911 Gearbox Rebuild Underway. Beadblast?

[Scott Clarke]

I'm going to put together my 911 gearbox. I have the case stripped. Only studs and bearing races remain. Any reason I can't beadblast the case and side cover, as long as I clean the thing afterwards (I mean REALLY clean it)? I could mask off the studs and races. I plan on applying some sort of corrosion inhibitor afterwards (such as a product for marine applications).

On another note, the threads that secure the clutch throw out lever pivot seem to be a helicoil repair job. Is the thing in the photo an insert that can be removed and replaced?

Thanks,
-Scott

[Scott Clarke]

Pictures aren't cooperating. I'll try in the morning.

[randywebb]

Scott - factory helicoil (or timecert or whateve that is in German)

[Grady Clay]

Randy,

I wouldn’t media blast the transmission just because of the remote possibility of not being able to completely clean the interior. I can’t imagine anything worse for a transmission than constantly shedding some abrasive material in the interior.

An alternative would be to first assemble all four empty castings using old nuts & washers. You can plug the holes (input shaft, axle flanges, speedometer drive) with the expandable “freeze-plug” rubber plugs for a water pumper. Drive a rubber “cork” into the shifter and breather. Use an old shift pivot if you want to maintain the original finish on yours. Expend an old back-up light switch. This way you can safely blast only the exterior.

When done, I would remove all the studs from the casting and have them re-plated or replaced. As a final touch you can take a “spot face” drill and make all the clearance holes like new where the nut & washers seat. You will want to use new nuts & washers.

I would clean the threaded holes with a thread chaser (not a tap) and reinstall the studs with Locktite Red. Take care to wipe off the excess, it will bubble up the threads for several hours because of the blind holes. If you have any suspicious thread damage, now is the time for Helicoil repair. (If I were going to this level of repair on a type 911, mag 915 or 914, I would Helicoil ALL the threads in the case.)

Some of these choices depend on the show level of originality you are trying to achieve. The original finish on 901 hardware was a black oxide finish similar to a recipe in Machinery’s Handbook. The best finish is the nickel (silver or gold) plating currently available. Most of the M8 nuts were 13 mm across the flats except the two for the shift pivot were 14 mm. Most of the washers were “wavey” spring washers with black oxide finish. The shift pivot used a thin pattern split lock washer as did the clutch release bearing guide tube M6 nuts.

For maximum corrosion protection we used a thin mixture of Tectyl and solvent to coat the studs, washers, casting surfaces, and nuts during assembly. After assembly the transmission was completely coated by paint brush with Tectyl, taking care to not get any in some critical places. Those are at the lip seals, on the mating surfaces of the axle flanges, transmission-to-engine, starter-to-transmission, and the shift shaft, speedometer drive threads and back-up light switch terminals. It takes about 24 hours to cure sufficiently (more is better). In winter we would set the transmission on top of a steam radiator overnight.


The threaded insert for the clutch fork pivot is a factory part, not a repair. The original very early pivot design was pressed in the casting and needed to be up-dated with the insert.
You should sacrifice a deep, fully broached, 12-point, 14 mm, 3/8 drive socket for wrenching the pivot. It needs to have the end turned to get maximum grip on the very shallow hex.

Best,
Grady

[Scott Clarke]

Thanks to both of you for the insight on the pivot insert. Grady-
thanks for all of that info, and guidelines for the hardware. The studs on my gearbox seem to be yellow cad, and not black oxide. Mine is type 911, and not 901. I was affraid that the abrasive material would be an issue with gearbox wear. I've spent hours scraping and scouring with Scotchbrite pads, and it still looks like crap. It doesn't need to be perfect, but I'd sure like it to be presentable. Here are the pictures for anyone wondering what we're talking about.

[Grady Clay]

Scott,

Sorry, dumb ‘ol me too early in the morning.

What all are you doing to your type 911?

I don’t see anything wrong with your clutch fork pivot insert.
It is a steel insert with a stainless steel Helicoil as original.
If OK, I would leave it alone.

Can you post some good images of the special M8 nuts used
on the 901 transmission. I’m particularly interested in showing
the locking “Helicoil” in the nut. Porsche call this a “Mid-Grip-
Mutter” P/N 999.090.101.02.


Here is the fairly unique washer for the 911. It is a nominal
15x8.4x1.5 (8.4 DIN 433). This example measures 14.75 x 8.60 x 1.50 mm.
The P/N is 900.031.001.02.

For comparison is this more common M8 washer. It is a nominal
17x8.4x1.5 (this example measures 16.68 x 8.60 x 1.50 mm).
The P/N is 900.025.007.02.
And P/N N 011 525 19 which measures 16.80 x 8.60 x 1.50 mm.

