356C 741/2C transmission question.

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M Penta
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356C 741/2C transmission question.

#1 Post by M Penta »

Hi 356ers,

long time no post - have not been working on the 356 much, just driving it!

Since I love to find new things to worry about, I am wondering if it is worth checking ring gear bolt type/tightness and inspecting diff carrier.

There are no obvious symptoms of any problems with the transmission, which I believe has never been opened up or removed from this low mileage car. It shifts well, oil looks good and seems quiet. The only transmission noise I can here is a very faint whine at float, goes away under accell and decel. Is some transmission noise under this circumstance normal?

The car is a 64C (217330 built in late 63) with 741 2C transmission, which I believe should have the 12 bolt diff?

I have read up on diff removal, but is it necessary to remove the entire assembly for visual inspection of carrier and/or replacing bolts? Or can bolts be accessed with diff in case?

It may or may not be worth doing this as a preventative measure, just curious what the pros here think.




Thanks!

Mark

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David Jones
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Re: 356C 741 transmission question

#2 Post by David Jones »

Mark, for what it is worth. My 63B broke the bolts for the ring gear a long time ago in 1971 and I replaced the diff carrier with one out of a 57 VW. My 59 coupe trans has never been apart as far as I know from the service records I got when I bought it in 94. My 59 cabrio likewise.
My 64C built in Jan 64 vin #216833 had a broken diff carrier bearing support when I bought it. About 2 years after I got it back on the road the ring gear lost a tooth so it broke twice. It was rebuilt with a 6 bolt ring gear on a 6 bolt diff carrier.
I am of the opinion that if your diff is not broken then don't try and fix it. It is like the IMS bearings on the water-cooled Porsches. 10% of them actually broke so you can take your chances. My two 59's are going to have to live with their 6 bolt diffs until/if they break.
The ring bolts can be accessed by just removing the one side plate, but to examine the diff carrier for cracks the whole assembly has to come out.
If I had known I would live this long I would have pushed the envelope a little harder.
Cymru am byth
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M Penta
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Re: 356C 741/2C transmission question.

#3 Post by M Penta »

Thanks for the reply David,

Yours was one of the threads I read about. Too much research on the internet can do that, just like the IMS thing..

I was not sure if the 12 bolt diffs are as susceptible as the 6 bolt diffs to bolt loosening, or if my transmission may have the dreaded lock plates under the bolts. It just seems like it could easily be a catastrophic event if the carrier breaks or bolts fall out. Hopefully there is plenty of warning with bad sounds first..

Good to know that the bolts can be checked in place, and at least a visible inspection of the carrier for any obvious damage.

Some other projects I have planned for the winter may involve removing axle tubes and/or trans, in which case I will give it a look.

I sure do like the Cadrobi conversion if anything does need work in there.

Thanks,


Mark

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Re: 356C 741/2C transmission question.

#4 Post by Doug McDonnell »

12 bolt diff "The 741 trans for the 356 C debut in August 1963. Identical to the 741 A, but having a 12 bolt ring & pinion. 741/2C, BBBD gears; 741/OC, BBBC gears. Numbers 70931 (Aug. 1963) to 86517 (March 1965). 77 data points." From Vic's article Since you have a transmission that is working fine I would follow the "If it ain't broke don't fix it" advice. Or send it to Vic over the winter if you really want everything checked since you are East.
1965 356C 2000 BMW 740i Sport 1967 Honda CL77 There is never enough time to do it right, but always enough time to do it over.

M Penta
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Re: 356C 741/2C transmission question.

#5 Post by M Penta »

Thanks Doug,

I was not sure if my early C built in 63 may still have had the 741A. Thanks for the tip, I'll check the number.

I will leave it alone for now, but will probably have a look when I remove the engine and trans for other work.

These are my two (hopefully unwarranted) worries when driving it: Broken crank and loose diff bolts/failed diff housing. I'm sure it will be in the back of my mind when I am driving over the GWB on way to ECH.

Mark

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Re: 356C 741/2C transmission question.

#6 Post by David Jones »

Mark, your car was built in 64 not 63 so for sure has the 12 bolt diff. Unless you gave the wrong Vin#. As I said my 64 was built in January of 64 and has an earlier Vin than yours.
If I had known I would live this long I would have pushed the envelope a little harder.
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Re: 356C 741/2C transmission question.

#7 Post by M Penta »

Hmm, not sure why I thought it was built in 63. 217330 is correct vin. I'll have to check the COA if I can find it..


Mark

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Re: 356C 741/2C transmission question.

