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Posted by: FourBlades May 2 2011, 01:51 PM

When a roll cage bar/chassis tube goes through a firewall, what do you do?

Do you cut the hole close and then weld around it, use seam sealer, just live with a small gap?

I am trying to decide what to do with bars connecting the cage to the front and rear suspension points.

Thanks,

John

Posted by: rick 918-S May 2 2011, 02:07 PM

Use a lazer pointer to locate the position for the hole. There are more ways to do it than you could think of in an hour but I would start with a small hole to be sure I was lined up where I wanted the tube to pass through. Then once you have proper center for the hole on the firewall use a hole saw. You will likely need to tack a patch over the small hole you made so you have something to drill the centering bit into. Cut the hole slightly larger than your tube. If your racing follow the rule book. If your doing this for you and want a clean looking passage use a split peice of vacuum hose to fill the gap.

There will other ways explained her too I'm sure. Many ways to get this done cleanly.

Posted by: Racer Chris May 2 2011, 03:52 PM

Aluminum cover-plates (two halves) riveted in position.
No need for a precision through hole that way.

Posted by: enderw88 May 2 2011, 03:56 PM

So it appears from both replies that you should avoid a structural joint between the cage and the sheet metal it may pass through? This seems reasonable since the elements in the roll cage should only be in tension or compression. Joining them to the sheet metal as they pass though would apply bending loads as the chassis deflected.

Posted by: URY914 May 2 2011, 05:58 PM

I did what Chris said. Hole can by oversized and with covers on both sides no one ever knows.

Posted by: Krieger May 2 2011, 06:00 PM

I cut some thin aluminum plates and used seam sealer to glue them in/fill gaps.

Posted by: Krieger May 2 2011, 06:02 PM

To clarify what I did I cut a top half and a bottom half. I think this is what chris is saying too. I used sheet metal.

Posted by: J P Stein May 2 2011, 06:18 PM

Here's what I did.


Attached thumbnail(s)

Posted by: stewteral May 2 2011, 06:31 PM

Hi John,

Given that the 914 moncoque is a "Flexy Flier" on a good day, when it came to sealing the holes through the firewalls, I took a different approach:
1) Chassis stiffness comes from a complete rollcage that reaches fore & aft to brace the load point at the top of the suspension at each corner.
2) Rather than trying to weld to the Flexy Flier, which I'm betting would creak, groan and make continually annoying noises. I got some THICK wetsuit foam sheeting, wrapped around the tubes, glued it all together creating a membrane barrier and pop-riveted the outer edges to the PURE 914 sheet metal.
3) The dirt & dust stays out and once I get on track with the V8 roaring, I don't hear ANY unusual chassis noises at ALL! Obviously, this is a brilliant plan!!

Terry

Posted by: Krank May 2 2011, 08:02 PM

Although I have never installed on, nor been apart of an installation, I would think the priority would be to seal the hole. After all, it is a "FIREWALL" aint it? Smoke, fire, fumes all have to be isolated from the passenger compartment by law and by good sense.

Posted by: kg6dxn May 2 2011, 08:13 PM

My holes are tight. 1/8" bigger than my tube. I used black silicone to seal the gap. Looks good and I don't need an extra cover plate.

Posted by: J P Stein May 2 2011, 10:03 PM

Interesting.....let's see a pic.

Posted by: kg6dxn May 2 2011, 10:23 PM

Too dark tonight for a new pic but here's the best one I could find. You can see in the background the tube going through the firewall. It's welded to the hoop and the rear shock tower through the factory hole. The firewall was cut with a hole saw...

Posted by: FourBlades May 3 2011, 06:46 AM

Thanks for all the great replies.

I am looking at what the original builders of my IMSA car did and it looks very rough. They cut way oversize holes and then riveted aluminum or very thin
sheet metal around it. I am trying to decide if I should leave it "original" or
make it a little neater and more functional. They did not seem overly concerned
about holes between the passenger and engine or front trunk area.

I'll post some pictures tonight.

John

Posted by: FourBlades May 3 2011, 06:48 AM

I like how you did this JP.

John

Posted by: stugray May 3 2011, 09:33 AM

Just an observation:

1 - For the fronts, the heater hose holes are perfectly lined up to put a support from my front suspension shock tower plate to the base of my rollbar. I am considering putting a support in there, but there is some confusion about whether I am allowed to do this or not in vintage racing (I'll wait a season or two).

2 - 914s have THREE firewalls: One between engine & driver, one between gas tank & driver, and one between front trunk & gas tank.
Since I removed my gas tank and replaced it with a fuel cell in the front trunk, WHICH wall is my firewall now?

Stu

Posted by: URY914 May 3 2011, 10:14 AM

Bulkheads. They are called bulkheads. A fire wall is between the driver/passenger and the engine.