4 speed transmission internals

[George Davis]

I'm rebuilding my 4 speed transmission.  Here are a few thoughts and some pics.

It's not a bad job, as long as you don't drop the tranny while removing it.  It's fairly heavy and a trans jack or a strong friend are pretty critical.  The upper bellhousing bolts are accessed from inside the car through the upper access panel in the transmission tunnel.

The trans is pretty straight-forward to disassemble, at least as far as I took it.

Trick 1 is that the countershaft (layshaft) has to be lowered in the case before the input shaft can be removed.  It's not hard, just remove the countershaft bearings, which are slip-fit, and the shaft drops.  Then remove the input shaft, mainshaft and countershaft.  All bearings are slip fit.

Trick 2 is the needle roller bearings for 2nd gear.  This is not the usual caged roller bearing, it's a split bearing and the rollers are not caged.  If one is not careful when removing 2nd gear, the bearing halves and rollers fall out and go everywhere.  To avoid this, mount the mainshaft vertically, then carefully lift off 2nd gear.  When you have room to get your fingers in there, hold the bearing halves in place and finish removing the gear.  As long as the bearing halves are held in place, the rollers can't fall out.  While holding the bearing halves in place, turn the mainshaft horizontal and take off the bearing halves.  Don't tip them or the rollers will fall out.  The other roller bearings are caged, so they're easy to handle.

I'm working on the trans because it hangs up in 3rd gear, but only after coasting.  I suspected excessive end-play on 3rd gear and believe now that I was correct.  3rd gear will have excessive end play if the slotted nut on the front end of the mainshaft comes loose, as mine was.  The lock ring that secures the nut does not actually prevent loosening of the nut, it simply limits the rotation of the nut to a few degrees if it does come loose.  But a few degrees rotation is enough to allow more end-play on 3rd gear.  I'll use loctite when the nut is retightened, in addition to the lock ring.

Replacement of the speedo shaft seal is covered in a separate post.  There is also a seal on the main shift rod, but I didn't disassemble the shift linkage in the top cover of the tranny, so this seal won't be replaced.  Since it's at the top of the trans, I'm not worried about it.

My overall (amateur, first timer) observations are that this is a simple and robust transmission.  The only fault I find is that the lock ring does not positively prevent loosening of the mainshaft nut.

Pics follow

Input shaft

Download Attachment: trans-inputshaft-sml.JPG
36.84 KB

Countershaft (layshaft)

Download Attachment: trans-countershaft-sml.JPG
45.93 KB

Mainshaft, showing slotted nut

Download Attachment: trans-mainshaft-sml.JPG
46.5 KB

Transmission case, showing reverse idler gear

Download Attachment: trans-case.JPG
67.03 KB

Trans top cover and shift forks

Download Attachment: trans-topcover.JPG
73.84 KB


George Davis
'69 280 SL Euro manual

[jeffc280sl]

Nice pics and good advice.  Thanks George.

Jeff C.
1970 280SL 4-speed

[ja17]

Hello George,

I use a standard hose clamp around the shaft to keep those pesky needle bearings in place during removal and installation of the cluster. Look for that big flat cluster key to be worn or damaged from that loose slotted nut.

Joe Alexander
Blacklick, Ohio

[Ben]

great info and phtos George. I did mine a few years ago, and have a spare now that I will do next summer.

Question........what bearings did you find in there ? I made a list but discovered that one of my shaft bearings was a roller bearing, cant recall where though. The new one was a standard type !

Also you may notice if looking at the generic exploded diagrams that the input shaft bearing is shown incorrectly, 180 degrees reversed. All the drawings I have are like this !

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

[George Davis]

Jeff, you're welcome!

Joe, thanks for the advice, I'll double check the key.  It seemed ok, but I'll look closer.

Ben, all four of the main bearings are ball bearings, as are the replacements I ordered.  I assume they are angular-contact (thrust) bearings, I'll double check that when I reassemble.  And yes, I've noticed the exploded parts drawings have the input shaft bearing backwards - an early case of cutting-and-pasting the wrong picture!


