Lowering compression and uneven combustion chamber volume

[Bhud]

Any of you smart people here care to offer an opinion?

I've got a cylinder head (twin, 4-stroke, aircooled). It had a lot of damage from flying bits of broken piston ring. These were big pieces, and they lodged inside the head. I got them out. It's not the original cylinder head for the bike (I've still got the original though). I ground out the damage using a Dremel.

Now the cylinder head looks great. However, some aluminium has been lost, and one side is bigger than the other. It's not an interference engine, but now it's more of a "hemi" than it used to be, if that makes sense. Therefore, if I reuse this head (and I don't have to, but I'd like to, as it has less cam bearing surface wear than my other one), it will have less compression in one cylinder than the other. Not massively less compression, but still it would be noticeably less. Now, I must admit, I can be a bit tool-happy at times, and this was a rare opportunity to use my Dremel, etc.

My question is: is this undesirable? Is it likely to cause a problem, and should I revert back to my spare head rather than use this one?

[Pete.]

Of course it's undesirable. How much of an effect depends on how much metal you removed. You could use water to figure out your effective compression ratio by comparing the combustion chamber volume to combustion chamber + swept volume. Or you could build it up and use a compression tester.

Is it a low-tuned engine anyway? Non-interference and air cooled suggests so.
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Bhud]

From factory, it's 9.2:1. So yes, it's low.

I'm not going to measure it or fuss any more by doing anything else to it. My line of thinking: if there's a loss of power, that's one thing; if it blows up, that's quite another. Separating the 2 issues: loss of compression vs uneven combustion chamber volume. Do I use this or my spare head (which is completely standard)?

[Pete.]

I'd put the undamaged head on if the cam bearings were still round enough to keep the oil pressure up. Then again, I would not have dremmeled the head back smooth, only taken off any raised chunks. A few digs in the donw wouldn't hurt, so long as the valve seats were not bashed about.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Bhud]

You might have done if you had a spare head...

[stinkwheel]

Maybe you could measure the chamber volumes and even it out?
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Bhud]

Yes, this is what I've been thinking.
I have a little leeway to reduce the volume in both domes. This will necessitate measuring how much cc each of them can carry, and trying to get them as close as possible by removing material from the lesser, then remeasuring and repeating the process, then finally skimming them back as much as possible to increase compression.

As a hobbyist, I'm not at the level of being able to put down more material (aluminium) using TIG or other means. The valves and valve seats are fine in both heads. I can play with this head a bit. But I don't know what I'll decide to do yet - I'm curious to see if I can make this head good, but also getting a bit moody about bikes. I'm rebuilding the entire engine as far as reasonably practicable, and replacing wear parts which are still available. It's a good platform for experimentation insofar as it's a basic bike, but there are issues with parts availability as well. Will have to have a good think. Obviously, the sensible thing would be to use the spare head as Pete suggests.

[stinkwheel]

If it's dropped the compression by too much once they're evened out, you could always raise it a bit by skimming the head or using a thinner gasket somewhere, just make sure the valve to piston clearance is sufficient.

That said, running an engine at lower compression generally makes it more reliable.

Anyway, pics of the die grinder mayhem. I'll show you mine...
https://lh3.googleusercontent.com/pw/AIL4fc8DwWR-Hjbgk-Nl2ukBfPvQglKvOnZ45sNulIHk5Db_UX_lL9QXgDmQ2njg8Nw44YbgZ1gkc2Yi79Xi2zigp1-ojR4vEMEij8LrZhY-KOEYZ65fmaDFMiu__y0rw_SGwgvxrfACNArK__DGM6zn0zCk=w1160-h870-s-no

https://lh3.googleusercontent.com/pw/AIL4fc_N73KP6lVK5wriefgnhzPUDD2TJpl7PtIGsYQ9rMSTxrkvuCxbatzQEzLnskT6d0eNZ6syE4UpBDSnjXlH82njnHnxhi3OrOT4oETJ4EcnBLHTlP92BVF0J30p4PWz1Xm4a0BpLTi5nOyaDimw0FF5=w1160-h870-s-no

Obviously, mine's a single so less concern about balance and I was already fitting a higher compression piston anyway. Had the seat re-cut.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Bhud]

That's nice. You don't appear to have removed much, and the main problem seems to be the damage near the valve seats, which you had recut.

I don't think there's anything to be gained from sharing my photo, sorry. I have little faith. Some people are allowed to get things wrong, but others are not, and it just ends up in forum grief. I can hazard a good guess as to the reasons but they will never change.

Anyway, I'm going with the spare head. Cheers

[stinkwheel]

One thing with that head of mine, it's not going to drop the exhaust valve seat, which has been know to happen on occasion with these motors.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[jeffyjeff]

Removal of material from the head will increase the compressed volume and reduce cylinder compression. I would assemble the engine and do a compression test. If cylinder compression is within mfr. specs, I'd feel OK about running it.
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History shows again and again how nature points out the folly of men - BOC