View Full Version : Backfire

20-01-05, 07:52 PM
Hi Guys,
Some time back I asked what you thought the cause of the backfire was on my 305 Chevy.
You willy sages correctly diagnosed an exhaust leak and I now have to renew the manifold gasket.
My question is that the manifold bolts look very small (about 6mm dia) and I'm very concerned that I'll break them trying to take them out (which will mean a head- off job to drill them out).
I was thinking of getting the engine up to temperature and then cooling the bolts by pouring water over the heads, hoping that the temperature differential will slacken them but if anyone has any better ideas I'm all ears.
I can't see how to get WD40 or similar into the threads as only the heads of the bolts are showing.
The headers are tubular, not cast.
Harry :confused:

20-01-05, 08:00 PM
Cooling them down quick will make them brittle, so you might snap them off quicker, Soak it in WD40 or other penetrating lubricant over night and if you have a flat punch and a hammer and can get to it, a few sharp taps on the top of the bolt trying not to damage the nut may also loosen it. I'm sure the oters have a better tip though!

Purple AK
20-01-05, 08:32 PM
Although the bolts only have a 3/8 hex head they will have a 3/8 thread, so not too much chance of them shearing :thumb: Probably better to try undoing them with the engine hot ;)

20-01-05, 11:12 PM

I agree with Chris. Those bolts are pretty stout and don't shear off easily. I struggled with the same problem (exhaust leaks near the center of the manifold) before I learned an old racing trick. Soak the composition exhaust gaskets in the bathtub overnight before you install them. This softens the material and lets it conform the the imperfections of the manifold flange better. It worked great for me!


21-01-05, 04:33 PM
Thanks for the tips, especially the one from Rallysnake about soaking the gasket. The leak is in the centre of the manifold so it sounds like a common problem.

21-01-05, 04:40 PM
Hi there if you are running tubular headers check the flange with a straight edge you may find it has a slight bow in it (worth checking whilst your changing the gasket)

21-01-05, 05:07 PM
Summit Racing (and others) sell soft copper header gaskets with a raised deformable embossed ridge to ensure positive sealing.

SCE Pro Copper exhaust gaskets have a unique O-ring seal around the exhaust port for a tight, leak-proof seal. Made with solid annealed copper, they help even out heat flow between the head and header flange to minimize warping. They'll conform to rough or damaged surfaces to maintain a good seal, and keep header bolts tight.
[/quote:b900b6a0da]I used these on my leaking headers and they have sealed perfectly. I had previously used the composite type and found these hardened, cracked and blew out.