Overskimmed?

[Mr Rusty850]

Sorry, but I'm wondering if you could advise on the following:
I'm about to restart rebuilding an 850 engine and I want to tune it mildly at the same time. I have a 12G295 head which I was planning to use but when I've checked the thickness of metal left below the oil channel it's down to 40 thou. In David Vizard's tuning book he quotes 60 thou as a safe minimum but later talks about taking it down to 40 thou if you are prepared to risk scrapping the head. I've checked the volume of the combustion chambers and it would give me an acceptable compression ratio but what are peoples views on using a head with only 40 thou of metal left between the face and the oil gallery?

Also, if I took that risk and it failed would it lead to a hydraulic lock up or would it just misfire and emit clouds of exhaust smoke?

Thanks,
Barry

[mk1]

I'd feel fairly comfortable with 40 thou. There is of course an outside possibility of it cracking, but it is exactly that, REMOTE.

In the event id did, a missfire & loads of smoke would be the most likely first signs, there isn't enough oil feed up to the head to hydraulic it.

Hope this helps.

M

[Peter Laidler]

Agree entirely. But for a slightly different reason. Given the relatively low oil pressure generated, .040" will be adequate support. You could easily test the area for cracks using dye testing.

On the other hand, there's a school of thought within this forum that instead of doing what you're doing with an 850 engine, why not get the best of both worlds and go for a 998 or 1100 engine

[Mr Rusty850]

Thank you for the replies. I'm sticking with the 850 as it's got a fair bit of history going back to it's original purchase and externally at least it is still standard. I do find this forum extremely useful and appreciate members input.
Regards,
Barry

[mk1]

I like 850's too.

[Oneball]

I’d have thought it was cylinder pressure that’s going to break through a weak gallery as it’s way more than your oil pressure. You can modify the oil feed if matching it does break through.

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[251 ENG]

With ministox race engines the heads get faced massively, if it brakes through in to the rocker oil feed we plug it up .

Oil feed is then sent up the head stud hole .