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 Posted: 11-18-2015 04:19 pm
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Esprit2

 

Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 289
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The current composite cylinder head gasket's crushed thickness is 0.020" thicker than the old steel-asbestos-steel gasket's. That lifts the head 0.020" higher off the block deck, lowering the compression ratio by about half a point. 8.4:1 becomes 7.9:1. The maximum cut on the bottom of the cylinder head for truing the surface is 0.020". I'm just sayin'.

From the introduction of the new gasket onward, Lotus decked the block 0.020" lower and seated the liners another 0.020" deeper into the block to maintain the correct 'nip' (the liner's exposure above the block deck). For most owners of an older engine, that's not a very practical option. If you ever rebuild the engine, including replacing the original pistons, specify a half-point higher compression than you're really targeting.

All 907s were built with the original low-spec head studs, and they're known to stretch with sustained full throttle running (track days, motorsports, etc). Stretch leads to blown head gaskets.

The 'new' composite gasket requires the use of Lotus' later upgrade head studs (identifiable by a depressed dimple in the upper end), along with a revised, higher torque spec. The gasket's higher torque spec 'will' over-stretch the original 907 studs.

"IF" you don't replace the low-spec head studs, then stay with the original 'Lotus' torque spec out of consideration for the studs. You won't be getting the full advantage of the composite gasket, but you won't be over-stretching the studs either.

Note that the J-H head torque spec was too high, crushed the old gasket, and caused problems with blown gaskets. Lotus reduced the torque spec to a level that didn't crush the gasket, or over-stretch the studs. If you're going to stay old-school, it would be better to follow the Lotus torque spec instead of the one in the J-H Workshop Manual.

Lotus Cylinder Head Torque Spec:
(Tighten Cold, Oiled Threads & Washers)
70 Lbs-ft - Front & Rear Pairs
75 Lbs-ft - Three Middle Pairs
Work up to it in several smaller steps, working from the center outward in a criss-cross pattern.

Regards,
Tim Engel

Last edited on 11-18-2015 06:25 pm by Esprit2