Twin Cam Rebuild, Omega 83.5 intruder pistons, Deck height,

[holywood3645]

Hi all and back to my twincam rebuild

Below is the QED spec' for my Twincam bottom end with the exception of
the pistons/Bore. I have Omega forged 83.5 intruder pistons. I would
like an engine CR of 10.5/1. However I have a couple of options as to
how I obtain the CR. My main question is the intruder portion
protrude into the head combustion chamber (above the deck) Presently
my head is standard non sprint thickness (think the cc is around 37 to
38 per cylinder.#Not verified yet#) I have the option of machining the
intruder portion to zero deck height and surfacing the head to obtain
the CR, or machine the correct amount of the intruder portion and
leave the head un-skimmed.

Q1 Are there any major advantages to either method?
Q2 What is normal un-skimmed head chamber CC?
Q3 Normally is a Zero deck height the optimum height or ?



Bottom end spec'
"1700cc Short Engine
A basic short engine assembly comprising a rebuilt Ford 711M block,
bored to twincam (82.5mm) #mine are 83.5 Omega# bore size. A 76.7mm
crank is ground, laped and fitted (with appropriate bearings), along
with 4.9" con rods and a set of new QED cast pistons #83.5 Omega
fordged#. Jackshaft bearings are fitted as required and the block face
is then machined to give the correct height.
Optionally this can be supplied with a new iron jackshaft, steel
flywheel and ring gear, baffled sump and front cover assembly."

[types26/36]

?
Bottom end spec'
"1700cc Short Engine
A basic short engine assembly comprising a rebuilt Ford 711M block,
bored to twincam (82.5mm) #mine are 83.5 Omega# bore size. A 76.7mm
crank is ground, laped and fitted (with appropriate bearings),"

IMHO taking any metal from the head would be the last resort, I would try to achieve your goals by modifying the pistons, what is easy to remove from the head is not as easy to replace and you my well regret skimming the head at a later stage.
Just an other point, I note you quote your crank stroke at 76.7mm, you may well have a stroke of 76.7 but it would be with a non standard crank as a stock 711 crank is 77.62mm, just be aware as a wrong figure would play havoc with your calculations.

[cabc26b]

Machine the pistons - which with high compression rations, you end up having to do to clear the valves if you are using agressive cams ....

[Dag-Henning]

- guess you will have to machine the pistons to get the CR "down " to 10.5 . Do believe the intruder part of the Omega is 4 cc. With a brand new head, and "close to zero" deckheight, I believe I got 11,8........Don't forget the thickness of the gasket when you calculate......!

DH