Manifold Spacers Shrink with time and heat - sanding them smooth again

[VFR750]

The composite spacers used to isolate the manifold from the cylinder head are made from carbon fiber reinforced plastic. I have observed they shrink over time, and it is probably due to the heat from the head. Or may be an artifact from the initial injection molding. Doesn't really matter.

Unfortunately, the shrinking occurs in the thicker section, and is not uniform. The end result is the sealing between the manifold, gaskets and heads deteriorates.

I thought it was always strange how I'd install new gaskets and the carbs would run great. Then, over a period of time a faint idle popping would develop. Additionally all of the nuts lost their preload, and I'd re-torque them.

While there are many factors that can cause idle popping and loosening nuts, I think it is safe to say, no one would ever intentionally install non-flat spacers and expect the gaskets to work properly.

So this is a winter project: make them flat again.



The original spacers were probably .25" thick. One without any sanding measured .248"

Two were sprayed with white lacquer to highlight the effects of sanding.

You can see that sanding the upper spacer only removed material from the perimeter. This is true on both sides of the spacer.

The lower spacer is the result of 10-15 minutes of sanding both sides, and frequent checks with a micrometer to insure I was not sanding in a taper. Amazing amount of material has to be removed.

Based on these dimensions, the center of these spacer have shrunk in thickness about 0.025" verses the edges. That's huge.

If you are using thermal spacers on your manifold (carbs/MFI/EFI) you might have this problem.

[john walker's workshop]

84-89 carrera spacers do the same thing.

[VFR750]

Yup, These too.

Do they Stop shrinking?