Light Aircraft Association

Im looking for 6 new prop bolts for my VW but i have been advised that the bolts i require are very difficult to obtain.
Metric 8mm 8.8 Prop Bolts, 75mm in length inc head, 35mm of rolled thread.
I know its an unusual thread length but my VW has oversize lugs on the hub and i wanted to avoid cutting them down.
Any ideas as to where i could obtain these bolts??

Is it a Peacock conversion? Might be just as easy to get the 'nuts' remade in UNF threads. Alternatively, I might have a complete metric prop hub on the shelf, but I'm mid-house-extension at the mo and the garage is a mess.

Id rather not replace the whole hub if i can help it. Changing the lugs themselves is an option as they are contact fit and not welded, just means removing the hub as they are too long to knock out.
Just fustrated i cant get hold of 6 bolts!

They are not that hard to obtain. There are no such thing as "prop bolts" they are just bolts of the correct grade and size. I'm no expert, but If I recall correctly the 8.8 is the grade of steel (HT I think?). I got mine from the nearest main commercial nut and bolt supplier. Look in the yellow pages and then take one of the bolts along. If they are half decent they will match it. The bolt you get won't have the wire locking hole in the head, but these are easily drilled out even with a normal hand held power drill.

Happy hunting, Ivan
________
Sienta

The 8.8 stands for 8mm and .8 is the grade, this being the highest so for use as a prop bolt.
I think the problem is that there are standard thread lengths and the length i want isnt so i may have to have them made up for a price, i dont know. Unfortunately they cant just dye them down as they must be rolled threads.
Thanks anyway, il make some enquiries.

The code 8.8 relates to the strength and elasticity as I understand it. The first 8 is an abreviation of 80 kg/sq.mm ultimate tensile strength (the load at which the steel will fail). The second 8 relates to the point at which the steel will begin to stretch plastically - in other words, after which the bolt will not return to its original length but will be permanently deformed. This is a percentage of the ultimate strength. In this case, 80% of 80kg/sq.mm.

So an allen bolt of grade 13.9 will have a higher tensile strength but will be more brittle.

A normal bolt of grade 4.4 has a low strength but is much more elastic.

The point about rolling the threads rather than cutting them is that cutting causes sharp edges and micro fractures in the steel which act as stress raisers whereas rolling forms the threads in a much kinder way without cutting the steel and doesn't weaken the bolt as much. The material to be rolled must have a minimum elongation of 12% and a tensile strength no greater than 110kg/sq.mm. You may find a local engineering company with a thread rolling head and you can just buy bolts of sufficient length that you can cut off the old cut thread and have a new thread rolled on.

Good luck !

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Based in Llandow, 5 miles west of Raf St Athan. They have a computer system called I.L.S. that traces any part you could ever need. Find the AN or MS or NAS part number of the item and they WILL find some for you. Tell them that "Britannia Rob" mentioned them to you.
01446-771768

I've just found 6x used rolled thread 8mm metric drilled head bolts with 75mm unthreaded shank and 20mm thread. Any good? Might suffice until you can find some more?

...or...

http://www.boltdepot.com/product.aspx?c ... 20&cd=1252

Thanks for putting me right Roger, i was mistaken for thinking that the 8 stood for the metric size. So much for the advice of a JAR engineer!!!

Rob the bolts you offered me are too long but il give your contact a try next week, thanks for that!

I had some wide/thick (12mm?) machined alloy washers under the heads of the bolts between them and the crush plate. No good if you have a small spinner, though.

The bolt coding is something that I puzzled about for years until a machinist at work explained it to me. It seems to be one of those little known facts that are so useful - and obvious once it's explained.

Good luck in your search

Jonathan

Most of what is written here regarding bolt specifications misses the point (though some of it is OK). Roger P has posted good information.

The standard thread length for an M8 British Standard Metric Bolt (I don't have the ISO standards to hand but they are essentially the same) is 22mm (2d+6mm). I would be quite surprised if you could find anything from stock that varies significantly from this.

If they are standard bolts then 75mm including the head would be specified as a 70mm bolt. (If it's a standard bolt, the head is 5.5mm thick and they usually come in 5mm increments of length.)

If I have understood the problem correctly, the simple solution to your problem is either to use shorter bolts or to counter-bore the ends of the lugs to ensure the thread does not bottom out. The standard Peacock hub had counter-bored lugs as standard precisely to solve this problem.

Don't worry about the strength of the threads. The calculation for nut-bolt thread interface is a complex one, but assuming both materials are roughly the same strength (it's usually considered good practice to make the nut softer than the bolt) and the bolt is a good fit in the nut, then three turns of engagement usually ensures that the core of the bolt will fail in tension before the threads fail in shear. A standard M8 nut has 5.2 turns of engagement which gives a good factor of safety. If you have 5 turns engaged in your lugs you will be just fine. There is no reason whey the end of the bolt needs to project out through the back of the lug.

Don't cut the lugs down as they provide the drive torque into the prop. Whatever you do, don't 'rethread' standard bolts with a die to make the threads longer. As mentioned elsewhere this seriously weakens them.

For what it's worth, a prop to flange bolted joint is just about the least ideal bolted joint you could set about designing. The bolt is very stiff as compared to the body of the joint (it should really be the other way around) and it is subject to various internal stresses (shrinkage, thermal, etc.). Historically it was usual to use a Belville washer (circular dished spring washer) under the bolt head to take up some of these changes. I rather think you would be better off checking the bolt tightness at regular intervals.

Assuming your lugs are threaded right through, my recommendation would be to measure the thickness of the prop hub, pressure ring, washers etc. and then get bolts 10-15mm longer than that. You will then have 10-15mm in engagement but no risk of bottoming out.

In terms of lavatory reading, I can heartily recommend a book by Carroll Smith called something like "Nuts, bolts, washers and plumbing" which contains everything you might ever need to know about nuts, bolts and fastenings. He designs and builds race cars (NASCAR etc)
Nobody is ever likely to steal it!!!

That reminds me of the joke about the newspaper headline following a multiple s3x assault in a launderette where the escaped Broadmoor attacker ran off after the attack.

It read 'Nut screws washers & bolts'