Cylinder Head tightening.

[Trevor Wilks]

I have tightened down my refurbished 406 cylinder head according to the routine outlined in the 404/405 workshop manual, and a new gasket supplied by the Bristol Owners and Drivers Association.

What is considered best practice with regard to further tightening? I have been used to a further check on tightness after approx. 500 miles, but also remember that Alvis (Red Triangle) advised re torqueing their heads after running the engine up to temperature for the first time.

What do those who have done this job on a Bristol 6, consider to be best practice?

[Geoff Dowdle]

Trevor ,
The need for retightening a 6 cylinder Bristol is really dependant on the condition of the head stud , head stud washers , head stud nuts and the way it was tightened previously . The other thing to consider is the type of head gasket used, I don't know what BODA supply. When the workshop manuals were produced the only readily available head gaskets were the steel shim type from BCL which I have never used ,but imagine will not compress.
Now most people fit a thicker composite type of gasket which will compress slightly over time due to the torque of the studs and due to heat produced in the engine .
I usually tighten the head studs down at least three times before I start a rebuilt engine, then again after about 500 miles and of course re adjust the valve clearance .
The alloy in the heads seem relatively soft and the original head stud nuts and washers deform the machined surface on the head where the washer sits . I always machine that surface flat . Thick hardened washers and flanged nuts spread the load on the head over a greater area which reduces the deformation of the surface of the head , which retains the torque on the studs.
I also bevel the head stud holes on the head's bottom face , this allows a small cavity for any excessive gasket material to go , due to any inaccuracies in manufacture of the head gasket .
So my recommendation is to retighten the head studs.
Geoff

[Pandah]

I bought my 401 last November. The engine had recently been rebuilt and when I took over the car had done around 150 miles since the rebuild. My understanding was that the head was re-torqued at some time during the 150 miles but has not been done since. I do not know what kind of head gasket was used but assume that it is of modern construction, and therefore compressible.
The engine has now completed over 1300 miles and I suspect that it would be wise to torque the head down again. I have two problems, firstly what torque figure do I use? I can find no reference to any figure in the workshop manual, or anywhere else! Secondly, I do not possess either of the special box or ring head spanners which are apparently required. Some advice would be much appreciated!

[Geoff Dowdle]

Pandah,
If you don't have the correct tools I suggest you contact Tony Byford of Swindon who should be able to help you .
Tony is a specialist Bristol engine builder .
Geoff
Sydney , Australia

[Trevor Wilks]

Pandah,

Hope you got what you needed from Tony Byford. If not, please be aware that there are no accurate torque figures given, as the nuts on the opposite side to the exhaust are inaccessible with a torque wrench. Bristol therefore supplied a special double ended ring spanner when new which was 1/4" Whitworth at both ends and had a bend in it to allow access to the rear nut on the non exhaust side. This allowed only a "one flat at a time" approach as described in the workshop manual, up to the limit in strength of the individual doing the job, or of the spanner !! In most cases the spanner wins and you get an evenly tight result by using the exact same length of spanner for all nuts.

Thanks in part to Geoff's advice, my 406 engine now seems to be working fine after 140 miles. Will wait and see whether I can tighten further at 500 miles.

Trevor

[Geoff Kingston]

At the moment have only workshop run the 85c Engine I bought from Bristol Cars but in view of this topic I asked Brian at Bristol Cars for his advice, he suggested 150 and 500 miles , in the past I have checked after the first run to full temp , at around 250 and 500 miles but this was with the gaskets available at the time.
I am lucky as I have the correct spanners, before the car is run on the road I will do a quick check, if any of the nuts pull up then they will all be done as per the book, a bit of a pain but worth it.
Geoff.

[peterg]

I've been concerned that, since the engine rebore it had been running hot - 90 - 95 while the oil remained constant around 85 - 90. On its third run to the BOC BBQ it shot off the scale by the time I got the car back. Checking, there doesn't seem to be much water in the radiator. Are there engine vents that stop air locks when it is filled up?

Back to the subject, can the head bolts be tightened using a 3/8 socket drive?

[Geoff Kingston]

3/8 drive 1/4 w socket will do the nuts on the drivers side of the engine, 1/4 w obstruction spanner is needed for passengers side, it can be done with a straight spanner but it makes a slow job even slower.
Original spanners are rare and expensive, Greg Lowe had some good quality reproductions made, don't know if he has any left, they were not cheap but they were exactly the right size and shape .
Some years ago in a BOC mag there was a feature on making custom spanners to use sockets and torque wrenches, all the associated calculations were way over my head so if in doubt stick to the original tools and advice in the manuals. Having said that flea bay is a good source of whitworth obstruction spanners that will do the job quicker than a straight spanner and a few years ago I managed to buy some custom/home made spanners which had belonged to someone that had worked on these engines in their heyday, they look crude, are strong and work beautifully , they were not cheap as several owners were obviously after them but illustrated how ingenuity will find a solution often at lower cost than the commercially available options.

[peterg]

Those nuts really are inaccessible. Has anyone had success fitting a 3/8 socket with a flexi socket extension into that space?

[Geoff Kingston]

If you are referring to the passengers side of the engine I think an obstruction spanner is the only answer, a flexi drive even if you could find one that fits would not impart sufficient feel for you to be sure the nuts are as tight as they need to be.
On the exhaust side of the engine as long as your 3/8 drive socket is under 20mm diameter it should fit, use a 3/8 drive extension as a 1/2 to 3/8 will not fit between the pushrod tubes, I would never use a flexi drive for this type of work.
Geoff.

[Trevor Wilks]

My 406 has recently passed the 500 mile mark after its rebuild, I was able to tighten down the head by approx. half a flat at nuts 3,4,5&6 progressing to 3/4 of a flat by nuts 11,12,13 & 14. Nuts 1 & 2 hardly moved. Valve clearances have now been reset and engine has run well throughout having followed Geoff Dowdles advice below. I agree you really need an obstruction spanner similar to the one originally provided by Bristol to get at the nut nearest the bulkhead on the side opposite the exhausts.

Originally Posted by Trevor Wilks

Pandah,

Hope you got what you needed from Tony Byford. If not, please be aware that there are no accurate torque figures given, as the nuts on the opposite side to the exhaust are inaccessible with a torque wrench. Bristol therefore supplied a special double ended ring spanner when new which was 1/4" Whitworth at both ends and had a bend in it to allow access to the rear nut on the non exhaust side. This allowed only a "one flat at a time" approach as described in the workshop manual, up to the limit in strength of the individual doing the job, or of the spanner !! In most cases the spanner wins and you get an evenly tight result by using the exact same length of spanner for all nuts.

Thanks in part to Geoff's advice, my 406 engine now seems to be working fine after 140 miles. Will wait and see whether I can tighten further at 500 miles.

Trevor