Exhaust gasket dimensions, unused and compressed

[arcticXS]

Hi!
First a little background to my question: These days I am fiddling with the cylinder head for my engine build, and I am looking at machining a couple of straight alloy inserts for the step in the exhaust port. On the -72 or -73 head I have, the ID of this step is 39.50 mm, and the big diameter for the exhaust pipe flange is 50.00 mm. The exhaust port is around 34 mm upstream of the step, and my headers are 1 1/2" OD, which gives approximately 35 mm ID.

Now, if the gasket ID is significantly larger than 35 mm, and the insert is flush with the gasket surface, I will still have a sort of step or groove where the headpipe meets the cylinder head. If, on the other hand, the insert is made a little longer, and the gasket sits AROUND the machined insert, then there will be a smoother transition from port to headpipe. So if the length of the insert protrudes about the same as a compressed gasket, the insert would butt up against the headpipe, and at the same time the gasket gets crushed just enough.

So here are my actual questions:
Thickness of a new gasket? (Are Mike's different from OEM?)
Thickness of a used gasket?
Inside diameter of a new gasket?
Inside diameter of a used gasket?

I assume that a gasket with inside diameter of around 40 mm and outside diameter somewhat less than 50 mm would be perfect for what I suggest.....If any other bikes have exhaust gaskets of this size, please let me know!

All replies welcome

 

[FrkyMnky]

I can get some dimensions for you tomorrow, but regarding Mikes vs OEM, there is a huge difference. Mikes are paper with a metal wrapping of some sort while the OEM ones are real copper/woven metal crush washers.

 

[5twins]

There's a better way. I made a filler ring out of some pipe .....

..... and then an insert from the ends of old stock headpipes. The two parts fit tightly in the head, filling the step, then the gasket and headpipe go over them .....

 

[FrkyMnky]

Another option, and the way the pipes I got from Dan at Pandemonium are made, is to use 1.5" tubing which will slide into the step and then have a collar that is welded on where the gasket needs to sit. Similar to what 5twins posted except its all 1 piece

 

[arcticXS]

There's a better way. I made a filler ring out of some pipe .....

..... and then an insert from the ends of old stock headpipes. The two parts fit tightly in the head, filling the step, then the gasket and headpipe go over them .....

5twins,
What size headpipes are you using? Mine are 38,1mm OD (1 1/2") and 35 mm ID. As the step ID is 39.50 mm, and the exhaust port is around 35 mm upstream of the step, I cannot see how the piece from the old headpipe could fit, unless its ID is smaller than around 32-33 mm. This then will create a restriction, is this intended or not?
My thoughts were to mainain a constant 34-35 mm diameter from upstream of the step all the way into the header, with as little disruption as possible, hopefully promoting flow.

 

[5twins]

I have MAC headpipes that are 1.5" O.D. I'm not sure what the I.D. is. To make the insert fit, you must peal off the outer layer of the original double wall headpipe. In my second pic, you can see the part of the insert that fits in the pipe has been ground and filed some. You can also see the inner layer of the double wall headpipe remains.

I guess you haven't heard about headpipe torque inserts. They are a performance mod that fools the motor into thinking it has smaller diameter headpipes, which it likes up near the head. However, a larger diameter headpipe is better farther away from the head for flow. With an insert, you can have both.

The torque insert part wasn't my intention when I made these up. I was just trying to come up with a better, leak-free mounting method at the head. This worked wonderfully for that. When I learned about the torque inserts later, I realized I had inadvertently made my own. The bike ran much better after adding these, becoming must freer revving. I'm sure plugging the exhaust leaks helped with that but I'd like to think the torque insert part did as well.

 

[arcticXS]

5twins,
If your headers are 1 1/2" (38.1mm) OD, then they must be flared out near the flange or head, or your cylinder head is different from mine. The step inside diameter in my cylinder head is 39.50 mm, and on your pictures, it looks like both ends of your insert has about the same diameter. I cannot understand how an insert with 39.50 mm OD can slip inside a pipe with 38.1 mm OD, or am I totally missing something here?
My headers (Heiden/MikesXS 1 1/2") have no flare, so the ID is around 35 mm all the way to the sealing surface. For any insert to slip inside the pipe, the insert OD must be less than 35 mm.

 

[5twins]

If your headpipes are like originals with an extension into the head off the sealing flange, you won't be able to do what I did. My pipes are like many aftermarket ones and just have a flared out flange on the end to seal against the seal ring, they don't insert into that step in the port. That was the problem with mine - that flange was too small to seal correctly because it wouldn't stay centered on the seal ring. My insert keeps it perfectly centered on the seal ring.

Take a look at the torque inserts on Mike's site .....

http://www.mikesxs.net/product/07-0769.html

They are made to be used on headpipes that have no insert into the port step of their own. They are made to be used on pipes like mine that only have a flange.

Filling that step in the port is a good thing. Racers have been doing it for many years. I'm just not sure how you could do it successfully on headpipes like the stockers.

 

[arcticXS]

5twins,
Thanks for the pics, links to MikesXS and other info, but I am still not getting the answer to the question I asked, for the pipes I have:
What is the outside diameter (OD), inside diameter (ID) and thickness of an XS exhaust gasket, when new, and used/compressed? Please specify if it is a MikesXS or Yamaha gasket.
(Quote from my previous post: "I cannot understand how an insert with 39.50 mm OD can slip inside a pipe with 38.1 mm OD, or am I totally missing something here?")

My pipes are aftermarket single wall 38.1 mm OD, around 35 mm ID, ALL THE WAY TO THE GASKET SURFACE. The flange/flare is actually a 10 mm high ring welded to the pipe. No stub that extends into the step.

So any "Torque Insert" or "EPO" will have to be less than 35 mm OD, in order for my pipes to fit, meaning the inside diameter of said TI/EPO will have to be 32 mm or less, significantly smaller than the exhaust port itself. I have tried to read up on recommended header diameters for the XS, including the use of "EPO" and "TPO". It seems to me these inserts are useful when using 1 3/4" and 1 5/8" headers, but 1 1/2" headers (35 mm or 1 3/8" ID) should work fine without any inserts, according to multiple members on this and other forums.