George Davis
'69 280 SL Euro manual

[George Davis]

Mainshaft assembly pics.  The first and second pics are out of order, the second pic should be first.

Download Attachment: Tranny-pics 002-text.JPG
64.54 KB


Download Attachment: Tranny-pics 003-text.JPG
74.6 KB


Download Attachment: Tranny-pics 004-text.JPG
71.15 KB


Download Attachment: Tranny-pics 005-text.JPG
59.34 KB


Download Attachment: Tranny-pics 006-text.JPG
81.22 KB


Download Attachment: Tranny-pics 007-text.JPG
70.14 KB


Download Attachment: Tranny-pics 008-text.JPG
63.1 KB


Download Attachment: Tranny-pics 009-text.JPG
61.95 KB


Download Attachment: Tranny-pics 010-text.JPG
63.48 KB


Download Attachment: Tranny-pics 001-text.JPG
67.46 KB

George Davis
'69 280 SL Euro manual

[bpossel]

Hi George,

This is great stuff!  Thanks for posting the info, along with all the nice pics!  Working on the car and taking pics at the same time is not always easy...  especially when the hands are greasy, you are deep in concentration on the job at hand... and then to take pictures on top of that...
Thanks again!
Bob

bpossel  (Memphis, TN.)
'71 280SL  /  '97 E320

[ja17]

Hello George,

Great job George, nice photos and documentation. This should be a George Davis "standard transmission tour"!

I use some wheel bearing grease to keep those needle bearings stuck in place during assembly. You can use a standard hose clamp to keep the gear cluster together snug, during the cluster installation into the gearbox. This will keep the flat key engaged in the slot of the thrust ring.

You can see how a loose slotted nut on the transmission output shaft will allow the gear cluster to come loose allowing the flat key to dis-engage. I find that the synchronizers are usually in good shape. A loose slotted nut usually causes the shifting problems. Worn Dog teeth  on the fourth gear may eventually cause a transmission to jump out of gear. Also as Dan C. has mentioned shim to reduce play in the output shaft or the transmission may also jump out of fourth. If you replace the fourth gear/output shaft,  save the bad one in makes the best clutch alignment tool possible.


Joe Alexander
Blacklick, Ohio

[George Davis]

Bob and Joe,

Thanks for the kind words!

I usually forget to take pictures during a project, but in this case taking pictures is all I can do while I wait for parts to arrive.  Other than the main bearings, synchro rings and seal kit, the hard parts for the 'box are not readily available.

Joe, thanks for the hose clamp tip.  It is rather difficult to hold the stack of gears on the mainsshaft together; the hose clamp will really help.  I'm happy to contribute this "standard transmission tour", your "tours" were my inspiration!

George Davis
'69 280 SL Euro manual

[Ben]

Yes this is excellently documented George, well done !

I will overhaul my spare transmission using this tour !

What will you do for the parts that are not readily available, re-use or find a scrap box and hope for the best ?? Also are the shims widely avaialable ??

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

[ja17]

Hello Ben,

The top shift Mercedes sedan transmissionns of the era shared a lot of parts in common. In fact if you unbolt the top cover from the sedan and bolt on the top cover of the SL, it will work in many cases.

Some gear ratios (mainly first gear) may be different. Check your BBB or Technical Data Manual. They list all the gear ratios for the standard tranmissions of all the models of the era.



Joe Alexander
Blacklick, Ohio

[George Davis]

Ben,

by "not readily available" I meant not available in the US.  I ordered bearings and synchro rings and a few other parts from Bud's Benz, they had them all except for the front seal, which they had to order from Germany.  Still no idea when that might arrive.  I may just try to match it up at a bearing and seal supplier.

Later I decided to replace 1st and 2nd gear and the 1/2 synchro body.  Those parts couldn't be found in the US, so I ordered them from SLS.  They don't stock these parts, but were able to get them in two weeks; they are on their way now.

In hindsight, I would have been better off ordering everything from SLS, but I didn't anticipate replacing any gears.

As a result of the long delivery times for all these parts, a project I hoped I could finish in 4-6 weeks will take more like 4 months.

