Doing a top end/upgrade. couple questions, heads, rings and deckheight extremes. Options

[Aaron Cox]

motor ate a head gasket. pulled it apart before it could flame cut a cylinder or head.

so its apart. 71 x 94, with LARGE cam. has mahle euro '8:1 pistons" in it.
deck was setup at .031 but was running a head gasket.

heads have never been decked/surfaced. pulled em and cleaned em, 2 chambers have a sparkplug crack starting - so they need to be welded.

while its apart, i plan to re-ring it, hone the barrels, get the heads done, and???

questions - what rings do you recommend for mahle pistons?
certain rings justify certain hone finishes right?

heads need welding.
valves are sealing great to the seats. guides are good. (motor has 35 k on it since full rebuild.) when the head gets welded, will this require new seats/guides and a resurface?
what is the proper welding setup- my machinist may want to weld em cold, but i was told to pre heat em first - your thoughts?

when reassembling - how tight can i go on deckheight? the euro pistons have a .034" dish to em. should i trim the pistons .034 to make them flat, and trim the cylinders accordingly also?

trying to get as much CR with these 94mm pistons as i can, and running 91 gas.

last question- cam. how does the cam affect deck height? how tight can i go?

going to CC my heads tomorrow - before machining.... what is the range for a stock 2L head? 55-65cc's?

*on reassembly - head gasket will be omitted, and cylinders will be lapped into the head. valves will also be lapped into their seats again.

[Brando]

questions - what rings do you recommend for mahle pistons?
certain rings justify certain hone finishes right?

Go with a good set of Hastings rings. You can buy them from jake, or ask FAT performance in Santa Ana. You can probably hone the cylinders yourself but if you've got a machinist doing work for you, why not have them do the cylinders for a few pennies more?

heads need welding.
valves are sealing great to the seats. guides are good. (motor has 35 k on it since full rebuild.) when the head gets welded, will this require new seats/guides and a resurface?
what is the proper welding setup- my machinist may want to weld em cold, but i was told to pre heat em first - your thoughts?

As DD said, there's some special procedures for welding aluminum heads, one of which is pre-heating the metal. After that you'll need new seats and probably have the heads flycut and decked to have a true sealing surface.

when reassembling - how tight can i go on deckheight? the euro pistons have a .034" dish to em. should i trim the pistons .034 to make them flat, and trim the cylinders accordingly also?

I believe the lowest you can go with deck height on a mildish cam is about 0.03", maybe 0.025" if you want to be kissing the carbon off of the combustion chamber. But without truing the cylinder seating surface on the case, you'll probably have 0.03" to 0.035" without the gasket.

Are your cylinders domed or dished? lower compression would be dished, higher would be domed (think 944 pistons or CIS pistons for a late 70s 911 engine). You can have them flattened if you wish, but that might take away that little bit of a bump in compression you're wanting.

last question- cam. how does the cam affect deck height? how tight can i go?

Your cam does not affect your deck height per say, but the opposite to a degree. With incredibly low deck height you're limited to cams that don't have extreme lift. Think of it as the piston being closer to the spark plug and valves in the cylinder, then the cam pushes the valve inwards towards the piston. With low lift this is not as much of an issue, but with high lift and the proper amount of duration -- Piston: meet valve, Valve: meet piston. Steel and aluminum don't blend this way.

going to CC my heads tomorrow - before machining.... what is the range for a stock 2L head? 55-65cc's?

Tolerances per the Porsche 914 specifications indicate 56-58ccs per chamber. You can go lower for more compression. Play with a static compression calculator to see what volume combustion chamber would be best for you. Keep in mind, you're taking material away from the heads to lower the chamber volume. You can always subtract, but adding is a bit more difficult (and costly).

Hope that helps a bit, bro. And I should be getting your lapping tool and valve removal tool tomorrow mornin.