Lotus Elan

Prop Shaft Bolt Spec

PostPost by: RichardHawkins » Sat Mar 25, 2017 6:36 pm

I would like to replace my prop shaft bolts, and was intending to use M8 rather than 5/16", a previous owner was a bit heavy handed (head bolts and cam cap studs were stretched) but I cannot see any indication of the grade of bolt, on the old bolt heads.

Does anyone know what grade of bolt was originally used?

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PostPost by: Chancer » Sat Mar 25, 2017 7:03 pm

Propshaft bolts are fitted bolts with a close tolerance shank driving through a close tolerance reamed hole in the diff and propshaft flanges, you should never even use standard bolts let alone metric, its unlikely that M8 bolts would pass through as their nominal size is slightly larger, in any case you must never ever use setscrews (bolts threaded along the length) an M8 setscrew probably would pass through the hole hence my warning.

Any movement no matter how tight the bolts are and there will be fretting, the holes will wear oval and the bolts will loosen and/or shear off.
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PostPost by: mbell » Sat Mar 25, 2017 7:43 pm

I'd think grade 5 or 8 would be fine. The bolts are just needed to clamp the flanges together and the power is transmitted by the friction between the flange faces not the bolts.

However I fully agree with Chancer that the correct size bolt and critically the correct length shanks must be used. The shank should be deep enough to be in contact with both flanges. This ensure the flanges are locked together.

Most of the usual suppliers will have the bolts available if you can't find suitable bolts locally.

On my +2 5 speed both ends of the shaft bolt but their front flanges are thicker so require different bolts to the rear.
'73 +2 130/5 RHD, now on the road and very slowly rolling though a "restoration"
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PostPost by: Chancer » Sat Mar 25, 2017 8:46 pm

If you think about your statement Mbell you will see that the first paragraph is contradictory to the second, one of my first ONC lessons on limits and fits, tol?rances and interchangeability (BS4500a) used the example of g6 fitted bolts and H7 reamed holes in propshaft drive flanges and the cons?quences of not using the correct close tolerance drive bolts, I have personally experienced the results of someone who should have known a lot better using the wrong bolts in the drivetrain of a Caterham Blackbird.

I had to drag out my 1974 Zeus book to recall the tolerance numbers, the one I was given on the 1st day of my apprenticeship!

I do recall however removing a propshaft from a much later Ford, a Sierra or Granada perhaps and being surprised to find that blue threadlock coated setscrews were used into tapped holes in the diff flange by the factory so clearly on that vehicle it was the friction of the flange taking the drive, thats not the case on the Anglia diff and I have seen many a loose prop with incorrect bolts.
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PostPost by: mbell » Sun Mar 26, 2017 2:09 am

I have to admit to being a layman on the details of fasteners and fittings so not coming from this as an expert.

In the pursuit of increasing my knowledge what you are saying is that in the prop setup for the Anglia/elan flange bolts a significant portion of the power transmission and the friction between the flanges isn't sufficient to transmit the power. Therefore if bolts without the correct width and length of shank are used it is likely the two flanges would rotate with respect to each other damaging the flange bolt holes and possibly causing bolt failure.

Is this correct?
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PostPost by: CBUEB1771 » Sun Mar 26, 2017 3:32 am

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PostPost by: prezoom » Sun Mar 26, 2017 3:39 am

For me the best choice are AN bolts that offer different shank lengths. This puts no threads in shear and when coupled with K or Jet nuts makes for easy assembly/disassembly. Agreed they may be hard to come by in the mother country, but Coast Fabrication in California has what you need. Pegasus in Wisconsin also stock AN hardware. You can go a bit long on the shank and make up the difference with either thick or thin AN washers.
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PostPost by: Chancer » Sun Mar 26, 2017 10:02 am

mbell wrote:I have to admit to being a layman on the details of fasteners and fittings so not coming from this as an expert.