IMAGE: Trans911Washers01.jpg



The exception is where the ground strap attaches to the
transmission. There is a similar but larger OD plated washer
20x8.4x2 on both sides of the strap (very important).
I don’t have a new example of P/N 999.151.022.02 to show.



Back to the question of surface finish.

You don’t want to damage the transmission castings or
prime it fro future corrosion damage.

You want it to appear close to original color and texture.

You are going to want to coat it with a preservative (Tectyl)
when complete.


There are several kinds of surface cleaning/preparation. You
are dong the basic with solvent and a scrub brush/ScotchBrite.
Next is media blasting. A great deal depends on the media and
force (air pressure). Third would be in a shaker full of media. I
don’t have a clear answer but some research would be very
worthwhile. Whatever you do, don’t be too aggressive.

I’m sure there are some water base preparations that will dull a
too-bright finish and aid in the preservation while not causing
any long-term issue. We want the answers for someone to
search Pelican in 2105!

Best,
Grady

[Grady Clay]

I found one of those nuts.
My photography needs some practice.
IMAGE: Trans911Nut02b


Best,
Grady

[jpnovak]

Along the same lines... If I have a tranny out of the car getting ready to be freshened, can I blast is before disassembly> I would tape off the vent hole and protect any cad plated items (flanges, clutch fork). I assume this would keep the inside clean. Mine still looks dirty now that all the accumulated grease has been removed.

[randywebb]

Scott - if it is buildup, I used a dull chisel for a scraper on mine (a '71 box). Or maybe it is that whitish mottled corrosion?

[Scott Clarke]

Randy-
I've got both buildup and corrosion. I'm not getting good results with the scraping/scouring approach.

Noah-
How bad was your case prior to cleaning? Is that a mag or aluminum case? I could buy some walnut shells, and empty the bead from my cabinet, and see what happens.

Grady-
This gearbox suffered a broken input shaft, right behind first gear. I have procured a replacement from a 914, and intend to use second through fifth from that gearbox, and retain my original first (closer ratios, plus I'm stuck with the 914 second anyway). I will replace first gear syncro ring and syncro teeth. Also I have the dreaded simplified differential, so I'll be making the changes you detailed in another post (thanks for that!). Until the day it broke, my gearbox/diff worked very well, so I hope to use the same bearings, etc. I need to examine the 914 parts carefully to see if the syncros are as good as the ones orignally form my car. I guess I could reuse the 911 components except for the gears and input shaft?

[Grady Clay]

Scott,

You can swap around the gear dogs and, for example, use the
dogs from the 911 5th on the 914 2nd, etc. Same with sliding
sleeves – pick the best one for the 2-3 position.

Always replace the two 4-point ball bearings in the intermediate
plate. I know – expensive – but one of the weak points.

Take some good pictures for me of the “Simplified Differential”
update. All those parts should be in the donor 914
transmission. What do the tits on the flange bolts look like?


What is the condition of the clutch cable ring on the differential
side cover?

This would be a good opportunity to reset your differential
bearing pre-load and pinion-to-ring dimensions. Did you get
a serviceable 7:31 ring & pinion from the 914?

Do some trading and get the 3-4-5 gears to make AFMSX.
Those are standard on earlier cars and are great for what you
are doing. The X 28:24 is 60 MPH@3000 RPM with standard
diameter tires.


Noah, that looks great. You should re-plate the hardware and
disassemble the clutch linkage for cleaning and inspection. We
need to get Wayne to order Tectyl in 1 Gal containers. He could
make some money and all these transmissions get preserved.


Best,
Grady

PS: You guys forced me to search around and I found an old
gear chart with Riverside layout and gear changes on the
reverse. I’ll post it on the historic Riverside thread. The gears
are AINSW and indicates 8200 RPM in 5th at the kink going into
the Carousel.


IMAGE: RiversideGears01.jpg


IMAGE: RiversideGears02.jpg


G.

[Scott Clarke]

Grady-
The tits of the flange bolts were about 15% worn. I believe the gearbox to be original to the car, and the car appears to have 75,000 miles. I'll check the condition of the clutch cable ring, but it didn't strike me as being damaged.

My original gearbox was A,GA,O,V,ZA, and the 914 box is AFMSZ, so it sounds like I don't have the X you recommend. Is X between Z and ZA?

I don't have the complete 914 box- just the input shaft and gearsets. I'll have to source the differential parts. Anybody have an X and some diff components?

Thanks, again, to all for the great input,
-Scott

[Scott Clarke]

I found my gear chart. What if I use my O as a 5th, resulting in a Y?

