Author Topic: Alloy head bolts  (Read 2316 times)

Offline duTch

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Re: Alloy head bolts
« Reply #15 on: 05.03. 2014 09:57 »
Quote
. I have obtained some high tensile socket cap bolts which I have cut to the correct length

Heartening to know...!
 When I originally threw mine together in '79-'80, I used socket caps ( BSF reasonably available then), mainly 'cos finding proper ones at the time was nigh on impossible in my neck o' the woods.
 
On the last build, I wasn't too sure of the socket caps integrity, the unthreaded diameter, or lack of 'waist', so replaced them with 'proper' ones, maybe from Burtons (?), but the lengths were a bit shorter than I'd liked, but haven't caused any trouble (low 7.25 comp.)
 I also tap and air cleaned holes and threads , (blast beads everywhere), and now worth noting the cavity between the sleeves and around the pushrod tunnel was completely clogged.
Started building in about 1977/8 a on average '52 A10 -built from bits 'n pieces never resto intended -maybe 'personalised'
Have a '74 850T Moto Guzzi since '92-best thing I ever bought doesn't need a kickstart 'cos it bump starts sooooooooo(mostly) easy
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Offline BSA_54A10

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Re: Alloy head bolts
« Reply #16 on: 09.03. 2014 05:43 »
I can not see the point of using waisted bolts on these heads.
the usual reason is to allow the bolt to snap before the thead strips.

Also if you use allen headed bolts nearing down onto alloy it is essential that you put some really thick washers ubderneath the heads or they will sink into the alloy.
Bike Beesa
Trevor

Offline duTch

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Re: Alloy head bolts
« Reply #17 on: 09.03. 2014 09:00 »

 Noted Trev, I had reasonable washers under socket caps, but maybe not as robust as their tensile replacements.
 Other thing, I think the shanks of the waisted bolts are a touch thicker, so there's less lateral slop and the head sits a bit more 'precise' (weeeell... *????*)
 cheers
Started building in about 1977/8 a on average '52 A10 -built from bits 'n pieces never resto intended -maybe 'personalised'
Have a '74 850T Moto Guzzi since '92-best thing I ever bought doesn't need a kickstart 'cos it bump starts sooooooooo(mostly) easy
Australia

Offline Briz

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Re: Alloy head bolts
« Reply #18 on: 09.03. 2014 10:27 »
The point of waisted bolts is that the required stretch that provides the clamping force is spread over a length of shank rather than being concentrated at the root of the last bit of thread, which on an un-waisted bolt is the weakest point.

You need good washers under any head bolt on an ally head. There isn't really much difference in under-head area between allen bolts and hex-heads.

Offline BSA_54A10

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Re: Alloy head bolts
« Reply #19 on: 11.03. 2014 22:40 »
The surface area under an allan bolt is 10% less than the  inner  base circle of a hex head.
While that is a very small number, it makes a massive difference to the compressive pressure under the head sufficient to create a shear plane under the head perimiter.

As to waisting , I am aware of the theory however in practice bolts fail in torsion, not tension.
With only hand tools there has never been a man strong enough to stretch a 3/8" bolt pasts it's elastic tensile limit.

Back in the real world.
The head bolts on an A series are so lowly tensioned that in most cases you can get by with good quality ( not Chineese )  grade 5 bolts.
My diction was probably not the best what i waa trying to say is "I Don't see the need for waisted bolts in this application "
Bike Beesa
Trevor

Offline Briz

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Re: Alloy head bolts
« Reply #20 on: 12.03. 2014 23:09 »
Well, a 3/8 allen bolt has a 9/16" head diameter. 3/8" hex bolts have a 9/16" hexagon. OK; BSF ones are a tiny bit bigger.
But my advice was that thick-ish washers should be used under both. Its not good practice to install head bolts into an alloy head  without washers on any engine.

And as regards the stretch, I wasn't talking about bolt failure, I was talking about normal usage. For best clamping, a bolt must be stretched a certain amount. That stretch will occur at the bolts weakest point. Without the waisting, thats all concentrated at the root of the thread.

Isn't the stock head bolt torque about 32 ft-lbs? Thats a fair amount and probably enough to stretch a grade-5 bolt as far as you'd want to. The stretching info above applies to any bolt regardless of tensile strength. It'd still be safer waisted.

A torque wrench is a normal hand tool. I found 90 ft-lbs was enough to break a 10mm stud! It broke at the thread root.

Online RichardL

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Re: Alloy head bolts
« Reply #21 on: 13.03. 2014 01:55 »
Back in 2008, some "Richard" guy posted the following, which he had found while trying to figure out head bolts.

The follwoing (about head-bolt shank diameter) is quoted from Roland Pike as found at http://www.restorenik.com/daytona/RP_chp_22.htm:

After a year we had improved the port configuration and it ran so much better with one big carb that the twin carb option was dropped. We had found it useful to measure the capacity of the inlet ports and check on performance, about 142 to 150cc's gave optimum results. If an engine was down for power we often found the ports undersize.  Like all aluminium cylinder heads, these expanded a lot with heads and at the outset we experienced stretched or broken head bolts. These bolts went downwards through the head into the iron cylinder block and would usually break at the root of the last thread, which was the weakest point because it took all the stretching. To overcome this we quite simply put the bolts in a lathe and reducing the diameter of the pIain portion to 10% less than the diameter of the root of the thread. This meant that the thread was no longer the weakest point and that the plain portion could stretch without exceeding its elastic limit.

This was completely successful on the first attempt and no more trouble was experienced with the bolts. They could stretch when the head expanded and return to their original length as the head cooled own. When the new twins were going into production however, Alan Jones who was Works Manager at that time phoned me to say they were unable to make the head bolt as needed, despite my pointing out that we bad found them necessary, he just continued to say they were unable to make them. I went on with my work and forgot about the matter, but I did not have to wait long, within a few hours the motor cycle test shop foreman was on the phone to me complaining the head bolts of the new twins were breaking right and left, so I referred him to Alan Jones.  Mr Jones reiterated they were unable to make the bolts we had designed. Prior to this last call I had taken the precaution of calling the drawing office to say that Jones would not follow their drawings of the bolt thus securing an ally. Alan Jones got no sympathy when he had to pull all the bikes concerned back, dismantle the engines and use the bolt we had specified.

The following is copied from: http://www.enginehistory.org/british_fasteners.htm

Bill Allan adds "It should also be stated that some of the early nuts would have been machined from round stock, with an integral washer, so round bar stock sizes would have been involved in calculations. One other problem is that the original Whitworth heads (AF) were too large relative to the actual bolt shank, (that's why spanners/wrenches are the length/size they are: so you can't apply to much torque. In the first part of the 20th Century, the head sizes were reduced to the size one below. (British Standards specify that the AF measurement of any bolt not be greater than 1.75 that of the shank: 1/4 inch Whitworth is almost bang on the button) This causes even more problems when you need replacement Whitworth fasteners, for machinery over a 100 years old. It's also the reason why some old spanners/wrenches have two Whitworth numbers on them."
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