The Yamaha Road Star Warrior Forum

Ok so I was at the local HD dealership today, MAYBE looking at a muscle vrod, but anyway, was looking at a fat bob and the salesman was saying that to get more compression i.e. power you can replace the head gaskets with thinner ones.  Stock is .0080 and going with a .0040 gives the motor more compression.  I'm not exactly clueless in the motor department, but I've never heard of something like this.  Is this BS or is there something there?

Its an old school mod that still works.

The question is, doing the math, what do you really gain in the real world.  It doesn't take much distance to increase compression but there's often other factors affecting HP gains to the rear wheel.

Churchkey:

Its an old school mod that still works. 

Churchkey:

Its an old school mod that still works.

Arizona Warrior:

The question is, doing the math, what do you really gain in the real world.  It doesn't take much distance to increase compression but there's often other factors affecting HP gains to the rear wheel.

Churchkey:

Its an old school mod that still works. 

After head milling or installing a thinner head gasket there are several other issues that need to be addressed.

If the cylinders share an intake manifold the manifold to head mounting flanges will need to be machined. With a "V" design engine moving the heads closer to the crank centerline decreases the distance between the intake ports. This is not a concern if the engine uses individual intake manifolds.

The exhaust system is normally not a problem.

The most important concern is rocker arm geometry, push rod length & hydraulic lifter design. Do the hydraulic lifters have enough working room to live with the pushrod cup (using the above specs) .040 deeper in the lifter chamber? I would have to have adjustable push rods to get the lifters back to factory specs. Adjustable rockers are not the answer as the fulcrum point changes & the relationship between the rocker arm & valve stem changes. The motor will run howver valve guide wear will be increased.

Also:

If the lifter cup can handle living .040 deeper in the lifter bore there may be a problem at the high end of the rpm scale. If the lifters pump up & create valve float they will move the push rod end of the rocker arm .040 higher than normal valve float. Add to that the rocker arm ratio & lets use 1.5 as an average, the valve is now .060 closer to the piston. Add the .040 for the thinner head gasket & the valve is now .100 closer to the piston during valve float. This could have disasterous results but is dependent on combustion chamber design, piston design, factory valve to piston clearance & camshaft profile.

In conclusion: There is a bit more to this mod than just installing thinner head gaskets to increase compression.

#1.  I don't the answer to your question.

#2.  The reason for #1. is that I stopped reading after you said you were looking at a HD.

I'm not familiar with the specifics of the Warrior motor in this way dimensionally.  Would you hazard a guess the Warrior motor could possibly live with these parameters including the pushrod length, with heavier valve springs as long as stock pistons are used?  The compression increase would be marginal and HC piston kits are probably a better option but just curious.

Churchkey:

 

 

Arizona Warrior:

The question is, doing the math, what do you really gain in the real world.  It doesn't take much distance to increase compression but there's often other factors affecting HP gains to the rear wheel.

Churchkey:

Its an old school mod that still works. 

After head milling or installing a thinner head gasket there are several other issues that need to be addressed.

If the cylinders share an intake manifold the manifold to head mounting flanges will need to be machined. With a "V" design engine moving the heads closer to the crank centerline decreases the distance between the intake ports. This is not a concern if the engine uses individual intake manifolds.

The exhaust system is normally not a problem.

The most important concern is rocker arm geometry, push rod length & hydraulic lifter design. Do the hydraulic lifters have enough working room to live with the pushrod cup (using the above specs) .040 deeper in the lifter chamber? I would have to have adjustable push rods to get the lifters back to factory specs. Adjustable rockers are not the answer as the fulcrum point changes & the relationship between the rocker arm & valve stem changes. The motor will run howver valve guide wear will be increased.

Also:

If the lifter cup can handle living .040 deeper in the lifter bore there may be a problem at the high end of the rpm scale. If the lifters pump up & create valve float they will move the push rod end of the rocker arm .040 higher than normal valve float. Add to that the rocker arm ratio & lets use 1.5 as an average, the valve is now .060 closer to the piston. Add the .040 for the thinner head gasket & the valve is now .100 closer to the piston during valve float. This could have disasterous results but is dependent on combustion chamber design, piston design, factory valve to piston clearance & camshaft profile.

In conclusion: There is a bit more to this mod than just installing thinner head gaskets to increase compression.

 

 

Arizona Warrior:

I'm not familiar with the specifics of the Warrior motor in this way dimensionally.  Would you hazard a guess the Warrior motor could possibly live with these parameters including the pushrod length, with heavier valve springs as long as stock pistons are used?

Churchkey:

 

 

Arizona Warrior:

The question is, doing the math, what do you really gain in the real world.  It doesn't take much distance to increase compression but there's often other factors affecting HP gains to the rear wheel.

Churchkey:

Its an old school mod that still works. 

After head milling or installing a thinner head gasket there are several other issues that need to be addressed.

If the cylinders share an intake manifold the manifold to head mounting flanges will need to be machined. With a "V" design engine moving the heads closer to the crank centerline decreases the distance between the intake ports. This is not a concern if the engine uses individual intake manifolds.

The exhaust system is normally not a problem.

The most important concern is rocker arm geometry, push rod length & hydraulic lifter design. Do the hydraulic lifters have enough working room to live with the pushrod cup (using the above specs) .040 deeper in the lifter chamber? I would have to have adjustable push rods to get the lifters back to factory specs. Adjustable rockers are not the answer as the fulcrum point changes & the relationship between the rocker arm & valve stem changes. The motor will run howver valve guide wear will be increased.

Also:

If the lifter cup can handle living .040 deeper in the lifter bore there may be a problem at the high end of the rpm scale. If the lifters pump up & create valve float they will move the push rod end of the rocker arm .040 higher than normal valve float. Add to that the rocker arm ratio & lets use 1.5 as an average, the valve is now .060 closer to the piston. Add the .040 for the thinner head gasket & the valve is now .100 closer to the piston during valve float. This could have disasterous results but is dependent on combustion chamber design, piston design, factory valve to piston clearance & camshaft profile.

In conclusion: There is a bit more to this mod than just installing thinner head gaskets to increase compression.

 

 

An option that MAY be possible with the warrior engine is the head gaskets are  composed of 3 layers that are held together with 2 rivets.If you remove the rivets & separate the layers you MAY be able to just use 1 or 2  of the layers thus increasing compression but with out doing the math I don't think  in this case the gain would be worth the cost of new gaskets & the time required to replace them!!!

Mike

All joking aside, I don't ever plan on parting with the Warrior.  After years of never liking Harleys, I gotta admit that some of their newer models are more in line with my taste than the AMF's or Roadsters/sportsters/dynas were. IMHO only. 

I just had never heard of thinning a head gasket to gain power.  Seems to be a lot of work for a little bit of torque.  If I was looking at a total performance rebuild with 10k to spend on a motor it might factor more into my thought process.  I'd have to get a lot more info to want to do it and not worry about it blowing out.

Let me jump in these with a bit of mechanical expertise because I taught Woody everything he knows. Here you go are you ready?  What Woody said!