[logician]

Hi Scott,

I found two 911 gearboxes that had the same defect in the machining of the pivot bolt seat. There was a pin hole in the seat, allowing oil from inside the gearbox to leak into the bell housing and release lever. Fortunately, none of that oil made it to the clutch disk. Read the following thread:

'70 (pull) clutch woes -- help!

I sealed the hole with epoxy.

-Juan

[Grady Clay]

Scott,

No, don’t reverse the gear ratios. That might work with a 912, a 914 1.7 or in some short lived application but not in your situation. An X 4-5 should be an easy and inexpensive gear to find.

The reason the gear angle is opposite between 2-3 and 4-5 is that it is designed to reduce the axial load on the 4-point bearing supporting the pinion shaft. In normal operation the axial force from the gear counteracts the axial force from the pinion to ring. If you use a gear cut the other direction, the forces become additive. The normal loads are too close to the limit for that bearing. If you add to the load it accelerates the time before failure.


Juan, the type 911 was sort of a “stop gap” fix to fill in until the 915 was ready. I think the sole reason was to get the pull-type 225 mm clutch in production earlier than originally planed because owners were eating up push-type 215 mm clutches even with 2.0s. Remember, the 915 has it’s beginnings in ’68 with the 908 transmission type 916.

The transmissions that you found with this flaw, were they the casting with the S/N on the bottom rib or on a rectangular boss under the differential?

Best,
Grady

[304065]

Grady, that's a brilliant explanation of why one shouldn't "flip" gears-- I didn't visualize it until I remembered that the gears are helical, not herringbone or straight-cut. A helical gear induces a thrust load along the shaft. That's something to consider when I'm looking for used boxes. . .

[randywebb]

re Mg surface finishses....

from http://forums.pelicanparts.com/show...&threadid=59006

Email from a manf. of Mg parts:

The short answer to your inquiry is that there is nothing I know that will give you that “polished look” on magnesium for the long term. This is the same issue people have for polished magnesium wheels. If you “polish” magnesium and simply apply a clear coat, the clear coat will delaminate (turn milky) from the magnesium at sharp edges and stone damage sites in even quite mild (humid/wet) conditions. This is because of the following factors –

Polishing with certain abrasives can contaminate a magnesium surface with cathodic (iron containing) impurities. This has the effect of further increasing the corrosion rate of magnesium alloys and aggravates the situation. Safe polishing media for magnesium would be high purity aluminum oxide, silicon carbide, glass or diamond. Do not use em[o]ry powder. Never use a wire brush to “clean” magnesium components.

Magnesium is an alkali metal and requires a conversion coating to be applied before painting. The conversion coating will passivate the magnesium surface and improve paint adhesion. This will slow or prevent delamination of the paint under corrosive conditions. All the best conversion coatings for magnesium are colored and most contain chromates. The “best” available conversion coating that would maintain a “shinny” finish is known as the # 21 Chemical Treatment (If you send me a fax number I can copy you with the chemical formulation which does contain hexavalent chrome compounds).

Magnesium is an alkali metal and epoxy based paints are preferred.
Epoxy based paints suffer from UV degradation so should not be used as a top coat. Its probable best to apply a clear epoxy primer followed by a clear polyurethane for the look you are after. Cellulose paints on magnesium behave poorly.
Don’t know if this will help you – why not chromate the fan assembly using one of the better chromate treatments, rinse, dry the chromate film between 110 to 120 C, then apply a silver base/clear coat system.

[logician]

Grady, The two transmissions that had the pinhole at the pivot seat both had the serial number on the center rib. The two serial numbers were 7100779 and 7100028.

-Juan

[Grady Clay]

Juan,

Interesting, let’s keep our eyes open to more.

As far as I know, the only change in the type 911 series casting was when the S/N was stamped in a new boss under the differential. There were lots of other “minor” changes indicating that someone had analyzed the original castings and there was now a new mold for an up-dated casting.

There are three versions of the magnesium pressure “investment” castings: ’69 901/12 & 901/13, the 911/xx 1v. castings with the S/N on the rib and the 911/xx with the S/N on the new boss under the differential. I don’t think you can distinguish between the two by Type number, only S/N. There are the two 914 castings, one for the four and one for the six, differing mostly in the material and casting technique. The 914-6 was almost identical to the 911 1v.

One of the simple things that always surprised me was they went to the effort to make a protected place for the numbers on the 2nd version 911 and then put the numbers in an even more exposed location with the 915.

Best,
Grady

[randywebb]

Take good notes, Scott. I'm relying on you to help me with mine after I blow it up...