In the pursuit of increasing my knowledge what you are saying is that in the prop setup for the Anglia/elan flange bolts a significant portion of the power transmission and the friction between the flanges isn't sufficient to transmit the power. Therefore if bolts without the correct width and length of shank are used it is likely the two flanges would rotate with respect to each other damaging the flange bolt holes and possibly causing bolt failure.

Is this correct?


The drive is taken by the bolts in shear, if it were friction then the flange would need to be the same diameter as the clutch plate and made of friction material. Wheel bolts take the drive, its not the friction of the mating face, were that the case there would not be drive pegs on knock on wheels.

It is imperative that all the bolts share the drive force or the flange will come loose and/or the bolts fail just as if you only secured a road wheel with only one nut, this can only be achieved by the holes being centred precisely to a close positional tolerance, being drilled then reamed to a H7 tolerance and the bolts having close tolerance shanks, they used to be known as "fitted bolts" you can tell this by how tight they are when you push them in (a close clearance/transition fit) and often the 4th one will take a little persuasion.

Using the wrong bolts, and i am not talking about their tensile strength, will result in failure in exactly the same way that assembling a flywheel without the drive dowel or the correct fitted bolts if no dowel will shake loose.

I fitted a second hand steel flywheel to a forged steel crank on a highly tuned crossflow, I used the correct close tolerance cap head bolts, (I think there may have been 12) but had not realised that either someone before me hadn't and the holes had opened up or more likely like most of the Junk that is produced these days for our vehicles the manufacturer had no concept of tolerancing and the need for limits and fits. Testing at goodwood I was convinced that I had run all the main bearings or even broken the crankshaft, never heard a more horrible noise, I was suspicious though as I had not heard any big end Knock, just what I thought was main bearing rumble which at tickover (highest angular velocity changes) was at its loudest and sounded like a broken crank.

I suspected and found that the flywheel was rotating relative to the crank, the bolts were all still torqued up, I could not move it by hand but when I removed it I saw the oval holes and material from the flywheel had cold welded to the crankshaft.

I recovered the situation by reaming out 4 of the fixing holes in both the flwheel and to a short depth in the crank and using the hollow bellhousing dowels to take the drive, that way I could still use all the retaining bolts, had no problems after that but have heard and seen many other engines using aftermarket flywheels and steel cranks fail in exactly the same way.

I have also experienced nearly dropping the propshaft on a Caterham Blackbird where the wrong bolts were used in the Guido coupling from the gearbox to the front prop, in this case the drive was being taken by the threaded portion on one flange, that was bloody scary how long it drove without incident but how quickly it failed when it did with near catastrophic results.
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PostPost by: patrics » Sun Mar 26, 2017 10:47 am

Hello Richard,
you can buy the bolts you need from: nasbolts.co.uk

order the bolts, washers and nuts from same place.

The washers are different depending on where they sit - under the bolt head or under the nut.

Regards
Steve
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PostPost by: Craven » Sun Mar 26, 2017 11:14 am

H7/g6 G7/h6. Sliding fit not intended to run freely, but to move and turn freely and locate accurately.

This standard of fit would require exceptional Positional Accuracy over 8 holes especially with individually manufactured flanges.
I wonder.
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PostPost by: rgh0 » Sun Mar 26, 2017 11:39 am

I doubt that Ford originally match drilled and reamed for an Anglia to get the required fits to drive off the bolts without relying on the friction from the clamping force but I could be wrong as I have never seen the original Ford manufacturing specifications. i am certain that Lotus did not spend money to try to make the fits better than Ford did.

You can get much more clamping force with 4 small bolts than you will get with a clutch pressure plate. There is at least 10 tonnes of clamping force if the bolts are torqued correctly. The friction between the two flangrs will be sufficient to carry all the torque of the prop shaft. The bolts normally will only fail when they loose tension for some reason and the friction force goes away and the bolts are loaded in shear and then cyclic loads will cause them to fail..

