Thicker head gaskets to lower compression

[SML Boomer]

When my 1988 100XP was rebuilt the mechanic used "thicker" head gaskets to lower compression and compensate for lower octane unleaded gas. From what I am reading this was necessary on motors from 1985 and older that were set up for leaded gas. Since my manual states use of 87 octane unleaded was it necessary or beneficial to use the thicker gaskets? Would slightly retarding the timing have the same effect? If he retarded the timing 2 degrees and used thicker head gaskets would the impact on power be noticeable? Also, is the OMC part # 0318358 the standard gasket I would use if I changed it back?

 

[jtexas]

Re: Thicker head gaskets to lower compression

"1985 and older" set up for leaded gas? is this some kind of joke? He's off by at least 10 years.

 

[iwombat]

Re: Thicker head gaskets to lower compression

Leaded vs. unleaded only makes a difference if you've not got self-lubricating valve guides. Since your two-stroke doesn't have any valves at all let alone non-lubricating guides, it's a total non-issue.

If it was designed to run high-octane gas, then lowering the compression to run regular makes sense. Since it wasn't, I think your mechanic did something unnecessary.

 

[LubeDude]

Re: Thicker head gaskets to lower compression

Leaded vs. unleaded only makes a difference if you've not got self-lubricating valve guides.

What the heck are Self lubricating valve guides???????????

The lead was there to raise octain, dampen the shock of the valves hitting the valve seat and stop wear! "PERIOD"!!!!!!!

 

[iwombat]

Re: Thicker head gaskets to lower compression

It's a bit of a misnomer for sure, but every mechanic and automotive machinist I've ever knowns calls the cast-iron or the nickel-bronze type self-lubricating.

Edit:


Straight from the parts catalog:

Rowe Cast Iron Valve Guides
Self-lubricating, easy to machine. . .

 

[SML Boomer]

Re: Thicker head gaskets to lower compression

Hey Jtex! Glad I made you day a little more jovial. The Clean Air Act started in the early 70's with the 1st regulations proposed in 1973. Due to politics and court actions, the final legislation to phaze out lead completely did not get implemented until 1986. There were some exceptions for off-road and marine but I,m sure the outboard manufactures were in compliance well before then.
Reguardless of my questionable dates, the issue of the necessity of thicker head gaskets or retarded timing for an engine designed for unleaded, 87 octane is what I would like addressed by some of the many knowledgable people on this board.

 

[iwombat]

Re: Thicker head gaskets to lower compression

I think I already did address it. Two-strokes don't care if there's lead or not - period.

Octane _can_ be important (but not in your case). But, what methods are used to boost octane, lead or otherwise, doesn't matter.

 

[Dhadley]

Re: Thicker head gaskets to lower compression

The OMC engineers did panic a bit back when the government "got the lead out" (talk about a misnomer.....). They sent out bulletins about thick head gaskets, bigger jets, reduced timing and all kinds of junk. None of which was necessary. Use the standard gaskets with the proper timing and fresh 87 octane and you'll be fine.

 

[jtexas]

Re: Thicker head gaskets to lower compression

Hey Jtex! Glad I made you day a little more jovial. ... Reguardless of my questionable dates, the issue of the necessity of thicker head gaskets or retarded timing for an engine designed for unleaded, 87 octane is what I would like addressed by some of the many knowledgable people on this board.

not much chance of that not happening -- stick around awhile you'll see what I mean. I was trying to express incredulousness (incredulousity?) over what your mechanic told you (as opposed to poking fun at you). I'm old enough to remember leaded gas - but it's strange that someone who knows OMC motors wouldn't know they made the transition mid-70's at the latest.

offense not intended.

 

[SML Boomer]

Re: Thicker head gaskets to lower compression

Thanks everyone. I've learned much on this board over the past month. In this case free advice is worth it's weight in gold. Jtexas yor are right, the mechanic did blow some smoke but hopefully he didn't do anything that can't be undone. He only cut a few wires and left off some parts the factory must have put there by mistake.

 

[JB]

Re: Thicker head gaskets to lower compression

Hmmmmm. Be careful. They may have resurfaced either the head or block or both and really used the thick gasket to restore stock chamber volume.

Be sure that you understand what DHadley has to say. That is gospel.

As far as the general discussion about lead vs. unleaded, 87 octane vs. premium, there are all sorts of myths that some folk just won't let go of.

There is zero benefit to a 2 stroke of tetraethyl lead in the gas, so long as the gas has a high enough anti-knock feature (octane). There never has been. Lead in the fuel was helpful to some 4 stroke car engines to lubricate valve guides. My Corvairs were an example. I had to replace the valve guides to run unleaded.

The 2 stroke that has a a high enough real compression ratio to require above 87 octane is very rare, mainly because the piston is well onto its compression stroke before the ports are closed, allowing compression to begin.

 

[ottertail]

Re: Thicker head gaskets to lower compression

Thicker head gaskets? Where did he find such gaskets? I agree with other posts here, there is no need for changing the head gaskets to thicker ones. The discussion on the OMC panic has to do with Omc service bulletin 2155 dated October 1986. Here Omc was lowering compression by installing thicker gaskets and backing down the timing on as the bulletin states 1970 thru CO 60 thru 2.6 XP/GT. Your engine was built after this scare and was built with all the modifications at the factory. If you are concened asked the tech what gaskets he installed and post that info here. Some one will know if you should change back but would guess you will notice no difference at this point based on the info you have listed.

 

[Benny1963]

Re: Thicker head gaskets to lower compression

i run a set on my 1983 150 crossflow but only because i had to take
0.10 off my xp heads to tru them up from overheat warpage during rebuild
and may not have needed them then but just wanted to drop the comp
although a crossflow gains very little compression from cutting the head it the i brow design in the chamber that canges compression on a crossflow head ,and yes i learned it on here ,i run 87 octane with 28 degrees timeing
low comp gaskets on cut xp heads

 

[Sandhammaren05]

Re: Thicker head gaskets to lower compression

You can lower the timing 3-4 degrees and lose 100-200 RPM that way.
an alternative is to mix in some $40/gal. 105 octane racing gas, bought at an auto specialty store (at times when you want to keep the timing and compression high to blow your neighbor's doors off).

When my 1988 100XP was rebuilt the mechanic used "thicker" head gaskets to lower compression and compensate for lower octane unleaded gas. From what I am reading this was necessary on motors from 1985 and older that were set up for leaded gas. Since my manual states use of 87 octane unleaded was it necessary or beneficial to use the thicker gaskets? Would slightly retarding the timing have the same effect? If he retarded the timing 2 degrees and used thicker head gaskets would the impact on power be noticeable? Also, is the OMC part # 0318358 the standard gasket I would use if I changed it back?

 

[CharlieB]

Re: Thicker head gaskets to lower compression

Flame speed of fuels are measured in feet per minute (FPM) this was the reason for the introduction of lead in gasoline many many years ago. The lead absorbed heat and maintained a consistent flame temp which controlled the burning speed to reduce engine knock and ping.

As EPA began regulating lead amounts fuel manufacturers began substituting other elements into their fuels to achieve the same ends.

Valve seat erosion became a side effect of the reduced lead, engine manufacturers came up with hardened seats.

Two stroke manufacturers THOUGHT the reduced lead would be a problem, they thought wrong.

 

[Sandhammaren05]

Re: Thicker head gaskets to lower compression

I burned leaded 93-95 octane through summer 1985 in Tx. Loss of lead cost 100 RPM.

"1985 and older" set up for leaded gas? is this some kind of joke? He's off by at least 10 years.