big end bolts

[redback]

Hi All,
I'm new to 10-tenths but have been a guest a few times.
I've got a question thats more of a curiosity than anything. Does anyone know why the big end bolts on a Ford Zetec engine have a helix on the shank? This type of bolt is probably used on other engines too, but why?

[bobdrummond]

I'm not familiar with that engine. What kind of helix is it?

[redback]

Hi Bob, The helix is like that found on a vice, a course thread with the top of the teeth missing. I also noticed that the caps are not dowelled so maybe the helix helps locate the cap and the rod together?

[Dino IV]

Hi redback, welcome to 10 Tenths!

Sorry, I got no answer just additional questions on this one. I am glad I got myself a Technical Dictionary English-Dutch not so long ago so I could look up some of the terms. But I am still puzzled. Is the helix angled like on a normal bolt? Like this:

Or is it a much greater angle? And how high is it?

Maybe it is used to screw the bold through the monocoque for minimum clearance and maximum rigidity. But that advantage would loose out with mounting and remounting the engine time and time again. And team's use just a few tubs every year. But it could be if they use a replacable joint plate or something like that which could be renewed.
Or in case of a higher helix, could it be used to dissipate the heating energy in order not to create any unwanted stresses inside the bolt-tub area?

[KC]

If you are referring to the last few threads on the shank of the screw (not a bolt, a bolt requires a nut, a screw threads into something other than a nut), it is the manufacturing method employed. Most bolts and screws these days are made using a rolling method where a length of wire is rolled between two fast moving dies called roll-formers. These shape the head, shank, and roll on the threads. Some are made the old way on multi-spindle screw machines and the threads are actually turned on using a single pointed turning tool or a threading die. All of these will leave a trailing edge to the threads that is unuseable. This allows the cutting tool to disengage the working surface of the metal without breakage. This also makes the bolt or screw stronger as the threads actually weaken the material. Some companies use the shallow end of the threads to lockup the screw when it is tightened instead of using locking washers or chemical locking compounds.

Main bearing cap screws use a shallow depth thread to make it stronger, and make the tapped threads stronger by giving the threads a larger cross section. A fit is specified on the bolts for the tightness. A tighter fit is less likely to come loose but is much more difficult to disassemble and more likely to be damamged whenit is removed.

For American made screws and most metric, the included angle on the sides of the thread are 60 degrees for v-shaped threads. Acme threads and buttress threads are totally different designs.

[bobdrummond]

If I understand it right, you're referring to a knurled finish on the unthreaded part of the shank. If I am correct, then these knurls are to help the bolt fit tightly into the hole of the con-rod. These knurls have sharp corners which is a big no-no in fastener design. The tendency is for these to act as stress-raisers and lead to possible failure and/or to actually act as a rasp and enlarge the fastener hole. A far better solution is ARP's 'Wave-Loc' method which employs a ripple rather than a knurl. http://www.arp-bolts.com may help

[redback]

I just checked out the ARP site. The bolt I am talking about looks just like the ARP wave-loc. I was worried about the grooves being stress raisers but if ARP use this concept it must be race proven. I might but a set of ARP bolts just in case. I saw the Raceline sight and they put ARP bolts in their modified engines. Thanks to all for your help and thanks for the link Bob, you were spot on.