The 100 Forum

Hello friends,

I have a 1955 BN1 with a BN2 gearbox. While on a short drive yesterday, the car would not engage in second gear. I would begin in first, throw the clutch in, successfully, move the shifter down into the second gear position without any resistance, and then let the clutch out and find the engine to be revving idly with no gear engagement. Tried several times before limping a short distance home in first gear. By the time I got to my garage, I actually could not get the shifter out of first gear and so shut the car off. I should add that in one of my repeated attempts to engage second, I accidentally shifted it into fourth gear, which worked fine. This leads me to believe perhaps that it is not the clutch but something in the gear linkages. That and the fact that I could start out successfully in first gear. I would love to hear any thoughts to help me begin to diagnose the problem. I am more driver than mechanic. Many thanks to all.

Hello Stephan,
Yikes! That sounds very unnerving. If it were me I would start by draining the gearbox oil and look for debris. Then I would pull the tunnel and take the side plate (where the shifter sits) off for a peak inside the box. Be careful not to lose those three spings or detent balls, otherwise the removal is fairly straightforward. In my opinion, it could be a broken bronze shifter fork, stripped gears or some other damage to the side plate (basically the "shifter linkage" on this one). Hopefully it is something less serious but it seems you have nothing to lose doing these steps. Good luck! Markus

Thanks Markus. A bit daunting, but gives me some idea of where to start.

Hello Stephen,

and anything new? Good luck in any case. I am still hunting a squeak on my BN2 and praying it is NOT the gearbox!

Stephen,
Not sure if you have found your problem or not, but it is possible that the set screw holding the 1st/2nd gear shift fork to its shaft may have come loose.
You can remove the side cover from the gearbox (at least in a BT7) and check it out. If so it can be repaired right there.

Incredibly helpful, thank you. I haven’t had the time away from the office to get back into the garage but will check this as soon as I do. Thanks again.

Hey Domenic, nice photo ... and pristine looking gearbox!

Do you know what would make one hard to get into reverse? Carpet and tunnel removed, new bronze bushing under lever, plunger works fine, nothing obvious in either side plate or gears themselves, it's as if the fork isn't pushing the sliding gear back enough. The car shifts into reverse if you really mean it, but it's best to hold it there as it will pop out sometimes. Way too much force needed for my liking!

Hello Markus,
Have you removed the gearbox side cover to check for issues? Since it wants to pop out of reverse, I agree it may not be fully engaged and you would be able to determine that with the cover off.
It is strange that a spur gear (non-helical) would have a generated force high enough to make it pop out.
With the cover off and the detent balls/springs removed you can move the shift shafts by hand.
How long has it been since the oil was changed?
If you did recently was there a lot of bronze particles in it?

The difficulty in getting it into reverse would be one of four things.
1. The lining up of gear teeth on meshing gears so that it will not engage. But this happens quite a bit and drivers know that, so we just shift into first or second to re-align the gears then go into reverse. I'm sure you know that but just wanted to throw it in the mix.
2. The reverse gear has a bronze bushing on its inside diameter and that may for some reason not want to slide easily on the reverse shaft.
3. The shift rods pass through the fore and aft holes of the gearbox housing and there might be wear particles from both the gearbox and the overdrive (steel and bronze particles) that have entered this location and results in increased friction of the shaft sliding in the holes. When I rebuilt my gearbox a couple of years ago these locations were fairly dirty including the bore where the ball detent and spring reside. There isn't much radial clearance between the shaft and the bore, so any amount of debris there would affect the shaft motion to some degree.
4. The ball detent and spring may be damaged and/or the shift shaft detent groove may be damaged. you could see this with the cover off.

Hey guys, revisiting this thread. I haven't gotten around to pulling the carpet and tunnel to remove the side cover of my BN2 gearbox as recommended by Dominic above. In preparation I am trying to find a diagram of the BN2 gearbox that isn’t exploded so that I can see how the shifter connects with the rods and selector forks. All the diagrams in factory manual or on parts co websites (like moss, etc) are exploded and difficult to understand. Can anyone point me in the right direction to find a good diagram or photo of the assembled gear box with shifter connected?

Also curious if the side access panel can be removed from the gearbox without removing the shifter. It appears that it can’t, but would love to hear from you. I am far more driver than mechanic so any info is helpful.
Steve

Steve,
Yes you can remove the side cover with the shifter attached.
The picture here shows the inside of the cover. The red circled location interfaces with the bottom of the shift lever and the blue circled tang fits into the slot created by the shift forks when the gearbox is in neutral as my previous pic shows.
When you take the cover off there will be three detent springs that will likely stay in their bores, all three are the same part # so no worries there. The detent balls will have to be removed with a magnet. Unfortunately there are also two detent balls imbedded between the shift rods and they can only be removed by removing the shift rods (meaning GBX removal from car).
The three that are accessible are there to keep the shift rods in the gear that you select. The other two are there to form a locking mechanism to keep you from being able to select two gears at once.
The second pic was from a post I made about a year ago and shows how the two imbedded detent balls work.

The cover can be removed from a BT7 no problem, but I do not have experience with a 100/4 so measure beforehand.
If you do remove the cover and find sludge in the detent bores you can use brake cleaner to blast out those bores then some compressed air.
You are on a fact finding mission here. It may be one of the detents in one of the gear hubs has an issue, but at least with the cover off you can shift between all the gears to verify.

Domenic

Dominic, thank you for this. It’s got me a step closer, but can’t really get my arms around it. I think I just don’t understand fundamentally what is happening when I shift gears. I can’t envision how the shifter connects with the three rods and how it selects a gear. I thought if I could see it connected it would help.

Once you take the cover off and move the lever and see the action of the parts you will understand.

Maybe my stupidest question yet, but assuming I need to drain the gearbox before removing that side panel?

Yes, and you should drain the overdrive also. At this point the only part you would need would be the side cover gasket. You can buy one or you can make one using Fel-Pro Karropak gasket material available at O'Reilly's or AutoZone or Napa

Ok thanks again.