And why replace the gears?  I'll post pics when the new parts arrive, but basically the dog teeth on 1st and 2nd and the engaging splines in the 1/2 synchro are badly rounded off.  They are supposed to be sharp.  This gearbox has always been hard to shift into 1st and 2nd, and I expect this is part of the reason why.  It's possible that new synchros alone would improve things, but as long as I have the 'box apart, I figured I should do the job halfway right.  The dog teeth on 3rd and 4th are a bit rounded off, too, but not as badly, and the 1-2 shift is both the most critical in normal driving as well as being hardest on the parts.  Not to mention the cost - 1st and 2nd gears and the 1/2 synchro will come in around 1000 Euros.

I believe some of the parts for the earlier gearboxes are no longer available, so finding a decent used 'box as Joe suggests might be the way to go in those cases.  Or maybe fit a later gearbox.



George Davis
'69 280 SL Euro manual

[George Des]

George,

The four large bearings are pretty standard metric items available through a number of domestic bearing suppliers. You can get the FAG, SKF and other forign brands using the std bearing numbering system. The number is engraved right on the bearing body. The needle bearings are somewhat harder to source especially the split ones--I ended up getting these through SLS. I got a bunch of shims in various sizes from SLS as well. The seals are fairly common items. If you do a search on the web you will come up with a number of suppliers of these metric seals--they also use a common numbering system similar to the bearings.

George Des

[CraigD]

George,
In your previous post, you mention that the gears have "rounded off teeth".  Could you explain why rounded teeth make it harder to shift?  It might seem to an uninformed person (like me) that rounded teeth might make it easier to get the gears aligned and meshed???

Thanks,

Craig
'70 280SL Euro, Manual,  Leather
Silver/Black

 

[George Davis]

Hi Craig,

Sorry if you already know this stuff, but I'll give the whole story.

When a gear is engaged, the outer sliding ring (slider) on the synchro assembly is slid toward the gear you want.  As it slides, the synchro ring (aka blocking ring) which is between them does two things: it acts like a clutch to make the gear come to the same rotating speed as the slider, and as long as there is a speed difference, it blocks the slider from further movement.  Once the gear and slider are rotating together, the synchro ring now rotates slightly and allows the slider to continue sliding so that the dog teeth and inner splines on the slider can engage each other.  

Odds are the dog teeth and inner splines won't be exactly aligned, so they are made with sharp edges where they come together, something like this:  >  <.  Image those two shapes moving toward each other.  This pointed shape allows them to meet and slide into alignment smoothly by pushing each other to the side.  Once they wear and become rounded off, they can't do that as easily as they can when they're sharp, sort of like this: )  ( .  Up to a point they can still push each other aside, but it becomes more difficult the more they are rounded off.

Hope that helps!

George Davis
'69 280 SL Euro manual

[Ben]

Thanks for the info !

I actually keep bearings and seals in stock for our industrial motor/gearbox repair business so I can get all those standard items. Previously I only had to order the seals, gaskets and the roller bearings from MB.

I decided not to replace my part worn 1/2 synchro, due to cost, but that was a mistake, hence I purchased a used box. I have not opened it yet as its in storage and I wont see my car out of the bodyshop for at least another 6-8 months !

Still I intend to have the transmission ready and waiting for it !!

This thread will be a terrific help !

BTW since I finally sorted the auto trans on my W111 I now have a scrap tranmission which I would gladly take apart for photos if someone wanted to write up something on that, altough I realise that a DIY fix on an auto isn't too practical !

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

[George Davis]

Ben,

I hope this does help.  I wanted to do the same as you, that is, have an extra trans to rebuild and then just swap it in.  But I couldn't come up with one so I'm doing the original.

With 6-8 months to go, I'd suggest starting fairly soon.  It could probably be done in a long weekend if you had everything on hand to start, but as you inspect you may find more things you want to replace.

I'll point out one other thing: I'm not worrying about the shim washers.  Ideally one should measure and selectively fit them to get all end-plays into spec.  Unless I find something way out, I'm just going to reuse the originals.

