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Re: Length head bolts / leaking head gasket

Thanks John, found the bolts on ebay!


Michiel W
Question is how long this bolt 36-11 is, is it the 51 mm version or a shorter version??



The longer ones are just 3 mm longer then the shorter ones! According to the measurement on ebay.
I would have thought the difference would be much bigger!

Michiel, the 36-0011 seem to be 41+ mm , so your 44 mm ones are even longer then the long version! Your 51 mm ones seem really long then!

Re: Length head bolts / leaking head gasket

I also fitted a full copper head gasket (annealed it first) and on the first couple of runs noticed a 'damp' area on the fins where the join is, this got less ad less each run out soon there was nothing there and it's completely sealed.

Re: Length head bolts / leaking head gasket

Thanks! I have an iron head, so I will be looking for 5 longer bolts. Or six!!:grimacing:

In the Netherlands I can find copper spray from Motip, and copper gasket cement(VHT).
I guess they are both the same. The copper spray is advertised for screw joint, but heat resistance.

Cheers Menno

Re: Length head bolts / leaking head gasket

['Menno, a set M20 headbolts, sold by Njavery on Ebay, consist of 4 short and 6 long bolts.
Something I wrote a few days ago']

As noted in my previous post the wartime parts list shows 5 long and 5 short bolts...

The post war parts list shows 5 long bolts, 4 short bolts and the head steady bolt listed under a separate number...I'm assuming that the different number indicates it is a different length to both the long and the short bolts. If not it would be the same number..

So, to be absolutely correct the wartime bikes should have 5 long and 5 short bolts...The postwar bikes would then have 5 long, 4 short and one other of a different length...That leaves 6/4 being incorrect...(unless there's another parts list somewhere)..Ian

email (option): ian@wright52.plus.com

Re: Length head bolts / leaking head gasket

The 1939 and 1940 civy parts list illustrate the bolts with dimensions, 5 of each.

Long 36-11/1 1 3/4in

Short 35-1086/1 1 7/16in

Rob

email (option): robmiller11(a)yahoo.co.uk

Re: Length head bolts / leaking head gasket

Long 1 3/4" with a washer and gasket leaves only 11 mm into the cast iron cylinder!, no wonder the threads strip so easily!

BSA must have realised that at some point and increased its length, as I have a few 2 inch headbolts lying which looks M20 to me. These 2 inch bolts might need a bottom tap being run through the threadholes but give of course much less risk of stripping.

--

Ian, I can't fully follow you, don't you think the 6/4 configuration is a better option than the 5/5 as you replace one short one with a long one to compensate for the headsteady thickness.?

email (option): m.wijbenga@hotmail.com

Re: Length head bolts / leaking head gasket

Hi Michiel..I'm not suggesting what is or isn't best...I'm pointing out that there are two options shown...5/5 or 5/4/1....

There is no 6/4 option shown in the parts lists....Ian

email (option): ian@wright52.plus.com

Re: Length head bolts / leaking head gasket

I received my NOS bolts 5 long 5 short to replace the badly pitted ones .I removed the old ones and have 9 short and 1 long , the long one being in the head steady, looking at the end of thread on some of the bolts they are bruised and looks as if they have just to say held in .

My problem now is where are the long bolts located in the head ? :confused: , what is the correct configuration 5/5 , 6/4 .

Sat on the bike on the left of the head between the 2 outer head bolts there is a smaller bolt , curious to what purpose it serves :thinking_face:

email (option): davemb42@yahoo.com

Re: Length head bolts / leaking head gasket

hi Menno,
another thing to check is the torque of these bolts because they move around a bit before settling. If I have had the head off, it seems that after every ride I need to re-torque them for a few rides or so (worse on LHS). Always done when engine is dead-cold of course. This seems to be common for all the British iron I have and might solve your leaks.

Re: Length head bolts / leaking head gasket

Dave
The small bolt you mention on the left of the head is basically a plug. It's to be removed when you time the ignition. A marker's inserted through the hole & contacts the piston so you can see what part of the stroke it's on. Make sure there's a lip on the marker so it doesn't fall into the barrel.

