741 Transmission & Ring gear, what to check?

[Juha Vane]

A little background, first.
My 356 was built for track use and the transmission have ratios 1B, 2B, 3E and 3C.
Not perhaps the best combination for normal use.
I bought a 741/0A transmission with unknown history, somebody been inside but I don't
know if it has been used after opening. Ratios in this box are 1B, 2B, 3B and 4D.

Everything look so nice inside so I don't want to take the shafts apart, but have read
that the differential carrier develop cracks. Also that the bolts shear, this on topic:
viewtopic.php?f=topic&t=21774
The 6 bolts in this trans are 8.8!, so if I'm correct, I should replace these with the correct ones.
What else should I check?

And I "can't" take it to somebody who knows, we have so few 356 here so very little, at least
known to me, knowledge here. Have saved the gaskets and will measure them so not to change
the ring gear settings, hope the previous did it also.

[Vic Skirmants]

Those bolts are JUNK! Definitely replace. The differential carrier crack can usually be seen easily at the area where the spider gear shaft is held in place by a pin. Although sometimes the crack is internal and does not show up easily on the outside of the carrier.
A 741/OA transmission would originally have had a 4C top gear.
Good luck.

[Edwin Ek]

Juha,

Transmissions are relatively easy and cheap to ship. Sounds like you need professional help, and previous work was shoddy.

You have been a Registry member for some time, so you probably know the recommended transmission rebuilders here in the US. Pick one and solve these problems for good.

Ed

[David Jones]

Remove the drive shafts and anything else that is not needed for setting up then take a few days vacation over to the UK and take the trans as checked baggage then drop the trans off with one of the known good 356 guys there. PRS comes to mind. Cheap flights I understand are available and surely a transmission as checked baggage should not cost more than the flight. Way cheaper than sending it over to the UK.
If all you intend to do is overhaul the diff then none of the above applies, just buy the parts from Vic. I have an NOS diff carrier if you need it.

[Juha Vane]

Thank you all!

The ring gear bolts was all loose, only the home made locking plate held them!
Make me wonder why people don't check things, like here "bolt is a bolt, isn't?"
Could not see any cracks inside or outside in the diff carrier. The trans has been
used with the wrong ring bolts as the surface under ring gear is slightly polished,
nothing major but definitely some movement has been there.

I washed all parts and tomorrow have a look on them, I probably will do this by myself.
Have an other question anyhow: should I use the paper gasket as it or should I add
some sealant, Hylomar, Permatex High Tack or something else, what?

Would love to purchase parts from Vic but wonder it it get very expensive, we have
24% sales tax here and shipping can get expensive. But I'll ask him.

[David Jones]

Juha, the side plate gaskets for the diff are the same as VW which you probably know. A full set of transmission gaskets is expensive but again shipping from UK should not be expensive and the cost of the individual gaskets may be quite reasonable. Also individual parts may cost less than you think. Remember the same bolts for the diff were used in VW's as well. Lots of VW parts in scrapyards even in Finland surely.

[Jacques Lefriant]

Hi Juha
Marco at Eleven Parts may be able to give you recommendations on possible sources for parts and help. Going to 12 bolt is possible a new CWP would be the best choice taping the other 6 is possible but probably will not buy you much. The late 6 bolt VW diff will help with the cracking but requires clearance for the mainshaft. My advice is to find someone that can do a proper Ring and Pinion setup. New 12.9 bolts that preferably grooved for the lock tabs is the minimum you should consider. You can use extra sealant with the paper gaskets. I would venture there any a very few professionals that can give you good advice and it will be hard to do without the parts in their hands. Mike at PRS services is one them. Giving engine rebuild advice can be done by committee but transmissions are a different deal.
j

[Jacques Lefriant]

Hi David
Porsche stopped using the diff bolts that VW used. Vic might know the progression I think the shared ones were the ones that were drilled for lock wire. some genius did not want to pay for the drilling and used the lock plate under the bolt which failed after a while. Porsche had to recall in the early 60s bolts that were too brittle. the last ones Porsche used seem to be adequate even in 6 bolt configuration but the ones you can get now do not have the 12.9 marking. the axle tube gaskets are the same as the VW tunnel gearbox also. I doubt if you can find the improved VW diff housing in Europe since I have customers from Europe ask me for them and even in the US split case VW stuff has dried up. For a while I was able to still get new VW tunnel case differentials but that too has dried up. The good news is that with Mike's help we were able to get Quaife manufacture a TBS(torque biasing diff) that accepts the Porsche ring gear bolt pattern and centers the ring gear you just have to use the Cadobbi side covers and the larger bearings with it. I should have them in a few weeks since they are sitting in theUK now. The bad news is that I did this too late for it to do much good in terms of the people that will be able to benefit.
j

[Neil Bardsley]

Thank you all!

