Tighten the cylinder head bolts right?

[elan+2s]

Sorry for my bad English.

I want to mount the rebuilt cylinder head of my Lotus Twin Cam. Now I have questions about tightening the cylinder head bolts. In the workshop, the tightening torque is only indicated. It does not quantify the steps in which the screws are to be tightened. Moreover, I find no indication of whether the cylinder head bolts must be retightened after a certain period of operation.

Can anyone help me?

[billwill]

That is strange, because the workshop manual does show the sequence of tightening the bolts.

My book is not on hand, so I can't immediately quote the sequence, but will do so if no-one else fills it in first.

[types26/36]

deleated

[types26/36]

deteated

[rgh0]

I Tighten the bolts to the recommended torque in 6 stages using the tightening sequence shown in the manual. Make sure the bolts and threads are clean and lightly oiled which is the basis of the quoted torque setting. The bolts can bind at the top of the head holes as these close up under bolt loads so make sure the bolts are running free in the head and in the block threads before assembly. Also check the bolts will not be bottoming out in the block once tightened

1. Position head and gasket and nip up bolts finger tight, check every thing is properly aligned
2. Tighten bolts to 10 ft-lbs and check alignment of everything again.
3. Tighten to around 30 ft-lbs,. Check each bolt is turning about the same amount to get to the 30 ft-lb point. A bolt that turns substanially less may be binding and you should remove clean and check and reinsert
4. Tighten to around 50 ft-lbs
5. Tighten to the final torque
6. Go around the bolts a couple of times checking they are all still at the final torque.

After running in the engine recheck bolts are at the final torque. I do that by setting the torque wrench at the final torque and then ensuring the bolts move a few degrees tighter when I check tighten them. They normally move a few degrees at or slightly above the final torque setting.

cheers
Rohan

[Quart Meg Miles]

Would a professional engineer please justify the tightening sequence as I don't find it intuitive.

The published sequence tightens the middle pair first then proceeds to tighten diagonally related pairs which I feel must stress the head and gasket with a twist.

I now tightening both bolts on one side of the middle pair (after tightening those) then the matching pair the other side to keep the head as flat as possible with only an end-to-end curve. What am I not understanding?

[rgh0]

The rules I have been taught are:

1. You typically work from the inside out to prevent trapping stresses in the part being bolted down.
2. You generally do diagonally opposite bolts in turn to avoid setting the component down to much in one concentrated location and potentially distorting the gasket or loosing the alignment between the two components and gasket.
3. You also do the bolts up in stages to avoid setting the component down to much in one concentrated location. The more stages in tightening the less that 1 and 2 above are important.

The Lotus sequence complies with these rules in combination with the tightening stages I described previously.

cheers
Rohan MIE Aust CP Eng ( well you asked for it )

[Gray]

The more stages of tightening the better within reason. Generally from the point it feels it is torquing the head down. Probably from around 30lbft.

Gray

(CEng MInstMC MCIBSE)

[elan+2s]

Thanks for the many replies.

Rohan has described exactly what I wanted to know. The order in which the screws are tightened was acquainted with. These are found indeed in the workshop manual.

Thanks again and I wish you a nice weekend

Egbert

[collins_dan]

How many miles should you go before re-torquing? Thanks, Dan

[rgh0]

I do the head bolt torque check after I have finished running in the engine at around 500 km.

For the race engines I would check them again after the first couple of race meetings also as occasionally they will move a little.

For a road engine I normally check after another 1000 km but they never move in my experience again after the first check at around 500 km.

This is all based on the composite style gaskets I use. The original copper and steel sheet style may behave differently but in general the recommendations should still apply.

cheers
Rohan

[brettengelaz]

Rohan has it right, torque in stages in a cross-diagonal pattern, and check/re-torque at intervals.. I re-torque my mild-tune (175hp) race engine every other outing. For a street engine, maybe every 1000mi or so.. I'm sure the learned experts here will have a suggestion.. While being Sisyphean effort, it's also a great time to do an overall inspection for loose/leaking/broken stuff too..
Cheers,
Brett.

[Quart Meg Miles]

Thank you for you erudition, Rohan, Brett, Gray et al. It's all common sense though I still feel the progressive tightening takes car of the stresses and misalignment more than the diagonal sequence. But I'm a mere B Sc (Hons) in electronics so take no notice.

The head alignment overall is a problem as I once fitted the head and not until I had started the engine did I discover that the chain fouled the casing. That's a lot of work to fix in itself unless one risks the gasket, and I can't remember if I did. Probably not.