Quick Q

[Bryan Boyle]

thread size for the exhaust studs on the head side and downpipe to manifold?  5/16x18?  Thinking cast iron is softer than steel, so coarser threads are used vs threading in steel...

I am refreshing a good manifold (my original one has 2 cracks on the pipes through the flanges...so...tossed in the metal bin for recycling...); had to make a plate up to cover (even though I know that there is no communications between the exhaust and the warming chamber...).  3 of the 4 studs were broken at the surface...so, drilled out and retapped (1/4x20 threads according to the one that was ok...), made up a plate out of 16 ga steel, and cap nuts/washers to hold it in place.

If anyone has a USEABLE intermediate pipe from the downpipe to the muffler, I'd be interested in hearing...otherwise off to the local exhaust shop to bend up one.

[BDA]

In my experience, coarse threads are always used in cast items.

[Bryan Boyle]

In my experience, coarse threads are always used in cast items.

'swhat I figured.  That and soft aluminum...

[BDA]

I was never really sure why coarse threads were used over fine threads in castings. I figured it was because castings might be more likely to have imperfections. This webpage gives some good reasons for to use one thread pitch over another:

https://www.usfastenersources.com/pdfs/coarse-vs-fine-thread.pdf

[jbcollier]

Coarse threads have physically thicker “threads” for greater strength.

Yes, some morons use fine threads in castings, even aluminium castings.  Triumph TR6 rear hubs attach to the aluminium upright using fine threaded studs.  Yes, they have been known to strip.  Easy to heli-coil though.

[BDA]

That's the first time I've ever heard of fine threads in a casting and the result is pretty predictable, I guess.

[Grumblebuns]

I think I have an extra. Let me check and get back to you.

[Blitzen]

My studs were 5/16-18 into the head but 5/16-24 for the manifold.  But don’t know if they were original.   My replacement studs from RD were the same.

[BDA]

Yeah, that’s the way I’ve always seen it - the coarse end goes into the casting and the fine end has the nut.

[Bryan Boyle]

Yeah, that’s the way I’ve always seen it - the coarse end goes into the casting and the fine end has the nut.

I was just going to dispense with the use of a 5/16x18-5/16x24 stud and go with an x18 cap head screw to attach the downpipe to the manifold.  There isn't much free space between the mounting holes in the downpipe flange and the pipes themselves to get a nut on a stud poking thru the flange...so, this may be a better solution.  I'll hack it out and see; I've the cap head bolts downstairs.  And a sheet of exhaust gasket material to make a gasket for the downpipe to manifold...

[Grumblebuns]

OK, I have a stock down pipe still attached to the exhaust manifold. The reason why I had to remove the manifold and downpipe as a unit was the frozen nut on the back stud. Let me work in trying to remove that nut and separate the pair. Asking $35 for the pipe, won't be needing it since I may go with headers, still undecided.

[jbcollier]

Repeated heat cycles and high temperatures means this is a prime location for corrosion.  Very common for studs, nuts and bolts to fuse, or erode away and shear when you go to remove them.  What survives best, and most easily removed later on, are good quality studs with brass nuts.  You can get brass nuts with 5/16NF threads but 7/16” across the flats for easier fitment.

[Grumblebuns]

IF I decide to reuse the stock headers, I will be using new studs and brass nuts.

Bryan, from the pictures attached, the section you are looking for is welded to the 2 into 1 collector. It might be quicker to have a muffler shop fabricate the pieces you need rather than use a potentially rusted out piece. I would check with RDEnt to see if they have new reproductions of the exhaust section.

Joji Tokumoto

[Bryan Boyle]

IF I decide to reuse the stock headers, I will be using new studs and brass nuts.

Bryan, from the pictures attached, the section you are looking for is welded to the 2 into 1 collector. It might be quicker to have a muffler shop fabricate the pieces you need rather than use a potentially rusted out piece. I would check with RDEnt to see if they have new reproductions of the exhaust section.

Joji Tokumoto

Yeah, that's kind of toasted.  Thanks for looking, Joji. 

Checked with Ray...only sells the pipe as part of the set...and I'm taking his header off...maybe the intermediate pipe will fit...if not, will take it to a bender and have them repop at the proper angle (it's not 90 degrees, fwiw.)