Head to water pump bolt

[rgh0]

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.

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 ??

All I know is how I have designed multiple gasketed pressure joints for multiple materials ( aluminium, cast steel, cast iron , stainless steel, exotic high nickel alloys) and temperature ranges from -200F to + 1500F and that is how it is done and it works. Whether it works for a Lotus head and the type of gasket and bolting you use it up to you to assess .

An old engineering saying " There is no perfect answer but many wrong ones"

cheers
Rohan

[2cams70]

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 ??

My guess is that they went too far with the original bolts in trying to give the head to block joint some flexibilty. Given a choice i'd use the later uprated bolts or the ARP bolts. With the ARP bolts you can use the original torque specifications and still have the bolt operating elastically.

[Craven]

Perhaps the increase in head bolt size/design was coincident with the later raised compression ratio, mind you those early head bolts will yield even at the specified torque.

[The Veg]

UPDATE:

Got a new set of ARP bolts, which I measure at approximately half a millimetre thinner than the studs. And now everything lines up nicely. Glad to put this behind me, and now you can all keep fingers crossed for nothing to leak (much) when I finish getting the engine back together and start running it again.