George Davis
'69 280 SL Euro manual

[J. Huber]

Great explanation George! Even I almost understood it...  :oops:

James
63 230SL

[George Davis]

It's finally done...

All parts finally arrived, except the front seal.  Fortunately, it's a standard seal and a local bearing supplier had one.

Prior to reassembly, I did measure the clearances for the bearing shim rings.  It's not hard to do with the main case empty so you can stick a feeler gauge through the bearing holes.  One set of shim rings had a bit more clearance than the book spec, but heck, the transmission ran for nearly 40 years that way so I just went with it.  The rest were good.

Assembly involves first laying the countershaft in the case, but without the bearings.  Then the mainshaft assembly is put in place, also without the bearing.  In the pic notice the hose clamp on the main shaft.  The clamp holds the assembly together while you're handling it, otherwise the parts will slide apart, which is not good.  (Clamp suggestion courtesy of Joe Alexander!)

Download Attachment: FInalTrannyPics 005.jpg
69.77 KB

Then slide the input shaft in from the front of the case and marry it up to the mainshaft.

Download Attachment: FInalTrannyPics 006.jpg
66.83 KB

Now it gets fiddly.  The mainshaft and input shaft bearings must be installed, more or less simultaneously.  Having 4 hands would help, but it can be done with two.  First remove the hose clamp, slide the bearings on as far as possible, and figure out some way to tap them the rest of the way on if they don't slide on easily.  A length of pipe that just fits over the shaft is helpful.  No pic of this operation, not enough hands!  After the mainshaft bearings are in, then install the countershaft bearings.  Those were a hand-pressure slip fit.

Now is a good time to make sure the synchro sleeves slide and nothing binds.  They sliders are not very easy to move with just fingers, but it can be done.  I could not slide the slider to engage 2nd gear, and one synchro ring really gripped first gear so much it was hard to break loose, so I had to take it all apart again.  I probably let the main shaft gears separate a bit too much during assembly and something got a bit out of place.  Reassembled the main shaft assembly and swapped the first and fourth synchro rings, put it all back together and everything was better.

The remaining assembly is straight-forward.  Install the speedo drive gear on the rear of the mainshaft, then the rear cover with new seal, then the output flange.  Install the bellhousing.  I did not have a gasket for the bellhousing to case joint, and in fact there was no gasket when I took it apart, but plenty of evidence of leakage, so a gasket sealant was used.  Then install the clutch release bearing guide tube.  Loctite was used on all fasteners.

The pics below show the clutch release bearing and fork installed, with two clips that hold the bearing to the fork.  The other pic shows the old and new style release bearings.

Download Attachment: FInalTrannyPics 007.jpg
67.67 KB

Download Attachment: FInalTrannyPics 002.jpg
64.28 KB

Then reinstall the top cover, making sure the reverse fork engages the reverse linkage inside the case.  Now make (to quote from Young Frankenstein) damnsure! you can select all gears, and that things turn properly and in the right direction for each gear.

Time to put that doggy back where it belongs!

Download Attachment: FInalTrannyPics 010.jpg
73.63 KB

There are some really bad ideas in the preceding pic: a floor jack can't go high enough, and the trans is nowhere near stable enough even if it did.  The wooden block assembly was used, with the block on the jack saddle, and the trans on the block assembly.  That kinda sorta worked, but not really.  I had delusions of doing this all by myself, but there was just no way it was going to happen.  Too unstable, had to use both hands to stabilize the trans on this wobbly mess, which meant no hands to jack up/down or move the jack.  My wonderful wife finally happened by and joined me under the car on the cold, cold floor as my jack operator.  Together we wrestled the box in with only minor injuries.  If you take away nothing else from this write up, take this: that transmission is heavy, and dropping it on your head will really louse up your day - use a proper transmission jack.

The rest was easy, we've all had the driveshaft out and replaced rear motor mounts and flex disks a few times, right?

