Cylinder Head Lapping Question

rohmell

Active Member
Jun 2, 2010
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New York
I have an engine here that has slight leakage of the '80cc' slant cylinder head near the front, toward the exhaust post side.

There are some posts here that say this type of head is prone to warpage, so probably that is what is happening to this one.

The head has a raised lip or ring around it, so if I were to lap the head, I would just be lapping this inner ring ,and not the larger square surface of the head where the head gasket rests upon.

I thought that the seal is made by the entire gasket flange, but it looks like the raised ring of the head indents and pushes down upon just a tiny bit of the head gasket, only around the inner circumference of it, and pushes it down into the cylinder bore, instead of pushing down upon a flat surface.

Wouldn't it be better to remove this raised ring and then lap the head so that the square surface that makes most of the contact with the head gasket is the part that is being lapped and flattened?

More gasket surface area that is flat makes a better seal, no?
 

scotto-

Custom 4-Stroke Bike Builder
Jun 3, 2010
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Ridin' inSane Diego, CA.
A couple of pics would help here as I'm trying to picture the ring you are contemplating removing. Normally the surface of the cyl. head mates flat on the top of the cyl. with the gasket sandwiched flat in between.
 

2door

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I know what ring he's talking about. Most of the ones I've encountered were only a couple of thousanths thick. I've lapped them off and never had a problem. I've probably increased the compression by a little but not much. There usually just isn't that much material there.

While you're lapping the head you want to check the mating surface of the cylinder too. I use a piece of glass with 400 grit wet cloth paper to check the cylinder. Just make sure to clean the grit out before reassembly.
Tom
 

rohmell

Active Member
Jun 2, 2010
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For Scotto,
See pic with arrow pointing to raised ring:
cylinder ring arrow.jpg


Here is another pic, looks like it's more that a few thous thick.
head ring another.jpg
 
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GearNut

Active Member
Aug 19, 2009
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San Diego, Kaliforgnia
That ring is the gasket surface of the cylinder head. Some folks only sand down enough to insure that the ring is truly flat, others sand the head enough to remove the ring completely.
The reason for the ring is to concentrate all the squeezing force of the assembly into a small area. This increases the psi load on it thusly squishing it with more force.
 

scotto-

Custom 4-Stroke Bike Builder
Jun 3, 2010
6,508
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Ridin' inSane Diego, CA.
For Scotto,
See pic with arrow pointing to raised ring:
View attachment 36125


Here is another pic, looks like it's more that a few thous thick.
View attachment 36126
Great pics. Well unless the cylinder has provision for the ring to seat in, I'd mill that ridge away and improve the mating surfaces, thus eliminating any leak from the combustion chamber. If it was meant to have that ridge, the cylinder would reflect that and would use a circular gasket (prolly copper) instead of the typical square shaped aluminum gasket with the four bolt holes in it.

Kinda looks like they forgot to machine that head surface flat before it was installed as none of mine ever looked like that.......who knows?

dnut
 

scotto-

Custom 4-Stroke Bike Builder
Jun 3, 2010
6,508
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Ridin' inSane Diego, CA.
That ring is the gasket surface of the cylinder head. Some folks only sand down enough to insure that the ring is truly flat, others sand the head enough to remove the ring completely.
The reason for the ring is to concentrate all the squeezing force of the assembly into a small area. This increases the psi load on it thusly squishing it with more force.
That's a pretty cheesy way to seat an aluminum head gasket......sheesh.
 

rohmell

Active Member
Jun 2, 2010
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New York
The reason for the ring is to concentrate all the squeezing force of the assembly into a small area. This increases the psi load on it thusly squishing it with more force.
Yeah, well it's concentrating the squeezing force upon air, and the inner part of the gasket is being forced down into the cylinder.
 

DaveC

Member
Jul 14, 2010
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Boise, ID
The gasket is made from what is called "dead soft" aluminum. It's ment to be crushed for a seal. Big problem is uneven torque.

You can use the old gasket but new ones are cheap. Use Permatex Copper Spray-a-Gasket and use a torque wrench($20 for an inch pound one from Harbor Freight) Torque up in stages, running it right to torque will cause leaks. Try not to go in a circle, a "Z" or "X" pattern is better.