#8 Post by David Jones »

Mark, Here is a COA from a recent sale of a 64 on BAT. I would show mine but I never got one, but I did look at the Kardex at a Porsche parade some years ago so got the build date from that. Looks like yours would be around end of February to beginning of March 64.
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1964_porsche_356sc_coupe_153186727395d565ef66e7d1964-porsche-356-sc-coa-2.jpg
If I had known I would live this long I would have pushed the envelope a little harder.
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Re: 356C 741/2C transmission question.

#9 Post by M Penta »

Right as usual David, March 5 1964.

Anyway, it seems that no one goes in to inspect for that reason alone, so I will try to put the possibility of loose bolts and cracked diff housing out of my mind for now. I have read plenty of posts where people drive with bad sounds to make it to where they are going. If it ever happens to me the car will be stopped as soon as it is safe to do so and get towed home.

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Re: 356C 741/2C transmission question.

#10 Post by David Jones »

Well if it goes bang let me know as I have a complete trans and a new diff carrier I can let you have though it would probably be cheaper to get one from Adam.
If I had known I would live this long I would have pushed the envelope a little harder.
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Eric Wills
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Re: 356C 741/2C transmission question.

#11 Post by Eric Wills »

Late 741 B/C transmissions had a slightly larger diff housing and stronger bolts. Porsche did not machine as much material from the VW diff housing and used a main shaft with clearance for the larger differential on the later 356s. Any of them can crack, as I just scrapped a C2 diff with a crack in the usual place a few days ago.
Here is a couple pics of the common problem. The picture of the broken bolts are from an early 741 with larger main shaft and smaller diff. The cracked diff is from the same trans. The completely broken diff was from a 1956 trans.

Note: this post is not intended to scare anyone into inspecting your differential. Just a visual aid for what is being discussed.
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Jacques Lefriant
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Re: 356C 741/2C transmission question.

#12 Post by Jacques Lefriant »

Hi Mark
The 6 bolt diffs like Eric posted were marginalized by clearancing for the mainshaft but the bolt issue had two factors 1. inferior bolts and 2. most significant the lock tab under the bolt head that contributed to the loosening like in the second picture. your box should have the better bolts and locks whether it is 6 or 12. Porsche also missed on the 12 bolt cover that the bearing portion could detach I think someone just posted pictures of that failure. These problems were the reason the Cadrobi conversion was developed as a alternative diff.
j
 

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Re: 356C 741/2C transmission question.

#13 Post by M Penta »

Thanks for pics Eric, just the visual aid I wanted to see haha.

Thanks for the reply Jacques. I was not sure if the 12 bolt diff had the lock tabs or washers. Sounds like there is a lot less to worry about with this diff. Yes, the Cadrobi sounds like a no brainer to me, If I ever have any issues in there I will certainly inquire.

I don't beat on the car but I do have fun with it as that is what it is designed for. I don't know why I am such a worry wort these days, maybe it has to do with being in reliability engineering for the past 10 yrs. These are the very things I am conditioned to worry about.

I will be driving it to Limerock for the historic festival, and then my longest trip to ECH in October. My biggest concern is actually other drivers and traffic...

Thanks,

Mark

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Re: 356C 741/2C transmission question.

#14 Post by Mike Horton »

I had a failure in my '57 T1, which also took the R&P. I had bought that car, as an insurance co. total wreck. It had just 56,000 miles, but had been flat spun into a concrete curb, the hit directly on the right rear wheel, tucking it under, and hence, the rollover. I was a college freshman then, and replaced the right side axle, and tube with used 36 HP VW parts. A couple of years later, while on a date, the diff bolts broke, and I had to call the girl's mom, to give us a ride. That hard side load, of hitting the curb, was likely the root cause. I replaced the diff carrier, with a used 36 HP part, bought a used R&P from the dealership service manager, and the new bolts, and lock tabs, as well as the needed gaskets, and seals from the dealer, "upgrading" the old drilled, lock wire bolts, with the newer version. This was in about '66, and I later sold it, still running well.

I since, did remove the diff parts from a'63 normal, with an unknown to me history, Magnafluxed the diff housing, and "since I was in there", replaced the 6 bolts, "Locktited" them, and lock tabbed them in place. It can be a lot of work, and it IS, your car, your call.

I think I'm with David on this one.


Yes, my '68 912, with the 901 5 SP, has the simplified diff, with the extended tip on the side bolts, which hold the spider gears in place, but I have replaced those bolts, with new. They are now NLA, according to Brad,
Mike

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Re: 356C 741/2C transmission question.

#15 Post by Jacques Lefriant »

Hi Mark
my first real job(1967) was imputing data for the FMEA on the S1C booster for the Apollo missions i recall our target was 95% to man rate the mission. i give you better chances.
j
 

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