Author Topic: Quick question lads, cylinder head torque  (Read 6811 times)

Offline Stephen Arsenal

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Quick question lads, cylinder head torque
« on: 05.03. 2010 13:38 »
how tight must the bolts in the top end of the cylinder housing /cooling flange part be ? the are about 7 or 8 long bolts
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Online RichardL

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Re: Quick question lads, cylinder head torque
« Reply #1 on: 05.03. 2010 14:02 »
Stephen,

There is quite a bit of discussion on this topic. Click the "Search" tab above and search "head torque" (without quotes). One thing to not overlook: if your head is alloy you must use thick washers with OD to fit in slight counterbores around the holes. I use two grade-8 washers per bolt and they just fit. Don't try common washers because the diameter will be wrong. If you don't use some effective washers, the alloy around the hole will squash a little (ask me how I know) and make it difficult to remove the bolts and bad for the head.

To your original question, "how tight?", that's a matter of a lot of opinion, as you will read in the previous posts, so I'm definately not going out there as an expert on the matter.

Richard L.
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Re: Quick question lads, cylinder head torque
« Reply #2 on: 05.03. 2010 14:11 »
G'day Stephen,
                     We are talking head bolts aren't we ? 25 to 30 Lb/inch, lubed threads. Alloy head, new or anealed copper gasket. Works for me at 10.5:1. Re-tension after 500 miles.
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Online RichardL

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Re: Quick question lads, cylinder head torque
« Reply #3 on: 05.03. 2010 16:06 »
Ooh, Muskrat,

You've opened up a whole other issue. I've found that new (solid) head gaskets are not particularly well annealed.

Richard L.
Plan on signing up for the world-wide 2020 DISTINGUISHED GENTLEMAN'S RIDEon September 27, 2020 (if it's not cancelled and we are free to move about by then). Watch website at https://www.gentlemansride.com for details.

Offline Josh Cox

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Re: Quick question lads, cylinder head torque
« Reply #4 on: 05.03. 2010 19:21 »
I've been told by someone smarter than me to use 32 ft/lbs.

Looks like this will be another "more questions than answers thread"

Copper melts at 1083 degrees C.

Would heating (annealing at say 7-800 C) a new gasket up to a high temperature then installing on a bike and TQ'ing up, remove/reduce the requirement for a re-TQ ?.
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Offline MG

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Re: Quick question lads, cylinder head torque
« Reply #5 on: 05.03. 2010 20:35 »
Like Richard mentioned, the correct washers are essential for the alloy heads. I tighten the head bolts with 45Nm (approx. 33 lb/ft, iron head bolts are tightened "tiger-tight" ;)). Last time I fitted the head of the SS I gave each bolt a good tap on it's head with a large hammer and a copper drift after tightening, then allowed the joint to settle for 2 or 3 days and re-tightened the bolts prior to fitting the rockerbox. The head joint is tight ever since without any re-tightening (approx 1000 miles).
Annealing a new copper head gasket is a must. They are manufactured by stamping and therefore the copper is work-hardened accordingly. Just heat it to a bright-red colour and quench it in cold water.

Cheers, Markus
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Re: Quick question lads, cylinder head torque
« Reply #6 on: 05.03. 2010 21:17 »
I'll give that a try next time Marcus.
I did mean Ft/Lb's not inch.
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Offline MG

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Re: Quick question lads, cylinder head torque
« Reply #7 on: 05.03. 2010 21:23 »
I thought so, muskrat  ;). Boy, am I glad we got the metric system here.  *smile*
I have to add that the head and barrels had been skimmed last time, so getting it tight was definitely easier. But worked pretty well nevertheless, and after some days of settling you will notice some degrees of movement when re-tightening. Perhaps might even be a good idea to repeat this procedure a few times.

Steve,
are you aware that there is a tightening sequence for the head bolts? You have to follow this, otherwise you will hardly ever get the head joint tight (especially with alloy heads).
For the iron head, like yours, I used 55Nm last time which worked pretty well with the same trick of settling and re-tightening, too (no re-tightening necessary after 800 miles now). I think there is no official torque figure for the iron heads, at least I wouldn't know of any. I just chose the value by slowly increasing and using the elbow-meter. Using the torque-wrench finally makes sure all bolts are tightened equally and the head doesn't get distorted.
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Offline MG

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Offline a10gf

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Re: Quick question lads, cylinder top torque
« Reply #9 on: 05.03. 2010 22:51 »
Confirm MG's conversion, but is the correct denomination ft\lb, or what?
The other way around, 1 Nm = 0.73756 ft\lb, soooo... with numbers rounded for real life:

to get ft\lb to Nm: ft\lb x 1.35
to get Nm to ft\lb: Nm x 0.74

Back to topic on the cylinder head bolts, I believe the tightening sequence is very important. See http://www.a7a10.net/BSA/techpics/sequence.jpg
The bsa service sheet mentions head bolt torque: "Make them all really tight"... Still looking for what that is in exact terms. On my iron top, I use around 34 ft\lb - 45 Nm, tightening the bolts in sequence with 5 ft\lb. I've seen recommendation to oil the bolts before tightening (needed?). One important detail, any sign of wear on the threads of bolts, get new ones. Owners manual recommends re-tightening after 250 miles on a new engine (= with a newly annealed copper gasket). Stripping the cylinder threads must be quite catastrophic, so I'd believe the lowest useable torque is the goal. To whoever who does not have a torque wrench, get one.

This is what I know, would be nice to know the exact torque recommendations for iron and alloy head.

