924Board.org

!tom

Howdy there.

Although I've never taken my fuel distributor apart, from what I've seen here it's quite hit and miss getting them back together so they don't leak.

I was just talking to a friend of mine who specialises in fixing rare cars, and we were talking about the fuel distributor. He mentioned to me the seceret on how to re-assemble a fuel distributor that has been taken apart.

Apparently, they are originally assembeled at an extremely low temperature. Thus, when the aluminum comes back to room temperature, the thermal expansion will increase the tension on the bolts that hold the assembly together. This has the effect of increasing the tightning torque on the bolts, but without stripping out the threads in the aluminum.

I have no idea how cold the housing is chilled, but I would expect you should get a similar effect by heating the hardware that holds the assembly together, and it's easier to heat things up than it is to cool to very low temperatures. So, if you have a leaky fuel distributor, you could try to put the distributor in your freezer, your bolts in the oven, then quickly throw it together and hope for the best.
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78 924 NA
5-lug

endwrench · Joined: 07 Dec 2002 Posts: 1631 Location: Victor, Montana

Interesting. I don't think heating the bolts will help much as the castings will act as a huge heat sink and quench all the heat out before you get 'em tight but I guess it wouldn't hurt to try. Cooling the castings in a freezer makes sense though.

Todd
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'79 924NA. Rebuilt 9.5:1, MSDS header, Mega Squirt Injection, MJLJ-EDIS Ignition, 1.6L Whipple Charger and Intercooler, 10lbs Boost, 944 Trans, Custom HD Clutch.
"simsport" said....superchargers are better than turbos its official!....

!tom · Posted: Fri Nov 09, 2007 11:20 am Post subject:

I agree -- I would expect that heating the bolts wouldn't accomplish much.

The coefficient of thermal expansion of aluminum is much higher than that of steel, so changing the temperature of the aluminum makes a bigger difference than changing the temperature of the steel.

However, I would expect the heat to be drawn quickly out of the threads of the bolts, leaving the shank of the bolt hot during the assembly process. You do have a good point though -- it's not like they're head bolts 8 or 10 inches long that would be cooled at each end during assembly and remain hot in the middle.

If dry ice were easily available, this may be a good thing to try. It sounds like the assembly process at the factory were far more extreme than a conventional freezer.
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78 924 NA
5-lug

RC · Joined: 25 Mar 2007 Posts: 2637 Location: Australia

Hmm, quite interesting.

As I didn`t have this knowledge available when I did mine and also realized that its difficult to apply much torque to the standard slotted screws, replaced them with higher tensile stainless steel cap head (hex socket) screws.

That alone makes a substantial difference because when I helped a mate rebuild his Saab one there was slight seepage with the screws done up as tight as possible before breaking with visegrips on an impact driver. We replaced the screws with hi-tensile cap screws and fixed the leak.

Logically, the combination of both would provide the greatest clamping and sealing.

Roger

Lizard · Posted: Fri Nov 09, 2007 3:19 pm Post subject:

that would be great...

if the fuel distributors were made of aluminum, but alas they arent.
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3 928s,

RC · Joined: 25 Mar 2007 Posts: 2637 Location: Australia

Generally the earlier 924 (and 928) ones are steel or cast iron but the later 80+ ones with the frequency valve, and the 931 ones are aluminium.

Roger

!tom · Posted: Fri Nov 09, 2007 3:58 pm Post subject:

AndyR · Joined: 26 Oct 2007 Posts: 19 Location: Hawaii

If you have any friends in food-service, you can ask if they have a walk-in freezer.
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-White 1987 Porsche 924S (Hawaii)

-Black 2000 Audi A4 (Mainland)

dpw928 · Joined: 02 Nov 2002 Posts: 1860 Location: owasso, ok 74055

Using a paint to seal the surfaces is not recommended as it will likely plug the decoupler orifices that run between the upper and lower halves. On my 928 (non-aluminum) that has a stainless steel gasket, I used wheel grease to coat the mating surfaces. This will not plug the fuel passages but works as a low strenght sealant for the flat surfaces. This same technique worked for the early flathead engines that used steel blocks, aluminum heads and stamped steel headgaskets. Now I'm telling my age.

Since I haven't disassembled the 924's distributor, I'm not sure if the components are the same, but it has held up for years on the 928's.

Dennis
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81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

dgKnecht · Joined: 17 Nov 2002 Posts: 42 Location: Palatka, FL

THANKS!!! i'll give that a try this weekend, pretty cold mornings right now

i actually had less leakage with the wheel grease, but still seeping out between the two halves, the permatex gasket failed badly.
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78 924

--all of us have been through some bad times.....i once had a job where i had to drive a Chevette

dpw928 · Joined: 02 Nov 2002 Posts: 1860 Location: owasso, ok 74055

Another product you could try is indian head gasket cement. It is actually a shellac with metallic particles. If you use it, make sure to keep it far enough from the small holes so it won't plug them when tightened down.

Dennis
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81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black