If you have acorn nuts throw them away and get good metric nuts from ACE or a nut and bolt house
 

GearNut

Active Member
Aug 19, 2009
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That's a pretty cheesy way to seat an aluminum head gasket......sheesh.
It is actually quite a common practice by many motorcycle manufacturers, or so I have seen many times when doing top end rebuilds on 70's and 80's dirt bikes when I was a kid.
Although. now that I think about it, Hodaka never did it that way....
Hmmmm.....
 

scotto-

Custom 4-Stroke Bike Builder
Jun 3, 2010
6,508
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It is actually quite a common practice by many motorcycle manufacturers, or so I have seen many times when doing top end rebuilds on 70's and 80's dirt bikes when I was a kid.
Although. now that I think about it, Hodaka never did it that way....
Hmmmm.....
Not quite the same, I've done plenty of top-end rebuilds on my own bikes back then, these are cheesy soft aluminum head gaskets on the china engines, part of why these motors are cheap and disposable.
 

HseLoMein

Member
Oct 30, 2008
124
2
16
Boston, MA
You dont have to throw away the acorn nuts, if they are the acorn nuts that come with the kit, a pair of vice grips will crush the acorn part of the nut and make it a regular nut, unlike the american made acorn nuts, the chinese is a thin cap tack welded to the nut, where as the american version is cast then machined
 

2door

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Sep 15, 2008
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You dont have to throw away the acorn nuts, if they are the acorn nuts that come with the kit, a pair of vice grips will crush the acorn part of the nut and make it a regular nut, unlike the american made acorn nuts, the chinese is a thin cap tack welded to the nut, where as the american version is cast then machined
Which is exactly why we suggest getting rid of them. Cylinder head nuts are not the place to save a few cents.
Not only do they sometimes bind on the studs but sometimes the studs are too long and the acorn nuts will bottom out before tightening against the head.
For less than $5.00 you can eliminate any problems you might encounter with those pretty little chrome plated acorns.
Tom
 

Mike B

New Member
Mar 23, 2011
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That's right. Spend a buck and do it right. 6mm nuts are not that expensive.

You have to spend ten bucks to get a good petcock so your motor doesn't burst in flames from the gas leaking out of the POS petcock they give you.

laff
 

Eric2.0

New Member
Sep 22, 2010
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NY
Sweet thread. I took mine apart today and saw stuff that made me wonder.

I'm planning to sand that ring completely away and make sure both the head and cylinder mating surfaces are perfect. Some copper spray seal and top quality nuts and washers...perfect tourque too.

Let us know how it turns out, Rohmell.

Which brings up the question....Who or how can I get or make a better head gasket?

EDIT: nevermind...I found improved head gaskets and hardware at SBR website.
 
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rohmell

Active Member
Jun 2, 2010
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New York
A couple of passes on some sandpaper on a flat surface and the leak is gone, the ring is still there, I didn't machine it off.

...Another easy fix that should have been taken care of at the factory...
 

Eric2.0

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Sep 22, 2010
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A couple of passes on some sandpaper on a flat surface and the leak is gone, the ring is still there, I didn't machine it off.

...Another easy fix that should have been taken care of at the factory...
Ok, I'll do the same, just a few passes on some sand paper.

Yes, should of been done at the factory along with a test run, bu tI guess this is why we only pay 100 bucks for them and need to accept that we'll have to do more work to actually get it to run.

I'm starting to accept this reality, but I sure would pay extra if they got all this BS done at the factory so I could bolt and go.
 

ScreaminDemon

New Member
Mar 29, 2020
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Yeah, well it's concentrating the squeezing force upon air, and the inner part of the gasket is being forced down into the cylinder.
yes that ring is supposed to go slightly down into your cylinder. if you look your cylinder is slightly beveled to allow the head to slide down inside it. the gasket your using is to small and needs to be cut back so that the head and cylinder can mate like their designed to. seems that its a new design and i have seen it on every new stock head and kit that I've gotten recently. it helps to raise the combustion.
 

kelly dean

Active Member
Oct 3, 2019
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kentucky
the raised ring on the chinese 2stroke does not go into the cylinder the way many after market HC heads do. As a general rule i true the ring on the head and i also true the cylinder top on my builds using a glass table top with wet/dry paper taped down. I also throw away the aluminum gasket and replace it with a quality copper unit. With a stock head you have to be careful how tight you torque those down they are thin and will warp quite easily if over tight, i wish i could give you a torgue number but i just work by feel when i do them. happy trails....kelly
 
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