Why an O-ring?

[scottscar54]

So I had an interesting question I wanted to pose......

I have been working on cars for 35 years now. Every car I have touched with a a carburetor had a gasket between the intake manifold and the carburetor, and also one between the intake manifold and heads. The old carbed muscle cars also suffer from vacuum issues when leaks occur similar to the bike.

So that got me to thinking, why the o-ring on the intake boots instead of a high quality heavy gasket. I have made my own gaskets from the Fel-Pro thick gasket material more times then I can remember and it has held up even on the toughest of applications..... such as my supercharged big block with twin 4 bbl carbs and no vacuum leaks, and even high pressure coolant in a thermostat housing where if the gasket is not perfect, it will without a question fail and of course you all know what happens then.

What do you guys all think? Anyone know why motorcycle manufacturers went with an o-ring rather than a heavy duty gasket? The gasket material compresses when torqued down between two milled surfaces.

Was just curious as it does seem like gaskets would work wll there too but of course are limited lifespan as well and definitely wouldn't hold up to multiple removals and reinstalls like an o-ring might.

[92xjunker]

Same thought, here. I would assume a full gasket would work better, just keep the material out of the passage and the cleaner and exact cut would be the goal.

[arsenic]

That's a very good question. It made me think. I have no idea what their reasoning was.

[92xjunker]

Most likely....cost

[arsenic]

Most likely....cost

+1

[scottscar54]

That is the only the thing I could think of was cost and ease of production.... an o-ring mold must be a lot cheaper then building a stamp for the gaskets....

I don't see a reason from a mechanical point that should cause a problem.

I may give it a shot and see what the results are.

[steves]

I think some of the old bikes had gaskets there. I feel like the oil cooled GS bikes did. I also think the gasket breaks apart more easily under the heat from the oil cooled motor. But what do I know, I'm not an engineer.

[scottscar54]

Heat might break down thin paper gaskets.... but the stuff I use is the heavy Fel-Pro felt material that is very very thick and is what I have used for years on big block heads, thermostat gaskets, and carb gaskets on cars..... thinking that type of material holding up to the immense pressure and heat of a big block cylinder head it should hold up to the intake boot to head mounting.... or at least that is what it seems like

I just got done cutting out matched sets on some fel-pro I have here in the shop.

Key I found is to do each boot individually..... lay the boot onto the material and trace it out with sharpie.... then once the rough shape is cut out.... turn the boot upside down and lay gasket on top.... I then used an exacto knife and carefully cleaned up the bore of the gasket to match exactly to the bore of intake boot to eliminate any airflow issues.

Got them mounted up and they seem pretty solid. Didn't take long at all. Not sure there is a savings since the sheet of heavy felt fel-pro was $20 but the sheet I have that has been lying around my shop so no cost to me and because the gasket sheet is so big I can still do a ton of other gaskets and stuff with it. If it works right, the big advantage is being able to get the gasket sheet at any local auto shop so that would eliminate having to wait on an order of o-rings.

Will post info on how they work.

[steves]

Each carb boot is a slightly different shape, they are position specific. I'd be worried about the void that's left replacing the oring with felt.

Also I've tried using high temp rtv as a temp fix and that doesn't work.

[scottscar54]

Hi Steves.

I kept there seems to be enough material on the boot itself to create two solid concentric rings of sealing on either side of the o-ring groove with more than 1/8" of gasket width on the narrowest and more on the outside of the o-ring groove .

By going right up to the internal bore and trimming back, get a full width of gasket all the way across.

Correct on the boots being individual, which is why I did the gasket one at a time using each gasket as it's own template.

I have to wait on new o-rings anyway so seems like it's worth a shot to give it a try.

Will post what I figure out for results.

[scottscar54]

Gaskets are done and installed but ran into new issue with a bad coil.

Long story short, 1&4 have weak/no spark and resistances don't check out on that coil so waiting on new coil..... the 12v lead is getting 13 volts and and only dropping to 11v when cranking so good there.

2&3 work great though, turns out even holding the boot you can still get shocked LMAO... that one is definitely developing voltage correctly and the resistance checks were all good on that one.

Will know more once new coil gets here and I can button get all 4 plugs firing right.

Will update on the gaskets once it is all running right.

[scottscar54]

Finally got around to tinkering with the bike with the gaskets in.

They do work but I kept the flattened o-rings in to give more surface area and that likely creating a small vac leak because will get a slight bit of surging.

Looking at the surface of the intake boots, there is enough surface area that the gasket should work without the old flattened o-ring. Will tinker some more and update but if it works, may be a nice solution for that problem.