|View single post by StevenD57|
|Posted: 12-25-2014 03:52 pm||
|Tim Engel just posted a very interest piece of information the the TurboEsprit mailing list.
If many of you are not aware, the only head gasket that is available for the Lotus 907, 912, or 910 engines is a newer style composite gasket that is marginally thicker than the original metal sandwich used on the early Lotus motors as used in the Jensen-Healey. According to the data Tim has shared this gasket is not only marginally thicker but it compresses less too. This means that the stock Jensen-Healey compression ratio which was low to start with from the factory is actually negatively impacted with this newer composite construction head gasket. Also when Lotus switched to using this headgasket on their production engines, the cylinder head studs were redesigned for additional strength and the way the cylinder head is torqued down was radically different than what is listed in the Jensen-Healey workshop manual.
The Goetze composite gasket has a compressed thickness that is 0.5mm (0.02") thicker than the previous steel-asbestos-steel gasket (whichis no longer available).
All else being equal, that extra gasket thickness raises the cylinder head by that same amount, resulting in a larger combustion chamber. The later 910 Zeus engines were designed to work with the composite gasket, so the cylinder block's deck height was cut lower by that same 0.5mm (0.02") amount, and then the liner seats in the block were cut that much deeper in order to preserve the proper liner exposure above the block deck ('nip'). In the end, Zeus engines with the composite gasket end up with the cylinder head in the original design-position, and with the correct nominal compression ratio. However, using that composite gasket on any older 9XX engine results in about a half a point lower compression ratio (8.0:1 becomes7.5::1). There are no factory alternative, it is what it is.
In Lotus TSB-19 (Technical Service Bulletin), the Goetze composite gasket was introduced along with upgraded studs (identifiable by a depressed dimple in the top end), an increased torque spec, and a new torque method utilizing a Torque Angle Meter.
Jensen-Healey Torque Spec (torque wrench)
83-87 lbs-ft (11.48-12.03 kg-m) (too high, crushed gasket issues)
Lotus original Torque Spec (torque wrench)
70 lbs-ft (9.68 kg-m / 94.9 N-m) Front & Rear Pairs
75 lbs-ft (10.37 kg-m / 101.7 N-m) Three Middle Pairs
Lotus 1993-onward Torque Spec
(Goetze gasket WITH the upgrade studs)
Tighten the nuts, using a torque/angle gauge, in the following steps:
i) 20 Nm (15 lbf-ft), initial torque with a torque wrench
ii) +75 Degrees, torque angle meter.
iii) +40 Degrees, torque angle meter.
iv) Wait 5+ minutes.
v) +20 Degrees, torque angle meter.
The torque specs are not interchangeable. The old Lotus & J-H torque specs will NOT optimally clamp the new composite gasket. The new torque angle spec will crush the old style laminated gasket and over-stress the pre-1993 head studs. Do NOT mix and match parts & specs. Besides being tighter, the torque angle gauge method is much more accurate/ repeatable than a torque wrench, especially the typical shade-tree mechanic’s non-calibrated clicker torque wrenches.
If you insist upon staying with the original studs, then also stay with the original Lotus torque rating, which was reduced from the old J-H torque spec. It's less than Lotus spec'd for the Goetze gasket/ upgrade studs, but 'less' is better than over-stressing the old studs (lesser of two evils).