Lapped my head

Pablo

Master Bike Builder & Forum Sponsor
Dec 28, 2007
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Did the Norman wet paper, water and dish soap on glass thing.

Took just a few thousands off, maybe about .005"-.008". It 's a bit hard to measure without a proper height gage. I used the butt of a caliper. I took enough material off to eliminate the small dent/gouge shown below. I had only one freak out moment - I thought I sheared a head stud - but it was a loosened stud spinning until seated. Phew.

Anyways it started up like it's usual happy self, runs like a champ. Can't say I felt much power improvement during my short ride, maybe a tiny bit more low end grunt.
 

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Easy Rider

Santa Cruz Scooter Works
Jan 15, 2008
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Pablo,
If you want a cheap way to raise your compression, try using a NGK B8HS plug. That's what I did for hundreds of miles on my bike before I sold it. When I last checked it, the plug was in perfect shape and the color was a nice brownish tan color.
 

Pablo

Master Bike Builder & Forum Sponsor
Dec 28, 2007
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Pretty quick and easy to lap it. I was mainly doing it to make sure it is flat and remove the factory ding - plus have some relaxation. I run a NGK B7HS
.....and it too seems perfect....I'm really wondering if anything fro B4HS-B8HS will work fine.
 

jasonh

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Jun 23, 2008
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Have you done any calculations to determine how much you raised the compression?

I'd like to know how a spark plug can raise your compression...maybe the plug is bigger or something?

Oh and I believe that raising the compression should make the motor a tiny bit more efficient. Maybe keep an eye on your fuel consumption?
 

Pablo

Master Bike Builder & Forum Sponsor
Dec 28, 2007
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Have you done any calculations to determine how much you raised the compression?

I'd like to know how a spark plug can raise your compression...maybe the plug is bigger or something?

Oh and I believe that raising the compression should make the motor a tiny bit more efficient. Maybe keep an eye on your fuel consumption?
I don't know the volumes so I can't calculate it. I colder plug could have more volume at the insulator....

I don't keep track of fuel consumption that closely. I am also under the impression that everything else in good order, raising CR can get a boost to low end torque.
 

MrLarkins

HS Math Teacher
Jun 17, 2008
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i've lapped cpu blocks for better heat transfer, but what are the physics behind lapping your cylinder head?
 
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jasonh

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Jun 23, 2008
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shaving a wee bit off the bottom of the head will make your combustion chamber a little bit smaller, thus increasing the compression of the motor. This will give you a little more power, and a little more efficiency.

Plus, making sure it is flat and true will help reduce head gasket leaks.
 

Pablo

Master Bike Builder & Forum Sponsor
Dec 28, 2007
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i've lapped cpu blocks for better heat transfer, but what are the phycis behind lapping your cylinder head?
The physics?
1) Flatter for better seal (not that there was a problem)
2) Smoother for better seal (got rid of some Chinisesisms - see photo)
3) Slight material removal (with a slight compression ratio increase With better fuel air squish, perhaps efficiency improves a tad - as long as there is no pre-detonation.)
 

Pablo

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Dec 28, 2007
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Thanks - good input.

I went for a very long ride today after I fixed the fuel valve.....for the first time in a long time the beast just wasn't running right on the way home....sure enough the head nuts needed a lot of turns to get them back to 15 ft pounds. Basically they were loose! I should have retorqued them before my ride :duh: :headslap:
 

MrLarkins

HS Math Teacher
Jun 17, 2008
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shaving a wee bit off the bottom of the head will make your combustion chamber a little bit smaller, thus increasing the compression of the motor. This will give you a little more power, and a little more efficiency.

Plus, making sure it is flat and true will help reduce head gasket leaks.
i must be seeing that wrong in my head...but it seems to me that removing material would make the chamber larger and thus have lower compression...:confused:
 

Skyliner70cc

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Mar 8, 2008
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Pablo,
I hope to heck it was the head bolts and not the bottom of the threaded head stud that backed out and loosened your head. This is a very common problem which is why I JB weld or red locktite the bottom headstud threads into the block.
 

jasonh

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Jun 23, 2008
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Pablo,
I hope to heck it was the head bolts and not the bottom of the threaded head stud that backed out and loosened your head. This is a very common problem which is why I JB weld or red locktite the bottom headstud threads into the block.
I'd hate to see what happens when you have a JB Welded stud snap off in the block...

A properly sized and torqued quality stud should not back out.
 

Pablo

Master Bike Builder & Forum Sponsor
Dec 28, 2007
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The loosening is normal (I guess) with these heads and new Al gaskets. After about 3 cycles of heat-cooling-retorquing the torque holds. I'm betting there are a lot of folks who need their head nuts torqued.