The reason you should use bolts with the shank that goes through both flanges is that you get some elastic deflection at the flanges interface with torque and that small deflection can lead to fluctuation in stresses in the bolts and ultimately fatigue cracks in the roots of the threads and failure. Also if the bolt is not torqued correctly it will be less likely to fail from fatigue.

Flywheels and knock on wheels are a somewhat different situation and not directly comparable.

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PostPost by: CBUEB1771 » Sun Mar 26, 2017 12:32 pm

rgh0 wrote:I doubt that Ford originally match drilled and reamed for an Anglia to get the required fits to drive off the bolts without relying on the friction from the clamping force but I could be wrong as I have never seen the original Ford manufacturing specifications. i am certain that Lotus did not spend money to try to make the fits better than Ford did.


There is another issue in that concentricity of the yoke-to-diif flange assembly is controlled by the central, circular features turned into both parts. A reasonable diametral clearance for the flange bolts is necessary to prevent an interference between the bolts and these circular features. I like to use the MS21250 bolts because they are held to tight manufacturing tolerances. For example the shank diameter for a 5/16" bolt must be between 0.3110" and 0.3120" meaning the the average shank diameter for a MS bolt is larger than the average for a commercial automotive bolt.
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PostPost by: Chancer » Sun Mar 26, 2017 1:09 pm

Craven wrote:H7/g6 G7/h6. Sliding fit not intended to run freely, but to move and turn freely and locate accurately.

This standard of fit would require exceptional Positional Accuracy over 8 holes especially with individually manufactured flanges.
I wonder.
Ron.


Indeed and that is another element of tolerancing that I as a designer had to specify on my drawings when the function required it, geometric and positional tol?rances, and as a propshaft joint was used as the prime example during the HNC lessons on the subject I can assure you that it requires carefull consideration to all of them.

I am however a dinosaur, I knew that i was becoming one when speaking to the new generation of bosses of sub contract manufacturing companies about which tol?rances were important and could their machines meet them consistently, they woud reply everything that comes off this machine is spot on :( and I knew then that tol?rances, the need and reason for them meant nothing to them, its far worse today.

Modern machines can easily exceed even the tightest of tol?rances but "spot on" does not exist and that phrase tells me all I need to know about the person uttering them, I still have some involvment with a?rospace manufacture by virtue of the Airbus factory opposite me, one of the draughtsmen or whatever they call themselves these days (ing?nieur conception here) is my tenant and I run with his boss who is my age, he confirms that the new generation have no concept and when all goes well, when the CNC machines do as they should then its not Worth trying to teach them as stuff is close to spot on but nobody is involved with inspection these days just relying on an ISO 9000 paper chase and if the tol?rances are not specified, if limits and fits are not even considered then an 8mm bolt could 8.5mm and not go into the prick in a bucket 10mm clearance hole because that actually measures 9.5mm and the positional accuracy of all the holes in the flange are off to a similar degree.

Anyway, back to the OP, it really is not a good idea to be using metric fasteners in a drive flange manufactured for close tolerance imperial ones, nominally an 8mm will be 2.4 thou bigger than a 5/16" one anyway.
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PostPost by: RichardHawkins » Mon Mar 27, 2017 7:59 pm

Gentlemen,

Thanks for the replies. I thought it was a straight forward question. I didn't realise it was a-contentious subject.

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PostPost by: 2cams70 » Mon Mar 27, 2017 9:05 pm

Ditch the Anglia propshaft flange. No way was it ever designed to take the torque of a standard Twin Cam engine never mind a tuned one. The bolt PCD and flange contact area is too small. The smaller the PCD the higher the shear force on each individual bolt for any given torque transmitted. For 1600gt crossflow motors Ford always used a larger diameter flange and bolt PCD. All late Mk2 Escorts were larger again wth threaded in flange bolts rather than through bolts. Lotus should have switched to the larger flange when Ford did but it seems they didn't!
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