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 Posted: 09-03-2019 03:05 pm
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Esprit2

 

Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 460
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The modern composite head gasket has a crushed thickness that is about 0.5 mm / 0.020" thicker than the original steel-asbestos-steel laminated gasket. That raises the head further off the block by that same amount, reducing the compression ratio by about half a point (8.0 becomes 7.5).

When the Goetze composite gasket was introduced in 1993, Lotus started machining the block decks lower by that same 0.5 mm / 0.020" in order to preserve the 'spec' compression ratio. But when the gasket is used on engines built prior to 1993, they loose half a point of compression.

If you're going to order custom pistons, specify a compression ratio that's half a point higher than you really want.

Alternatively, you can have the block's deck cut that much lower, and then cut the liner seats lower in order to preserve the correct 'nip'... ie, the liner's stand-out above the block deck.

*~*~*~*~*
Replace the studs in the block that retain the head. The new gasket requires a higher torque spec that exceeds what the studs can handle... they will yield. Either install the later upgraded Lotus studs, or the ARP studs that are sold by most/ all Lotus parts vendors.

For identification...
The original 907 studs have flat upper ends.
The upgrade 907/ 9XX studs have a depressed dimple machined into the upper ends.

IF you keep the original 907 studs, then you have no choice but to use the original Lotus torque spec so as not to yield the studs. But that low torque will not take full advantage of what the composite gasket offers.

Note that the torque spec given in the Jensen-Healey Workshop Manual is too high for both the original studs and the original steel-asbestos-steel gasket. Mostly the gasket... it would crush, leak again, and you'd be right back where you started.

Lotus reduced that spec to:
70 lbs-ft ( 9.7 kgf-m) Front & Rear Pairs.
75 lbs-ft (10.4 kgf-m) Three Middle Pairs.
Oiled threads & washers.

... but the J-H manual's torque spec was never updated.
But then, the original gasket is no longer available.

Regards,
Tim Engel

Last edited on 09-05-2019 07:09 pm by Esprit2