|View single post by StevenD57|
|Posted: 12-27-2014 03:11 am||
|Follow-up information from Tim:
The torque specs are set relative to what the studs (nuts & bolts require), and that's what was done for the original 907, and the J-H manual calls for 83-87 Lbs-ft of torque with oiled threads and washers.
With steel and cast iron parts, there is rarely any problem with the resulting clamp force being excessive and over-stressing the clamped parts. With aluminum parts (ie, aluminum flywheels), sometimes the fastener's optimum torque is too much for the clamped part, and the torque spec has to be dialed back. The 907's original steel-asbestos-steel (s-a-s) head gasket would sometimes be crushed by the J-H 83-87 lbs-ft torque spec. Crushing would relieve the stress in the fasteners, relieve the clamp force on the gasket, and lead to blown head gaskets. Thightening the nuts more seemed logical to many, but it was actually more of the same problem, and made matters worse.
When Lotus introduced the 907 in their own cars (1974 Elite), their own manuals stated a torque spec or 75 Lbs-ft. That was a significant improvement, the incidence of blown head gaskets was greatly reduced, but they didn't go away. Gaskets would still blow, usually at either the front or rear. So Lotus further revised their own torque spec to 70 Lbs-ft for the front and rear pairs of studs, and 75 Lbs-ft for the middle three pairs of studs (tighten from the center outward in a criss-cross pattern, and in 3 or 4 smaller steps to the final torque).
That torque, and the original spec head studs and head gasket remained in place through the 1992 Turbo 910. It was adequate for any 907 ("high" compression was only 9.5:1), and the low compression 912. It was also adequate for the high compression 912 (10.9:1) that was street driven normally, and Lotus was in the business of building "street" cars. However, for a hotrod 907, a 912HC, and newer 910s with ever increasing boost pressure or chipped ECUs, sustained full throttle (track days) would stretch the head studs and result in a blown head gasket.
Higher torque wouldn't help since it was the studs that were yielding, and it was also shown that the gasket would crush with greater clamp load. So, in Jan 1993, Lotus introduced new, upgraded head studs, increased the torque spec accordingly, and switched to a Goetze composite head gasket that could take the clamp stress. It was a package deal.
If you use the Goetze gasket in an otherwise stock 907, including the stock head studs, then you cannot use the new, upgraded torque spec, since it would cause the old studs to yield. But the stock 907 (8.4:1 or 9.5:1 compression) doesn't need the Goetze's ultimate sealing capability in order to be effective in that 'scaled down' application. The Goetze gasket with vintage torque spec will still seal better than the old steel-asbestos-steel head gasket did with that same vintage torque spec. Simply installing a Goetze gasket with all else being the same will not lead to a future gasket failure. However, it will result in a half point of compression loss, if the engine is otherwise stock (ie, the head and/or block deck have not been skimmed). Half a point means 8.4:1 becomes 7.9:1, and 9.5:1 becomes 9.0:1.
As Kees pointed out, shaving the head will lower the head in the assembly, and offset some of the thicker gaskets effort to raise the head. The maximum the 907 head could be shaved is 0.020", which is exactly how much greater the Goetze gasket's "crushed thickness" is thicker than the original s-a-s laminated gasket's.
For someone like Kees, who has his own inventory of the old s-a-s laminated head gaskets, upgrading to the later Lotus head studs, or ARP aftermarket studs is not much of an option. If the higher torque those studs allow for is actually used, the s-a-s head gasket will be crushed, and will leak... ie, a blown head gasket. The s-a-s gasket is the weak link.
With the current Goetze composite head gasket, or JAE's aftermarket knock-off of the composite gasket, the gasket is no longer the weak link since it can handle all the clamp force dished out by any of the available Lotus or ARP upgrade studs, and the higher torque specs that go with them. The composite gasket can take it, but not the problem becomes correctly matching the torque spec used with the head studs that are installed.
You can use the composite head gasket with the original low-spec J-H / Lotus head studs and either the original J-H 83-87 Lbs-ft torque spec, or with the Lotus' later, reduced 70 & 75 Lbs-ft spec. You can also use it with Lotus upgrade studs (Jan 1993 onward) and the new torque angle spec (which equates to something like 109 Lbs-ft), and the gasket can take the greater clamp force.
Lotus head studs and nuts use a coarse thread, while the ARP aftermarket studs use a fine thread. Fine thread is actually the more technically correct for the application. For any applied torque, a fine pitch thread's lower helix angle will produce more clamp force than the coarse thread's steeper helix angle. So, if you use ARP studs, it is critically important that you follow the ARP torque spec, and use ARP's special thread lubricant... not just 'oil'.