Re: Length head bolts / leaking head gasket

Bob
Dave
The small bolt you mention on the left of the head is basically a plug. It's to be removed when you time the ignition. A marker's inserted through the hole & contacts the piston so you can see what part of the stroke it's on. Make sure there's a lip on the marker so it doesn't fall into the barrel.
Thanks Bob , should have seen that ...... I'll go give my head a shake and slap.
I dread to think if anything major goes down on this machine I'll be like a duck out of water :worried: , maybe I should have stuck with what I know the 2 strokes.

email (option): davemb42@yahoo.com

Re: Length head bolts / leaking head gasket

Dave
Don't worry, you can learn this stuff easily & the knowledge stays with you.
I notice you also asked about where the long & short bolts go. Basically, the long ones go in the holes where the head is thickest.
I agree with those who've said you should torque the bolts after the first few rides, with the engine cold.
When I was running mine on a daily basis, I'd occasionally slacken the bolts & re torque them. I found that bolts tended to freeze in the barrel, & worried that the torque setting would be correct simply because they were stuck, & not necessarily bearing down on the head & gasket. Probably overkill on my part?

Re: Length head bolts / leaking head gasket

Bob / Dave,

Thinking back to my days of a skilled engineering fitter, we had instructions on our drawings wether to oil the thread or run it down dry before you torqued them down. Oiling the thread made a big difference to the torque figures. Torque figures are given so you do not damage anything, could be maximum stretch on the bolt you are torqueing down with out permanently streaching the bolt. Or you are not permanently damaging something that is clamped between the faces. Sure Ian will chime in on this one he can word things better then myself.
I always used to just run my M20/21 head bolts down dry, but used to run a tap down the threads first if they were a little sticky. Last few times a have used copperslip on the threads. First wondered if the copperslip would go hard due to the heat round the bolts near the exhaust port but it's not a problem. I have never torqued a M20/21 head down i just run them down till my elbow clicks. Good tip is if you fit one of the new 2 part gaskets run the bike for 10 mins and tighten down again, you will be amazed how slack the bolts have become. Keep tighteng up after every short ride till the bolts dont move. Best thing is to fit a solid copper gasket if you can get one.
Dave you will get to grips with the bike they are simple, you just need to understand how the things work before you pull anything in bits.

Tim W

email (option): t.j.walker@btinternet.com

Re: Length head bolts / leaking head gasket

Bob
Dave
Don't worry, you can learn this stuff easily & the knowledge stays with you.
I notice you also asked about where the long & short bolts go. Basically, the long ones go in the holes where the head is thickest.
I agree with those who've said you should torque the bolts after the first few rides, with the engine cold.
When I was running mine on a daily basis, I'd occasionally slacken the bolts & re torque them. I found that bolts tended to freeze in the barrel, & worried that the torque setting would be correct simply because they were stuck, & not necessarily bearing down on the head & gasket. Probably overkill on my part?
Thanks Bob
Aye its going to be a learning curve :grin: .Funnily enough I did look at what you say "the thickest part" but they did go tight so I shyed off from putting the long bolts in , maybe this is why it had 9 short bolts fitted previously ? . I think I'll do what Tim has suggested and run a tap down the threads as they were sticky when I was putting them in so the Torque setting no doubt will be out , I read somewhere that the Torque is 28ft lb

Another question as I'll have to buy a tap, is what is the actual Thread size ? also I'll get a new copper gasket.

email (option): davemb42@yahoo.com

Re: Length head bolts / leaking head gasket

Tim W
Bob / Dave,

Thinking back to my days of a skilled engineering fitter, we had instructions on our drawings wether to oil the thread or run it down dry before you torqued them down. Oiling the thread made a big difference to the torque figures. Torque figures are given so you do not damage anything, could be maximum stretch on the bolt you are torqueing down with out permanently streaching the bolt. Or you are not permanently damaging something that is clamped between the faces. Sure Ian will chime in on this one he can word things better then myself.
I always used to just run my M20/21 head bolts down dry, but used to run a tap down the threads first if they were a little sticky. Last few times a have used copperslip on the threads. First wondered if the copperslip would go hard due to the heat round the bolts near the exhaust port but it's not a problem. I have never torqued a M20/21 head down i just run them down till my elbow clicks. Good tip is if you fit one of the new 2 part gaskets run the bike for 10 mins and tighten down again, you will be amazed how slack the bolts have become. Keep tighteng up after every short ride till the bolts dont move. Best thing is to fit a solid copper gasket if you can get one.
Dave you will get to grips with the bike they are simple, you just need to understand how the things work before you pull anything in bits.