After everything is reassembled, time to fill and bleed the clutch.  I used a plastic syringe filled with brake fluid.  Make sure the pedal is fully up, then open the bleed screw and pump fluid into the bleed screw.  Close bleed screw.  Check for fluid in the reservoir, then do the syringe thing again with less fluid so you don't overflow the reservoir.  Test the pedal.  No go.  Suck brake fluid out of the reservoir into the syringe, get back underneath and pump it in through the bleed screw again.  Try to minimize the amount of air you push in.  When nearly all brake fluid has been pumped in, stop and suck a wee bit back out.  I got a bunch of air that way, I guess it sort of gathers just below the bleed screw.  After that it worked just fine.

Download Attachment: FInalTrannyPics 012.jpg
58.97 KB

Then I filled the transmission with ATF, got the car down and hoped like heck it would work.  To my great relief, it did.  Took the car for a little test drive, and will drive it to work soon.  I'll report on how the 'box is after a bit of driving, but for now I'm just happy to be done and relatively unhurt.

George Davis
'69 280 SL Euro manual

[J. Huber]

Hey George. Very impressive. Maybe you'll need a nice long drive this summer to break it in!

James
63 230SL

[Naj ✝︎]

Hey, GD,
 

quote:time to fill and bleed the clutch. I used a plastic syringe filled with brake fluid. Make sure the pedal is fully up, then open the bleed screw and pump fluid into the bleed screw.

This is a good one! [:p]

naj

68 280SL

[Ben]

Good stuff George, love the syringe trick too !

Can you elaborate at all on where/how you measured for the shims etc.....or did you just put everything back where you found it ?

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

[George Davis]

James,

there is a long drive planned for this summer.  Any chance you'll be in the SF area in mid-August?

Naj and Ben, thanks.  I tried to use a pump oiler first, but it didn't like brake fluid.  Spotted the syringe and it worked like a charm!

Ben, I'll try to describe things a bit, but I don't have pics.  First, on the main and input shafts, the bearings are located by a ring in a groove around the outside of the bearing.  The rings bear against the main case and fit into recesses in the rear cover and bellhousing.  The shim rings also fit into the recesses to take up excess space to control axial play.  If you reuse the rings, then you can just reuse the shim rings, assuming you believe the factory did it right.  My new bearings did not have rings, so I reused the old ones.

Shim rings only go on one end of the countershaft, on the rear face of the outer race of the rear bearing, where it fits into a recess in the rear cover.  This is actually easy to measure.  Install the countershaft and bearings, then place the main case onto the bellhousing so the case is on top of the bellhousing.  Tap the countershaft down lightly to seat the front bearing in the recess in the bellhousing.  Use a good vernier caliper or similar to measure how far the rear outer bearing race protrudes past the rear face of the case.  Then measure the depth of the recess in the rear cover the same way, but be sure you include the gasket.  The difference gives you the axial play.

I hope that made some sense.  It'll be more obvious when you have it apart.

George Davis
'69 280 SL Euro manual

[Ben]

Yes it makes sense and its pretty much the way I did it !

Ta !!

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

[jeffc280sl]

I'm in the process of reassembling a rebuilt 4 speed manual transmission.  I've taken quite a few pics to suppliment the great work of George Davis.  I don't know where I would be without his writeup.  I do want to stress the importance of following his and originally Joe Alexanders suggestion to use a clamp to secure the rear section of the main shaft when putting it back together.  George wrote about it in his 12/26/06 comments.  There is a key which fits into a slot on the bottom and sticks up maybe 1/16 of an inch.  There is a similar slot in a top washer to cover the part of the key which sticks up.  If during reassembly the back gear slides out on the shaft just 1/16th of an inch this slotted key can get out of place and your transmission will have major problems. I would install the rear snap ring bearing last leaving the clamp in place.  Invert the tranny and hold it in a vice so the weight of the rear gear holds the key in place.  I've torn my transmission apart 3 times already because of this issue.  Sadly I'm getting pretty good at taking it apart.  Going to give it a rest tonight and approach it with a fresh perspective tomorrow.

If anyone has any question about the manual transmission now is the time to ask.  Its still apart and I can take some detailed pictures.

Jeff C.
1970 280SL 4-speed