11 replies to this topic

[IainNeon91]

Hi all, just wondering if this head could be bolted straight on?

Link:

http://www.ebay.co.u...d=p5197.c0.m619

Only it says it's been reskimmed, but do i still have to get it skimmed to match my CR??

Cheers,

Iain.

[Cooperman]

You won't know what compression ratio a head will give until you measure and calculate. Only then will you know whether the CR would be within whatever limits you want to achieve.

[MRA]

If you are that worried about your CR then no don't get it, my reasons are simple this is a head for a 1098cc engine that was built in the 50's or 60's it has cap springs that at higher revs will simply ping off into oblivion, well actually they will get caught around your springs etc.

[IainNeon91]

I would probably change the springs to doubles anyway i think, would it still do the same thing?

As a general skim, i see a lot of people getting heads skimmed by 10thou? Is this because they have done the calculations or is it just a number?

Is there a recommend CR for a fast road 998?

Edited by IainNeon91, 11 March 2013 - 11:11 AM.

[MRA]

The valve caps and valves would need to be changed to increase reliability.

Valves and seats will have been lapped in together so would need relapping and all of this would increase your costs

you are better off asking them if they can do a modified head if that is the way tou want to go

[IainNeon91]

Yes that could be worth an ask, thanks for the help!

[Cooperman]

I would probably change the springs to doubles anyway i think, would it still do the same thing?

As a general skim, i see a lot of people getting heads skimmed by 10thou? Is this because they have done the calculations or is it just a number?

Is there a recommend CR for a fast road 998?

The 998 usually runs well with a Cr of 10:1.
Until you measure everything and do the calculations you really won't have a clue what CR you would have if you simply fitted it, and that's no way to get a good engine. The actual CR depends on several aspects of the components used. The first is the volume of the combustion chamber and this is determined byb the+. The second is whether the pistons are flat, D-topped, or dished. Over the years there have been so many different configurations that you don't know what you've got until you measure it all. The other factor is how far down the bore the piston tops are at TDC. For each 0.010" on a 998 there is an additional approximately 0.9 cc of combustion volume, so that needs to be measured and allowed for.
It can be a case that if flat-top pistons are fitted a head can have chambers which are too small and need to be enlarged. Less likely these days, but the original 998 Cooper raised top pistons can cause this to be an even bigger problem.
It must all be measured and the calculations done.

[Spitz]

I imagine, but ask to be sure, that the head has been "reskimmed" ( as the ad puts it ) just to ensure flatness. I think typically it's like .005"

That head seems a bit pricey......maybe a better bet would be to track down a 12g295 head....perhaps these are harder to find (?)

I have two 295's....both were skimmed by .080" ( larger initial volume than a 202 )

[Cooperman]

Whatever head you get you do really have to do the necessary measurements, calculations and any necessary machining to get the compression ratio correct.
There is no short cut to this other than never changing a cylinder head for a different one.

[IainNeon91]

I didn't realise a head change would be such a lengthy process!

Edited by IainNeon91, 11 March 2013 - 05:24 PM.

[IainNeon91]

I found this example of a calculation

(998/4) / (10.0-1) = 249.5/9 = 27.72cc chamber

for flat top pistons there is about 5cc for the deck and gasket, so this would mean i'd need 22.72 (if i have flat tops) and apparently a standard 12g295 is about 28?

That's a bloody good amount to be skimmed...?

Edited by IainNeon91, 11 March 2013 - 05:26 PM.

[Cooperman]

That's not far out. The normal copper-faced 998 gasket is 3 cc in round figures and allow about 1 cc for amount above piston to block deck level and 0.5 cc for the ring land volume. Thats 4.5 cc, but you would need to measure the actual distance of piston top to block deck.
With D-top pistons the 998 Cooper had a 28 cc chamber as standard giving around 9.5:1, so for 10:1 with flat tops expect to have to lose about 5 cc from a 28 cc chamber volume head.
Then you put exactly the right amount of fluid in a chamber and see how much needs to be machined off. Expect it to need between 0.060" and 0.090" skimmed of it, a bit less if it's been skimmed before maybe. You won't know until you measure, etc.
It's not really complicated, it's just part of engineering an engine correctly. It's what all engine builders do all the time.