4 Speed Gearbox Bearings--for Benz Dr

[George Des]

Dan,

I just disassembled a 4 speed manual box. Everything inside looks good--the gears aren't missing any teeth, no chips or scores, same with the synchro rings. The bearings, however, looked fairly bad, especially the upper two on the mainshaft and the input shaft. Looks like they rusted somehow. The two uppers are SKF 6306 C3 with snap rings. The rear one is open and the front one is shielded on the inner side only. On the countershaft it looks like two FAG 6305 ETN/C3. These appear to have phenolic bearing carriages instead of metal with the closed side facing to the inside of the box. Can you tell me if these are the stock bearings for this box and if so what is the downside of using a std i.e. open or single shielded SKF or equivalent bearing on the countershaft ?

George Des

[hauser]

George, I'm sorry I don't have an answer for you but I do have a question.  Are parts for the 4 speed box available and if so are they easy to find?

1969 280sl 5 spd        1999 ML320          Gainesville,Fl.

[Benz Dr.]

These are standard bearings although you may have to order the ones with the locating rings. I usually remove the plastic covers if they have them. The trans fluid will lubricate them properly.
The smaller needle bearing ones on the shafts are normally OK and don't need to be changed. Setting the end play on the shafts is very important or it won't shift well. The input shaft can't have any end play at all - most do and that's not good.

Daniel G Caron

[Ben]

I overhauled my 4 speed and bought all the bearings locally, all were ex-stock except the ones with the locating rings which took a few days, no big deal.

As Dan says the endplay is the major thing to watch out for.

Dan, do you set the endplay at the front end

Regards,
Ben in Ireland.
'64 230SL 4sp.
'03 CLK Kompressor

[Benz Dr.]

The front and rear covers have wafer thin shims that set the end play. Most of the time you can just put everything back in and you're OK.
The input shaft is a different deal on most of them. I'm not sure but it seems like they're often apart before and people loose parts or don't bother to mark things coming apart. Most of the input shafts need about a .020'' shim to remove the end play. If you can wiggle the shaft around it's too loose. Set properly it won't move at all or very little. Somehow this affects the shifting as everything inside the box will move the .020'' if it's not tight. Even that little bit taken out will make the trans feel crisp while shifting.

There are some parts available for the trans but the gears rarely break or chip. Using gear lube will harden the gear faces and may cause them to chip - it won't work all that well on the brass syncros either.
 Trans fluid has some unique properties which allow parts to lock up solid one second and then spin freely the next. Think of how an automatic trans works.

Daniel G Caron

[Ben]

quote:Most of the input shafts need about a .020'' shim to remove the end play

Dan, .... does the circlip sit close to the gearbox casing or are there shims between the two. I seem to recall a spacer somewhere that I never sw in the parts book. i must look at my notes again.

Reason I ask is I have another box here and I plan to open it soon as I still have some minor shifting problems in my current box and I know it has something to do with the shims on the front end. I replaced all bearings and brass rings last time.

Regards,
Ben in Ireland.
'64 230SL 4sp.
'03 CLK Kompressor

[Benz Dr.]

Hi,
the shim goes between the casting and the ring clip. Keep adding shims unil there's no end play.



Daniel G Caron

[erickmarciano]

will also need to rebuilt my trany but got one question .
when shifting from 1 st to 2nd at 5000rpm it grinds would I need to replace two syncros or one?my question is in the trany is contact  syncro to syncro  or syncro to gear ?
thanks
Erick

1971 280sl
1962 VW
1954 Allstate vespa
1958 gs150 vespa
1962 gs160 vespa

[Benz Dr.]

Most gear changes are smooth and no grinding happens at all. Shifting into first is a bit different and you will often have some grinding if you're moving more than 10 MPH ( could be more or lees than this speed )
You should never have ANY grinding while up shifting and this could be too much end play on the input shaft. Grinding while shifting down can happen on any gear and could indicate worn syncro rings.

How they work:
The syncro rings in these cars are made of a brass alloy and are fairly durable. The face of the ring is made on an angle and fits on the gear much like a valve sits in a valve seat. The syncro ring has very light grooves milled into the face ( seen as a complete circle ) and the grooves are seperated by grooves milled at 90 degrees to the circular grooves. It's the grooves that make them work.

On the outer edge of the ring there are many small teeth that corrispond to the teeth on the gear and the shifter ring. The shifting fork fits into the shifting ring and this is where you get the up or down shift from one gear to the other.
When you shift the two gears may not be turning at the same speed so to ease this process the synro ring comes into play. As you move the shifting ring it grabs the syncro ring which will slip sideways a small amount until all the teeth on the synro ring, gear and shifter ring all line up. The small grooves on the synro ( remember I said they're important) allow the oil to be squeezed out and still leave enough oil to lubricate the moving parts. At high speeds the syncro rings would wear out quickly without this lubrication. Only ATF is capable of providing the type of high pressure barier lubrication required.

Why they quit working:
The teeth can get worn off or the face can wear. A simple test is to drop the ring on the gear and try and turn it. It should spin slightly then stop and yet you should be able to lift it off. The tolerances for this to happen are very small - any wear and it won't work at all. More proof as to the importance of the grooves and slots.
Another common problem is too much end play on the input shaft. When you shift the input shaft will move in or out and the syncro rings won't get full pressure and may not grab and hold.
Another problem is popping out of gear especally in high gear. This is not usuaally a syncro problem but is a wear problem. I always check the end of the input shaft where it slides into the pilot bearing at the back of the crankshaft. If there's any wear the shaft can deflect and make the trans pop out of gear. If you have a lot of input shaft end play AND it's worn at the pilot bearing end it will really pop out of gear.

I told you there's a lot to it.......

Daniel G Caron