Trans/gearbox rebuild

[BD55]

Has anyone had a transmission rebuild recently? Although mine works fine, it is very noisy in 3rd and 4th and it will need some work at some point. My car is a C and the box, a 741 dual mount is original to the car. Just looking for advice and past experiences and approximate cost involved, and if there are any upgrades available? Cheers.

[Hugo 356]

No personal experience. Cogbox used to be recommended but I don't see a web-site for them so not sure if they're still around...

[neilbardsley]

I would call PRS and ask. Chances are whoever does the work will have to buy bits from them anyway

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[simon james]

I suffered an enforced rebuild about 8 years ago.
After various warnings I did it myself. I didn’t find it difficult or expensive. Little or no machining work was needed.
Best advice I can give is to crack test your diff carrier. Mine was (unbeknownst to me) cracked and caused a catastrophic failure one evening, forcing me to re-build the box.
Thankfully it (the diff carrier only) is based on a 30hp split case VW item, which on earlier 356 boxes is turned down to clear the mainshaft (removing strength in an already thin area) and in later boxes the mainshaft is necked to clear the diff. I chose the latter route, full size VW diff carrier and turned down my mainshaft.
Sounds like you have noisy bearings. Be wary of replacements, the originals are twin row.
The whole job was easier than re-building an engine!!!!

[BD55]

Thanks for the replies. I have built a couple of split case boxes in the distant past, so may have a go....but I am thinking I only want to take it out once so it may go to a pro.
Simon, so what you did was the equivalent of what they call on the Registry a 'cadrobbi conversion' or similar?
The parts for this are sold by Eleven parts.

[Mike Smith]

I think Simon is saying that he used the differential Housing from an Early Swing Axle VW Box
This is exactly what Porsche used except that their Pinion Shaft did not quite clear the carrier
All the early boxes were lightly machined on the outside dia. of the carrier to allow enough clearance
However this weakened the casting and it is very usual to find the Diff Carrier cracked at the point where they drove in a solid pin to locate the Star Wheel assembly (PRS use a later VW Pin at this point that puts less stress on the casting)
When Porsche finally realised their mistake they reduced the Diameter of the Pinion shaft at the local area where it passed over the Diff. Carrier (this is what Simon did)
The Cadrobi conversion uses a Late VW Diff Carrier that allows the later (larger) support Bearings to be used
It also allows the Drive Shafts to be removed with the transmission in the car
In addition the Crownwheel and Pinion Ratio is changed from 7:31 to 8:31 which raises all the Ratios throughout the box
Simon also mentions the twin Row bearings - These are the two bearings in the Intermediate Plate
If you read the number from the bearing and go down to you bearing supplier he will give you a Single Row Bearing with 8 Balls in a Plastic Cage - It WILL eventually fail
The bearings should be a Double Row of 12-Balls in a Steel cage
We never use Plastic Cages anywhere on a 356

[BD55]

Mike thanks as always for taking the time to share your knowledge, much appreciated. Can i ask, do you think the Cadrobi conversion is a good idea or have you ever used one?... This is probably going to be a winter job for me, so will contact Paul when the time comes.

[Mike Smith]

I think that the concept is good but it is sold in the wrong way
It is sold on the benefit of the Bigger Bearings and the idea of being able to remove the drive shafts insitu

Bigger bearings - I have never seen them fail except through lack of oil
Removing driveshaft’s insitu - I cannot think of a reason to want to do it, unless you have clouted a kerb REALLY hard and bent a driveshaft (I did do that in my youth, about 50 years ago)

For me the real advantage and the reason for my interest in the conversion is that I always think that 1st Gear (`B`) in a 356 is too low
Porsche used to supply a higher 1st gear (a `C`) but it is impossible to find now and was a little too high for road use (good for tracks with a tight hairpin- bang into 1st and floor it - now the race boys use 2nd in the1st position but, of course, that is unpleasant on the road)

There is a high 4th available, a TG and I use one in my old 356`C` but it gives you a large drop between 3rd and 4th and I find myself in 3rd most of the time around town but it is very helpful on motorways (mine came from a good source but is noisy)

OK, back to the plot - with the Cadrobi conversion and the 8:31 it lifts all the gears to the same amount (i.e. reduces the RPM for a given road speed) and does not interfere with the drop between the gears.
It’s a long time since I looked at all the detail but from memory 1st ends up a little higher than the Standard `B` gear but not has high as the optional `C
If you use the highest 4th gear available from Porsche (`C`) yet gain it will be a little higher but not as high as the TG

Note - when I am talking about `B` and `C` gears this is nothing to do with the 356 Models it is the Letter Porsche gave to the different gear options i.e. - a Speedster set is BBAB

[BD55]

Thanks Mike, great information, very interesting. So the 8:31 is readily available? Be interesting to weigh up the cost against advantages.

[roy mawbey]

Thats really interesting info Mike. If ever my gearbox fails PRS will get the job. Without doubt even though I have attempted everything else on my car in 51 years, the gearbox is not something I would attempt if it had to be split. PRS must have overhauled many 356 boxes and learn't how to do them properly by gaining the experience over many years. Doing that job for the first time even with the factory workshop manual from the 1950's would still stop me doing it. I remember being offered a 716 gearbox that I knew was okay for £30- back in 1970. It would have taken up garage space then so declined. Would have been a great spare to have though !!

Roy

[Mike Smith]

Roy - that was a good deal at £30 although the 716 is not a great box. Porsche seemed to be listening to the `bean counters` when they introduced it. All the selector forks were thinner including the Hockey stick operating rod, at the point where all the selection is carried out: add into that that they also went backwards and re-introduced the single mount.
None of it was a good idea.
However, it is easy enough to replace all the bits with parts from the next generation the 741 and all the 741 guts will also fit inside your earlier 644 split case, which also gives you the later Synco. Mechanism.

It was this versatility and the ability to be able to repair and alter things that, peaked my interest in VW and of course Porsche

[roy mawbey]

Mike, that's interesting I knew the 716 had what the factory said improved syncro mesh to the previous box but I never knew some of the 356B internals would fit my 356A box. This is exactly why I would never attempt doing any repair to it!!

Its a good thing you decided to work it all out Mike, without guys like you these nice cars of ours would be driving around in far fewer numbers!

Roy

[Adrian3]

Mike, have you, or do you know anyone with, any decent miles with a Cadrobbi conversion. Don't chide me for referring to the Maestro again, but he didn't like them for some reason!

[neilbardsley]

Some discussion here http://forum.porsche356registry.org/vie ... hp?t=25641

Jacques seems to have made a similar improvement

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[Mike Smith]

I as unware that this option was available in Harry`s era ?

As far as I am aware there is no one in this country running the Mod. and we are the only people who have actually put a transmission together with it.
I could certainly get a respected opinion from others who have carried it out in The States but please send me a copy of Harry`s comments

It is a relatively simple mod, and there can be no reason that I can think of that makes it in anyway suspect. - The diff carrier is stronger, - The Diff Carrier Supprt Bearings are much larger and the total tooth count is one more, which is good.

There is a theory says that 40 is a good number for the total tooth count (Crownwheel & Pinion). I do not know where this statement came from
Porsche available Diffs are 6:29 (35 Teeth) - 7:31 (38 Teeth) - and 7:34 (41 Teeth)

6:29 (35 Teeth) - was a Hillclimb ratio with a known protensity for failure
7:31 (38 Teeth) - is the standard ratio for almost all 356`s
8:31 (39 Teeth) - The Cabrobi Option
7:34 (41 Teeth) - was a Porsche option but I do not know for what

I can send you the calcs. and a Graph for each, assuming that my Homemade calculator still works?