Big end nuts

[Howard Wright]

Hi All

I’ve now got to the trial assembly of the 11/2 crank and Nippy rods in my standard case.  I was quite surprised not to have more clearance issues but in truth there is really only one.  The big end bolt heads. Following on from this thread I’ve installed M8 cap head bolts which work well on one side of the block but don’t clear the camshaft on the other. 

I was prepared to turn down the camshaft between the lobes but I’d need to take a lot off to clear the cap heads.  So I’ve ordered some button head “bolts” which are actually “set screws”.  I may be able to get away without turning down the camshaft.

Question... My structural engineering background tells me there is no difference in axial strength between  bolts and screws, but the bolt shank is stronger in shear.  I think the main force in a big end fastener is axial so any reason that I need to use bolts rather than set screws?

Cheers

Howard

[Chris KC]

Lateral location of the cap, perhaps?  

When you say bolt heads Howard, I take it you are inserting the bolts from the rod side and putting nuts on the cap side?

I for one wouldn't want to machine the camshaft. One possibility you might consider is helicoiling the 'rod' side and inserting bolts from below? Though I'm not familiar enough with Nippy rods to know if there is 'meat' to enable this. And of course you may not wish to modify them.

Surely the first port of call would be Nippy big end bolts, if only for inspiration.

[Hugh Barnes]

Page 7 of the Sports Parts List here - http://archive.a7ca.org/wp-content/uploads/PL_1169.pdf shows the BE bolts as parts BC92 and 93 but sadly don't illustrate them as such, so how they differ from the standard is not clear (to me at least)

[Howard Wright]

Thanks Chris and Hugh

I don’t have the original bolts but by looking at the rod they were round head located by a pin under the head.  Standard A7 bolts won’t fit as they are located by a ridge on the rod and aren’t long enough. I don’t rate my chances at finding new Nippy bolts.

Chris I like the idea of bolting up into a Helicoil but as you surmise there is not a lot of meat because the side of the rod and cap is scalloped.

Cheers

Howard

[Ruairidh Dunford]

This is from an Ulster - the only thing near to what you want that I have on file... sorry.

   

[Howard Wright]

Thanks Ruairidh 

That looks identical to the rods I have. Gives me a good indication of what the bolts are like.

Cheers

Howard

[Colin Wilks]

Right or wrong, on my 1 1/2" rods I am using M8 cap head bolts with a shank that fits snugly into the meat of both rod and cap. I have had to use a bit of emery on them to get them to fit the 5/16" holes but now have what I understand are referred to as "fitted bolts". I have chamfered the heads so as to clear the camshaft, and this leaves enough of the internal hex to allow for an allen key to orientate the bolt whilst tightening up the nyloc nuts. I was fortunate to have the original Nippy style bolts, which locate in the rod by a pin as others have said, and so had a pattern for the dimensions of the chamfering required. My only worry is if they were able to rotate in service and so foul the camshaft, but I think if this occurs the whole job's knackered anyway.
The bolts were fitted with temporary nylocs prior to white metalling. Torque is 19 lbs/ft.

[Bob Culver]

Original bolts are a very neat locating fit; capscrews I have seen were slightly undersize. So reduced 8mm seems a good idea.
I dont know what sort of astronomic revs are intended but many engines capable of quite high revs and with longer strokes and much heavier pistons utilised just 5/16 bolts and not apprently of exotic material.
Ricardo was very disapproving of the scalopped style of joint.

[Charles P]

I was prepared to turn down the camshaft between the lobes but I’d need to take a lot off to clear the cap heads.  So I’ve ordered some button head “bolts” which are actually “set screws”.  I may be able to get away without turning down the camshaft.

Turning down the camshaft is not a good idea.
Button head cap screws are a decent idea but as you say, set screws. 
Low head socket socket caps may save your day and are available in normal bolt style.
See page 26
https://www.holo-krome.com/uploads/7/2/7...g_2018.pdf
(opens big PDF so don't do it on your phone. )

And if I'm putting a socket head inside an engine I make sure it's Holo-Krome or Unbrako. Beware of some EBay vendors being fast and loose with the term "Unbrako"

[Zetomagneto]

Howard, can you put the bolts in the other way round, and use a K nut self Locking nut at the top, they are all metal self Locking with a greatly reduced head used in many motor sport applications.

I had thought about tapping the rod M9 and using ARP formula V bolts part no ARP 104 -6005
a google search will give you under head dimension.

Tapping size for M9 is 7.75 I have not got round to taking this any further but will have to address it at some future point.

I think the size between lobes on the Camshaft can be .620"  I wouldn't want to go any less than the diameter of the centre bearing

All the best
Gene

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Bob, Nippy rods have a scalloped joint! A jig would have to be made up to tap these as you only have half a hole at entry