tractor at the moment. 

[Freightrain]

Sounds like clutch is not disengaged enough and torque load is holding it.  Been there if I recall, especially if loaded on a uphill pull in first.  Have to really finesse it slowly.  I don't ever need 1st very often at all, it starts in 2nd without issue.  I'm only 30k lbs loaded.

Have you tried without the clutch and see if it allows the shift?  Likely find the same situation.  The shafts are spinning pretty fast and it won't slow down enough to releases the sliding clutch which is pretty heavy since it has reverse gear on it.  That is a lot of mass and wants to keep pushing on first gear and won't let you pull it out.  Unlike the rest which are just a small hub.  

[pdgx]

51 minutes ago, Freightrain said:

Sounds like clutch is not disengaged enough and torque load is holding it.  Been there if I recall, especially if loaded on a uphill pull in first.  Have to really finesse it slowly.  I don't ever need 1st very often at all, it starts in 2nd without issue.  I'm only 30k lbs loaded.

Have you tried without the clutch and see if it allows the shift?  Likely find the same situation.  The shafts are spinning pretty fast and it won't slow down enough to releases the sliding clutch which is pretty heavy since it has reverse gear on it.  That is a lot of mass and wants to keep pushing on first gear and won't let you pull it out.  Unlike the rest which are just a small hub.  

Clutch is fully releasing..  can hold clutch in for as long as you want, and it will not release once it sticks. 
No it will not release w/o clutch. 

[mowerman]

I think we’re trying to say your pilot shaft never stops spinning with clutch pedal to the floor… and yes that’s got to be very frustrating.. bob

[mowerman]

I had that problem with a pickup years back I think the pressure plate hadda broken spring or the disc had separated.. bob

[mowerman]

I sold it as is 

[Geoff Weeks]

Does it grind or have any trouble going IN to 1st at a stand still? I am thinking about a defective pilot bearing here.

[mowerman]

I’m still thinking this is something kind of simple

[Freightrain]

I would suspect in issue in 5th more then first with a pilot bearing.

If it was a detent, it should act up sitting still.  Can you shift it without engine running(once gears are lined up).  If it hangs up then, I would think detent.

[Geoff Weeks]

I could be wrong. Just something to consider.

My reasoning is thus:

If there is drag trying to turn the input shaft, it is going to continue to load the gears/dog clutch, making them hard to separate. 

It is by no means definite, but if it goes into 1st grinding , it would point to drag on the input preventing it from coming to a stop.

When shifting in to higher gears, both shaft are in motion the whole time (unless you are trying to put in 3 rd or 4th while standing still).

Torque gets multiplied the most in 1st gear, meaning any input drag is multiplied by the 1st gear ratio. It would see a greater force then higher gears on the sliding (dog) clutch. 

[Freightrain]

I get that reasoning.  Anything is possible.

That is why I asked if it hangs up with engine not running.

 

When I pulled my 673 nine yrs ago, the pilot bearing was junk.  Nothing left of lubrication and it was sloppy.  Wasn't noisy that I could hear but junk just the same.

[Joey Mack]

A loose set screw in the shift fork can cause this... How does reverse act ?

[mechohaulic]

On 3/7/2026 at 2:18 PM, mowerman said:

I was thinking maybe clutch not disengaging all the way 

got my vote on that

[mechohaulic]

single or double disc Mack clutch ??? what does the lever do when it hangs up??? : stick not fully move to proper position ?? lever worn jamming between two sliders ? shaft bushing worn it should happen more frequently 

[mechohaulic]

the part "it takes a couple loaded clutch activations " is the red light for me. a detent ball could care less if clutch activated rapidly. a double disc with one disc hanging would take notice. a lot depends on the movement of the shift lever during these episodes. lever move slightly after slider working ? is pressure held on the lever attempting to complete a shift ? 

[pdgx]

Sorry guys, on the road all day yesterday, will try to catch up.

There is no resistance into or out of any gear (including 1 or R) with engine off as long as gears are aligned. No issue at all with Rev when running. Everything moves smoothly as expected, til it doesn’t out of 1st.

The resistance to shifting from 1st does not feel to be a load caused by rotational gear friction… When it locks, you can repeatedly hit the shifter - hard, and it feels solid - no slippage even by applying / releasing drive load with the throttle. Feels like something physical blocking it, (except on the 2nd Tuesday of the week…..) No common action seems to cause it to release. 
Does not appear to be driveline torque, caused by a turn. Happens at any time.

‘I’m still thinking this is something kind of simple’ -
I hope you’re right. Just haven’t figured it out.

Again, it’s just a PITA, but starting in 2nd vs 1st when loaded is significant.

Thx for your input.