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Re: Quick question lads, cylinder head torque
« Reply #10 on: 06.03. 2010 10:25 »
Hi Mg , a10gf and anybody who read my conversion chart, it is rubbish and I have removed it for safety.
My torque wrench might be the same as yours a10gf as it has the same conversion figures moulded into the lid, none of the units match the figures I have on the spread sheet, and what are MKgs (Metre Kilograms ?) it was a down load from the internet so probably I have pressed a wrong key or something but can't explain why it has the LB/FT and NM headings unless I have altered the spreadsheet at some time.
So sorry for the misinfo

All the best - Bill
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Offline MG

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Re: Quick question lads, cylinder head torque
« Reply #11 on: 06.03. 2010 10:36 »
Bill,
Don't worry, we all make mistakes!
mkg is an obsolete torque unit in the metric system. It is meter kilograms, that assumption of yours is correct. If you multiply the weight of an object in kg by the gravitational acceleration (9.806 m/s^2), you get the gravitational force in Newton. Therefore 1 mkg equals 9.806 Nm.


Like a10gf mentioned, slight oiling of the threads and the underside of the bolt head is vital. The idea behind torque or angle-controlled tightening is to achieve a specific, calculated pretension on the bolt that prevents loosening and provides residual clamping force under operational loads.
The higher friction of a dry thread and bolt head reduce this pretension. To achieve the same pretensional force with a dry thread would mean applying a higher tightening torque. On critical thread joints usually the torque figure always is given in conjunction with lubrication infos.
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Online RichardL

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Re: Quick question lads, cylinder head torque
« Reply #12 on: 06.03. 2010 13:51 »
Markus,

First, I follow your logic and can tell that it is based on a strong engineering background, however, it tickled a memory of something I read in Roland Pike's autobiography. The chapter in question is at http://beezagent.blogspot.com/2009/02/roland-pike-autobiography-chapter-30.html .

Looking at the following paragraph taken from there, you will see at the end that they thought assembled clean was better/necessary. Now, that was not long after the invention of torque wrenches, so theory and technology have evolved and, as far as I would know, the ideas for best practice in this regard could have changed or, perhaps, they were just flat wrong. In any case, I think it is interesting reading and, most importantly, tells me that memory loss is still at bay (a bit, at least).

The A7 and A10 connecting rod bolts were originally BSF threads, we changed this to 26 threads per inch cycle threads as we thought larger area at the root of the thread would make a stronger bolt, we had to reduce torque when tightening the big end nuts proportionally, unfortunately someone in the drawing office forgot to tell the engine assembly and several bolts were broken before the error was discovered. Even more strange was the fact the broken bolts only happened to one of the several mechanics assembling the crank assemblies. I was asked to investigate, so sent Arthur Butler up to the engine assembly to watch how they did this torquing up. He returned a little later laughing with a small tobacco tin in his hand, It had plumbers tallow in it, apparently only one mechanic was putting tallow on the threads before tightening, with the results that nuts tightened appreciably more before the torque wrench recorded the correct torque figures. We accordingly tested some bolts with various lubricants on the thread and decided that important bolts such as connecting rod bolts should be assembled clean.

Ricahrd L.
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Offline MG

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Re: Quick question lads, cylinder head torque
« Reply #13 on: 06.03. 2010 15:01 »
Richard,
interesting reading indeed. Absolutely makes sense to me. They changed the bolts to a finer thread, which makes the root diameter larger, but naturally the threads themselves become shallower, which makes them prone to stripping by over-tightening. I am positively sure that this was the failure here, I can't imagine the bolts with the now larger root diameter really broke, they just stripped the threads with the now to high torque value. The threads get stripped by the axial tension which is controlled by the tightening torque and which increases with the use of lubricant, like I mentioned before. So assembling clean definitely helps here.


As there seems to be some interest in this topic, I will try and explain what is involved a bit:

In today's literature you will find tables with torque figures for different thread sizes and different kinds of lubricants. Also the surface treatment and the manufacturing process of the bolt itself and the base material makes a significant difference in friction behaviour (zinc or cad plating, blackening, cut or rolled threads, etc.). The tables are based on empiric data gained from bolting experiments.

The article shows us, that these things had indeed been considered and investigated by the BSA engineers, but I guess much of the information on specific bolt joints has been lost during the centuries.
The question of oiling or not is not a matter of better or worse, it is a matter of what pretension value had been determined for the screw joint and how this should be achieved by defining the friction characteristics and the appropriate tightening torque. If the engineers have stated to assemble a critical bolt joint dry, then you should do so. If it should be oiled, then use oil.
Nevertheless, the head bolts are not really crucial. It is first of all a matter of imposing enough surface pressure on the copper gasket to make it yield in order to creep into any recesses in the head or barrels. The plastic deformation of the head gasket is the main reason for the settling of the head bolts and the need for re-tightening. Secondly the pretension of the bolt joint has to be high enough to avoid lifting of the head when exposed to the combustion pressure. When running the engine the heat makes the copper softer and promotes the settling. The bolts loose their tension, the residual clamping force is lost and the head joint ís leaking.
Of course the torque value has to be small enough not to break a bolt or strip a thread, that goes without saying. The pretension and an operational tension add up when having the same direction of force, so over-tightening can also lead to a failure during operation by over-stressing the bolt!

The tightening procedure I stated is my personal experience. This way I managed to get both head joints tight without the need to disassemble and re-tighten after running the engine. I did oil the bolts slightly and it worked okay this way.
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Offline The General

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Re: Quick question lads, cylinder head torque
« Reply #14 on: 06.03. 2010 17:37 »
Chiltons BSA guide has this A group sequence and A50/A65 torque settings. I would guess A10 head bolts would be same torque :
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