The ring gear bolts was all loose, only the home made locking plate held them!
Make me wonder why people don't check things, like here "bolt is a bolt, isn't?"
Could not see any cracks inside or outside in the diff carrier. The trans has been
used with the wrong ring bolts as the surface under ring gear is slightly polished,
nothing major but definitely some movement has been there.

I washed all parts and tomorrow have a look on them, I probably will do this by myself.
Have an other question anyhow: should I use the paper gasket as it or should I add
some sealant, Hylomar, Permatex High Tack or something else, what?

Would love to purchase parts from Vic but wonder it it get very expensive, we have
24% sales tax here and shipping can get expensive. But I'll ask him.

You may wish to consider calling Paul at PRS in the UK. They build quite a few transmissions and will sell you parts. That will get rid of the import tax problem?

[Juha Vane]

Found the necessary number for ring & pinion setup, but for what purpose is
this number on ring gear = -0,16?

[Vic Skirmants]

That is information for the first builder at the factory. You don't need it.

[Jon Bunin]

Found the necessary number for ring & pinion setup, but for what purpose is
this number on ring gear = -0,16?

Juha, that is the gear manufacturer's setting value for the ring gear, marked as a deviation (-0,16 mm) from the blueprint value T (40,00 mm), so your ring gear would ideally be set at 39,84 mm from the pinion axis.
In later Porsche transmissions this is difficult to measure, so the ring gear is normally adjusted to within Porsche's recommended backlash range.

Here is a manual page of ring & pinion values for 1950s Volkswagens, which were the same for a Porsche 356 through 1965.

[Juha Vane]

Managed to purchase 3B and 4C gear set for 741 transmission with gears 1B, 2B, 3E and 3C.
When I took the transmission apart, the gaskets in differential bearing cover was 0,17mm and
the gear plate 0,23mm. In my 2 transmission gasket kits, one bought from Sierra Madre
and the other from Europe seem to be same origin. Same gaskets and same thickness.

In the gasket kit is gaskets for gear plate, 0,08mm, 0,15mm and 0,25mm.
For the differential bearing only 2pcs 0,25mm gaskets supplied. This is a bit thick?

Should I find some 0,17mm paper and make these side cover gaskets or what should I do?
I don't have the tools to set the gears and are doubtful if anyone here have them.
I have ink to check the gear pattern, but no way to measure bearing preload.

[Jacques Lefriant]

Hi Juha
you should as a minimum evaluate the S1/S2 shims. those are the ones between the bearings and the diff be careful to keep the orientation the same. they should be a nominal say 3.8-3.9 to compensate for the gaskets you must increase say to 3.9-4.0. Sometimes a .25 shim is used to get a better result then you might have a 3.6+.25 and 3.9. you can tell if they are worn when there is a ridge and they measure say 3.86 vs 3.9. seriously assemble the stack of internal pinion shaft parts with the pinion and the intermediate plate take the case the diff and ring gear with the side covers to either PR services or Peter Iverson in Denmark and have them set up the CWP.
j

[Juha Vane]

Hi Jacques,

The other bearing remain in the carrier but the other shim is 3,90mm.
It's not worn, no ridge on either side.
I contacted Peter Iversen earlier, but he must be busy with the 4-cam
stuff as I never heard of him.

I must find some correct thickness paper and make the 0,17mm gaskets.
Don't know where I could find the 0,10mm thicker shims locally.
Am I totall lost if I make the correct thickness gaskets, assemble just the
carrier with side covers and feel how much preload there is in the carrier
bearings? This transmission worked fine, not noisy, just the gearing was
not suitable for street use. After the preload is where it was, I can check
the ring & pinion with dye to verify correct pinion depth.
If then it's correct, I assemble the transmission. If not I have to seek
professional help.