Cylinder head depth

[dusty]

Folks,

Can anyone tell me the depth from top to bottom face of a standard twincam head. I'm trying to work out how many times my head may have been skimmed.

Regards
Jon

[CBUEB1771]

The Dave Bean catalog lists all heads prior to the Big Valve specification as 4.64" thick, the Big Valves as 4.60" thick.

[dusty]

I have 4.635" so it looks as though mine has had 5 thou taken off at some stage.

[Elanintheforest]

I saved this from a posting a couple of years ago.

Small Valve = 4.638/4.643" with another .045" allowed to be removed. Big Valve = 4.598/4.603" with only another .010" allowed to be removed.

Mark

[dusty]

The reason I'm asking is that I want to end up with a CR of 10.5:1 to match the Q360 camshafts I'm fitting. The head will also have the bigger inlet valve for the difference it makes!

I guess I'll need to take off about 30 thou.

I would like to be a bit more scientific though hence my other thread.

[patrics]

Hi,
I take that you have looked at the thinner head gaskets you can get before machining the head?

Regards
Steve

[andyelan]

Hi Jon

Isn't a Q360 basically the same as the Lotus Sprint / D Type. Take 30 Thou off your head and you'll have the equivelent to a "Big Valve" head which I would say should be fine for road use. If you want to start blueprinting, you'll be into equalising the combustion chambers, getting the cylinder block deck heights correct and the rest. You really need to read up some of the more specialised literature if you intend to go down this path.

Andy

[dusty]

It's not my intention to be too precise, I'm just a bit paranoid about taking 30 thou off my head without taking some measurements first, incase I've got it wrong and end up with a very high CR

[paddy]

See my post on your other thread. To get to 10.5 it looks to me like 30thou isn't quite enough.

Paddy

[andyelan]

Jon

If I were you I'd leave well alone and assemble the engine as is. Unless you're racing, you'll hardly miss a bit of extra performance during normal road use. Every time you skim a head you're using up it's life. There are enough times you have no choice but to skim, if its warped or corroded for example, if it's not necessary, leave it alone

Andy

[rgh0]

I always do the final adjustment of compression ratio using gasket thickness and / or piston intruder height. You cannot rely just on head thickness to determine compression ratio but need to measure up all components as many things may have changed versus the original lotus design - eg block deck height to crank centreline, pistons details such as valve cut out sizes, combustion chamber volume, valves head design, head gasket, all which can inlfuence the end compression ratio.

For a road engine with a target of 10.3;1 I would be happy if it was between 10:1 and 10.6:1 based on the final build components if I could achieve that just by playing with gasket thickness based on using standard pistons. I would only start looking at machining up special pistons or playing with head chamber volume if outside that range.

I would never take material off the head face just to achieve a certain compression ratio - as others have said the heads are to expensive to do that and risk shotening their life. I only take material off the face to achieve a good sealing surface and always the minimum amount possible to do that.

cheers
Rohan

[rgh0]

One more thing I forgot to add is that you can in practice take much more off the head if truely needed than the numbers quoted by Lotus. The limit is set by the water jacket under the exhaust port. You can normally go to until the casting numbers on the bottom of the head dissappear before you have to actaully start welding up where you break into the water jackets.

As the head gets thinner you may also have to do work on the valve seat location and combustion chamber size and shape depending on exactly the spec of engine you are building.

cheers
Rohan