Tim W
Thanks Tim

I'm taking all this on board and will say this forum is a minefield of information , all the information and advice are very much appreciated

Dave

email (option): davemb42@yahoo.com

Re: Length head bolts / leaking head gasket

There is no laid down torque setting for the M Series head bolts as they predated the general introduction of the modern torque wrench...

The 28-30ft/lb. figure I regularly quote in these posts is one arrived at by looking at the settings for other later engines where figures are quoted and which shared the same head bolt diameters. This was then 'fine tuned' by experimentation...
I would recommend the use of a torque wrench unless you have a finely developed 'sense of feel'..Many engineers and others who regularly work on machinery do have this ability to 'feel' the pressure applied and are able to repeat it with some accuracy over a number of fasteners. However, even if effective it could not be deemed the most accurate method...

Check to ensure both the barrel and head are flat on the gasket face..They can be surface ground or lightly skimmed using a flycutter in a milling machine if distortion is present. Remove the absolute minimum necessary to produce a flat surface and ensure a good surface finish is achieved...

It is also best to fit heavy duty washers under the bolt heads. Standard washers tend to collapse under the pressure of tightening....080" (2mm) thick would be reasonable for the application...

To avoid the job of having to repeatedly retorque the head bolts after fitting a new head gasket I would strongly recommend the fitment of an annealed 1mm thick solid copper gasket...After initial torqueing up only one check on the bolts is generally required after running the engine. Run up to full temperature and then let it go completely cold before retorquing.

In my experience I have found the modern composite gaskets unreliable and unpredictable in their performance...This I feel can be attributed to the fact that the central layer of the composite has been changed from the original asbestos to a safer but less effective alternative...

Personally, when using a solid copper gasket I find that the fully annealed gasket fitted 'clean', without the application of any additional sealants, performs without problems...However, I realise some prefer the 'belt and braces' measure of using some 'Welseal' or similar for peace of mind...

The threads for the head bolts should routinely be tapped out during a rebuild or when the head is off for maintenance to be carried out...The standard thread is 3/8"BSF but if the engine is not one you have worked on previously be aware that some or all of the threads might have been changed if there have been previous problems in this area.

Lubricants can distort the accuracy of the readings of a torque wrench as the coefficient of friction between the bolt and the bolt thread is affected by the type of lubricant used....

From my own experience in engineering if the use of a lubricant is desirable then a very thin machine oil or similar will have the least effect on the settings...This is what I use in conjunction with the settings noted above and I haven't had any problems with the threads to date....

Another, perhaps minor, thing to consider is that a torque wrench becomes less accurate as it operates towards the limits of its range.. Many years ago when I started to torque the head bolts I purchased a wrench with a range of 0-50ft/lbs. so that the required setting was more or less the 'mid range' of the wrench and thus as accurate as possible..

Tighten the head bolts progressively and evenly using the tightening sequence shown in the manual...This more particularly applies if an aluminium head is fitted as these are more easily distorted....Ian

email (option): ian@wright52.plus.com

Re: Length head bolts / leaking head gasket

On the subject of torque settings, my original M20 manual says to do the head bolts up "dead tight".

Re: Length head bolts / leaking head gasket

Thanks Ian .I'll start again and run the threads through . I did manage to get some NOS washers with the bolts which are I would say are a good 2mm thick .

Dave

email (option): davemb42@yahoo.com

Re: Length head bolts / leaking head gasket

Thanks Ian .I'll start again and run the threads through . I did manage to get some NOS washers with the bolts which are I would say are a good 2mm thick .

Dave

email (option): davemb42@yahoo.com

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