19 replies to this topic

[babsbrown]

Good evening guys,

I thought I had settled on using a 12g202 cylinder head for my build but having read so many good reviews about the 12g295 I'm thinking of using that instead.

Only problem is, as we all know they suffer from valve recession. I want to get it sorted eventually and get unleaded seats and bigger rimflow of GS valves fitted. But at the minute out of my budget.

Given the above is the head actually useable with the valves in the current state? They bounce up when dropped down into the seat and they seat with water in the chamber. The valve seats are actually pretty decent

 And cleaned up well with a lapping in

Cheers, Babs

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Edited by babsbrown, 19 December 2024 - 09:28 PM.

[dotmatrix]

not the most conclusive answer ever: but it doesn't look right

[slidehammer]

I would say if you are sure you are getting a good valve seal on all chambers, it is usable as a standard road head, but would never work as a performance item with that amount of valve recession. The inlets are recessed too, which would indicate to me that the valve seats have been recut at some point.

[Shooter63]

Have you cc'd the chambers yet? With the valves in like that you have gained a few cc's and if I remember right you are struggling to get the compression up to a decent level. Apart from that the head looks usable if not ideal.

Shooter

[babsbrown]

Cheers gents, @shooter I did CC the head and it came back around 28cc average each chamber. That would still imply I need a good 80 thou skim to give me decent CR.

But after sleeping on it, I think I'm just going to shove it on the shelf for another build and get it sorted in due course.

I'll get a 12g202 skimmed for now. I wish I'd have honestly bought flat top pistons now ,but we live and learn.


Cheers again

[babsbrown]

Have you cc'd the chambers yet? With the valves in like that you have gained a few cc's and if I remember right you are struggling to get the compression up to a decent level. Apart from that the head looks usable if not ideal.

Shooter

Hi Shooter, I'm not sure if I misinterpreted your message. Do you mean it would be ideal if the compression was OK?

Or not ideal but useable haha.

Really respect your opinion,

Cheers, Babs

[Shooter63]

Have you cc'd the chambers yet? With the valves in like that you have gained a few cc's and if I remember right you are struggling to get the compression up to a decent level. Apart from that the head looks usable if not ideal.

Shooter

Hi Shooter, I'm not sure if I misinterpreted your message. Do you mean it would be ideal if the compression was OK?

Or not ideal but useable haha.

Really respect your opinion,

Cheers, Babs

What I meant was, are you sure the chamber volume was 28cc, if ( big if) its not skimmed then as you know the original nominal chamber volume was 28.4cc, with the valves in like that which I think is due to the valve seats being re-cut, probably with stones as per the norm back in the day, a bit of simple maths tells you, you have gained capacity. On my work PC I keep a really decent compression ratio calculator, if you list the cc in the dish of the pistons and how far down the bore they are at TDC I'll post a sheet with how much you need to skim various heads to achieve a given ratio.

Shooter

[Cooperman]

With that head it would be easy to fit new seat inserts to bring the valves to the right position in the chamber and make it suitable for unleaded petrol.

I don't know where you are located, but Cambridge Rebores in Burwell, Cambs, would do it and are excellent.

[babsbrown]

Thanks for your advice gents.
One thing I was curious about is the fact that 998 cam head inlet valves are 27.8mm

If they were made of similar material and had the same metal properties of the exhaust ones. That would make for an interesting experiment to seen if they would fit.
Something I wouldn't risk though.

@cooperman if love to just pay to get it all sorted with unleaded valves but I don't want to take the gamble of the seats cracking. I already had one crack this year. But appreciate the recommendation thanks.

@shooter yes please that would be great.

It's 1098 + .040
1098 crank
998 gasket
8.3cc dish
13.5 thou piston deck height.

Thank you

[babsbrown]

My calculations had it that I'd need approx 23cc chamber for a 9:1 compression ratio

[Shooter63]

Thanks for your advice gents.
One thing I was curious about is the fact that 998 cam head inlet valves are 27.8mm

If they were made of similar material and had the same metal properties of the exhaust ones. That would make for an interesting experiment to seen if they would fit.
Something I wouldn't risk though.

@cooperman if love to just pay to get it all sorted with unleaded valves but I don't want to take the gamble of the seats cracking. I already had one crack this year. But appreciate the recommendation thanks.

@shooter yes please that would be great.

It's 1098 + .040
1098 crank
998 gasket
8.3cc dish
13.5 thou piston deck height.

Thank you

Do you want me to add the skimming required for various head types of just the chamber volume in general? I think I've got most types of head loaded in the system.

Shooter

Shooter

[babsbrown]

Ah yes please if you do the variety of heads that would be fantastic, as I have 202 heads too.

Thanks mate

[Shooter63]

There you go, I included the heads I know you have, I took a punt at the ring land volume as I've never measured a 1098 piston, but it won't be far out, I've used 3.4cc on the gasket volume as that is what a VR gasket measured at. It should give you a decent place to start. I've rounded to the nearest 10th cc. One thing to think about is that this is a static comp ratio so theoretical only. A dynamic ratio calculation will be more precise.

Shooter

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[babsbrown]

Excellent thanks Shooter, really appreciate the time taken to do that for me.

Having looked at your table I decided to get a 40thou skim for now on the 12g202 head. That will in theory get me 8.5:1 not exactly mind blowing performance compression ratio. But I had been getting a bit carried away and want this to stay a fairly budget build.

I also had the valves recut for a total of £100 which I was really happy with.

Your table is very accurate may I add. I CC:d the chambers and they were perfect.

Thanks again, Babs

[babsbrown]

Here's the head now. I'm wondering whether it's best practice to chamfer the waterway holes to take the sharpness off them. I did run a small file around the chambers to take the sharp edge of them. 

 

 

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