Dome diameter larger than cylinder bore question

[rustyhondas]

86r

My jug is 68mm

I got a head that measures 72.4mm across where the dome starts

I can hog out a copper head gasket to match the head

But I am thinking this won't work rite

The head should measure a .25mm larger or so but not several millimeters .
What would happen?[ATTACH=CONFIG]230765
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I wanted to see how she'd run with that head on the right with the big opened up chamber but I don't think
It will work , there is going to be a pocket all around the circumference of
The top of the cylinder between the head , will this do weird things?

[cr480r]

there is going to be a pocket all around the circumference of The top of the cylinder between the head , will this do weird things?

You don't want a pocket at edge of bore for end gases to collect, and you don't need the top edge of bore exposed to any extra heat. It may work just fine, but it's not ideal. I wouldn't use it.

[rustyhondas]

Don't you hate it when you don't hear what you want to hear,but you know it's the rite thing to do

[arlaunch]

In my opinion the cylinder head on the right looks like an experiment gone wrong. It is opened up way to much with the squish band being way to narrow. Not to mention what looks like pretty big craters from detonation.

[bkm]

It was probably off of an oem cylinder with a big bore sleeve.

Sent from my SM-G900V using Tapatalk

[rustyhondas]

Totally agree it most likely is off a "310" jug or 290 or whatever that b astard size liner is

The dome looks like this pretty much to me

What I was trying to achieve it to lower my static compression from 200 to something around 180 all while tightening up the quench gap

So I picked up a cheap head on eBay to play with but it turns out it's a big bore head unbeknownst to the seller or I

The squish still sucks, but it did get me down to 165psi

It's etched into it 23.5cc

Between my short reach compression adapter,wide oversized dome taper,and fat head gasket,making my results low

I think 23.5 might get me rite where I want to be ,maybe 23cc

My next move it to get my original head reshaped

Being that you don't like the look of that right heads dome ,and it basically looks like the end result of BDTs head service,what do you say to that?

I'm planning on having that done. What's the deal with the "CR"
Style dome shape

38 AS carb
.80 over
PT hi rev
Some forgotten porting I had done 20 years ago
90 octane non ethanol boat gas is what I want to run

Play bike

[cr480r]

If you have enough extra volume you might be able to just mill the big bore head until the outer squish diameter is reduced to where you want it. That's a much easier task for a typical machine shop that doesn't specialize in 2 strokes. It seems like once you start talking about changing angles, shape, and volume you usually get a blank stare. Especially if they can't index off the spark plug.

[rustyhondas]

That thought crossed my mind too

I don't know how much is acceptable to take away


Big bore head measures 16.8-16.9 mm thick from
Head stud nut lands to the gasket surface face

I can bring it down myself on this granite table with some oil and sandpaper

What a excellent solution if there is a minimum thickness I need to maintain is the question

And if the .030 or so I need to take it down to gain a few mm is going to get me back in the same boat with high compression

[rustyhondas]

I guess to heck with the compression increase, should be able to find some winning combination out of all of these and can lower the deck height to play with squish

Any last words on minimum head thickness

[cr480r]

Don't even worry about the head getting too thin with that lil cut. I have taken very deep cuts to fix heads that should have been welded with no issues. The problem with deep cuts is the spark plug gets closer to piston, and you have to use a toroid design to restore the volume. You don't even have to worry about that here. Measure the volume of the head and compare it to your old head to see how much room you have to play with. Was your old compression too high? Or do you just want try a narrower squish?

Edit: I re-read your post. I see now you are trying to reduce compression to 180

[rustyhondas]

Measuring like this old head with a million miles on it at 200psi /br8es plug ,running premium pump gas

Reads 35.69mm

New head before cutting measures 35.47

The sides of my piston do not come up flush with the deck ,the outer edge of the piston
Is about .8 mm from the deck



So cut the f-er and run a br9es ?or am I getting too close to the crown?

I can't run premium anymore because of the ethanol,and all I can get is rec90

I think she might rev out nicer too with lower compresh and this PT pipe

[yaegerb]

Only way you will know for sure is to run a UCCR.

[rustyhondas]

Cut her down to 68.5mm

Has 175psi

All is good

Thanks for keeping me going

[cr480r]

Let us know how it runs

[John Tice]

If you’ve got enough meat left in the larger bore head; the larger head needs to be re-machined to your existing head, & the combustion chamber must conform with your cylinder displacement. The cranking compression must be close to what your running, maybe 135-150psu??? $$$$$$$$$$$$$$$$$
JT