Lotus Elan and Lotus Elan Plus 2 Forums

2cams70 wrote:Sorry but I don't entirely agree with Rohan's explanation.
Aluminium's thermal expansion rate is around 2.5 times that of cast iron. Consider an Aluminium head bolted onto a cast iron block using "dumb bolts" - i.e very rigid and stiff bolts (eg. ARP studs). Under running conditions when the head heats up because the bolts are so stiff the head will have no where to go when it expands relative to the block. It will therefore have to compress the head gasket in order to make room for itself. This happens every cool down and heat up thermal cycle. The gasket is not made of rubber with infinite elasticity. Every time a head gasket is compressed and relaxed it looses some of its elasticity to recover and it will eventually fail.
Smarter bolts are ones having adequate "springiness" to allow the head to rise slightly when it expands under heat thereby relieving the head gasket from some of the stresses of having to compress and relax during every thermal cycle. The smartest head bolts are the "torque to yield" type as used by virtually every modern OEM manufacturer these days. These bolts can maintain a constant clamping force over their entire stretch range so the head gasket is best protected over many load cycles.
The Lotus head and the Ford block were originally designed to be used with bolts having a certain clamping load. If you increase that you risk causing distortion of either the block or the head face. If you overtighten any flange bolt it bows inward right? Think the same effect when you overtighten your cylinder head.
I'm not saying your cylinder head gasket will fail if you use ARP studs but I can see from an engineering perspective that they aren't ideal. It may be ok on a hobby engine where cylinder head bolt tension is checked on a regular basis, it's not expected to run reliably over many hundreds of thousand kms with many heat up and cool down cycles, not expected to survive full RPM excursions from cold, etc. I won't be rushing to use them in my engine however.

rgh0 wrote:You may be right but Lotus themselves went to a much thicker shank in the later engines. So I am not sure they thought it so critical. If so which bolt should you use - the early or late original bolts ??