Last year I sold an engine for Bruce Selous’s widow, and it had been rebuilt by Bruce’s stepfather Bill Hill. The cylinder holding-down bolts were not hexagon headed, and were Allen type socket screws, in the usual 3/8″ Cycle Thread 26TPI. Although very non-standard, they were not obvious at a glance, and I thought that they were a good idea. The standard type bolts do not allow the use of a ring spanner, only an open-ended spanner that can easily slip and chew up the hexagon on the bolt, and the adjacent area of crankcase. Clearly they were a commercially made item, but I haven’t been able to find any currently listed. Perhaps they had been made by Bill Hill by cutting down some longer screws of a different thread form, and then cutting the 26TPI 3/8″ Cycle thread on them ? I know that I have seen them on an engine on a previous occasion, and Klaus Kretzschmar once asked me if I could find some for him, but I had no luck at the time. Any ideas out there ?
Hi Brian,I reckon you”r right when you say the Allen bolts were cut- down and rethreaded (3/8 cycle) I have never seen cycle-thread Allen screws as big as this. Under my bench is a large tin of assorted Allen bolts. What length of plain bolt would be required to make some?. regards Geoff.
If contemplating altering a modern screw in this way do measure the diameter of the plain part because many if not all such screws have rolled threads. In the rolling process metal is displaced from what becomes the bottom of the thread and is squeezed up to form the crests, the result is a diameter greater than the shaft diameter it was formed from.
Rolled threads are stronger, the rolling process has a beneficial effect on the microstructure of the metal but when cutting a new thread you start at a disadvantage because the outside diameter will be less than the nominal thread diameter.
When threading a new hole the drilled diameter is usually made larger than the root diameter of the thread, this is done for manufacturing considerations and can be as much a 30% of the thread depth. Now that is fine if the modified and slightly truncated screw thread is going into a newly threaded hole because adjustments can be made to the tapping hole size in order to retain as much strength as possible but we are dealing with 60 year old threads so do exercise some caution!
With reference to efr’s caveats about thread diameters – what about a high tensile M10 cap screw ( readily available in the required length ) as a starting point? It could be turned down to the required 3/8″ oversize and then screw-cut. Admittedly, a bespoke solution requiring machining skills that might be beyond my own!
Out of curiosity, I hunted in my “hope chest” for some long Allen screws. Now most of these screws are pretty old and a mixed bunch so I cannot speak for more modern examples that may not have even been made in the UK but I found that the plain portion of these examples to conform quite closely to the nominal thread size. From their appearance I am sure they have rolled threads so it would seem that British manufacturers at least took the trouble to maintain the nominal diameter on the plain portion and reduce the diameter of the length to be thread rolled.
This might not hold true for more modern products there is after all the attraction to manufacturers of reduced manufacturing and material costs, not much on one screw maybe but if you are making thousands . . .
The only answer is take a micrometer with you and measure before you pay!
As far as using 10mm screws is concerned I see no reason why not as long as the head of the screw can be accommodated. Indeed an increase in the area under the head might be an advantage for load spreading when the screw is tightened down particularly as Allen screws tend to have a smaller area here than the equivalent hexagon head and washer.
There is little to be feared by Mr. Average “man-in-shed-with-lathe” about turning down the shank of a long screw. It only requires that the end of the screw be centre drilled, a centre in the tailstock can then safely support the end of the screw. From there on it is only a matter plain turning and anybody that can cut a halfway decent thread in a lathe should have no problem doing that.
When it come to thread cutting in the lathe by the less experienced I’d recommend cutting the thread to around 70% of the full depth and finishing with a die because: (1) the cutting tool does not need to be perfectly accurate (2) a partial thread will guide the die true, you don’t want a drunken thread on a long bolt (3) removing the bulk of the unneeded metal reduces the strain on the cutting edges of the die and improves finish.
G’on, ‘ave a go, ‘yer know you want to! 😉
I’ve had a look through my box of old cylinder holding-down bolts, most of which have mangled hexagons from years of use and abuse. As regards length of socket-head cap screws to make some 26TPI 3/8″ Cycle thread ones from, I would be buying some 10mm metric ones at 100mm long. The other important thing to take into account is the thickness of the washer/spacer that goes under the head of the bolt. Ones in my box seem to vary enormously from 1/8″ thick up to 5/16″ !! The length of the finished bolt needs to take into account the thickness of the spacers, as you MUST not have the bolt “bottoming out” in the holes in the cylinder barrel.
I think that I may have a go at making some, and once I have turned the shanks down from 10mm to 3/8″, I will probably use my tailstock die-holder to cut the threads. With adjustable split dies I can open the die right up for the first cut, and then gradually tighten it up until the thread is a nice ‘fit’ in the holes in the barrel. As I am not a toolmaker, just an average “mechanic”, I think that will be a lot easier for me than turning the thread. My 70 years old Myford has to have the gears altered to do 26TPI, and that is too much fiddling about for me! I did some screw cutting in the lathe over 20 years ago, when I did my Motor Vehicle Engineering B-Tech, but it is but a distant blur these days, and the college lathe did not have to have the gearing changed, just a move of a lever did the job.
The latest posts on this topic refer to the important matter ( for “Mr Average” ) of cutting clean, well-fitting threads. I’ve always found this hard to achieve using the commonly available split dies, even when opened up to their maximum – especially on stainless steel. Incidentally, the expanding screws on die holders I have purchased usually had poorly machined tips which did not engage the split accurately, and required re-machining. Even with this improvement the results are still inferior and I am considering enlarging the die holder diameter by a few thou … any comments on the wisdom of this idea? There is obviously a risk of die fracture! Suggestions on a suitable cutting lubricant would be welcome.
Efr’s suggestion of roughing out the threads first on a lathe before using the die is appealing. My only reservation is the lowest spindle speed on my lathe is 160rpm – far from ideal. I’m going to give it try though. I can cope with the fiddle of change wheels in the interest of a better result.
The choice of die material, and manufacturing quality, are factors in the equation. My carbon steel taps and dies, purchased on the grounds of a low volume of work, may have been a false economy. HSS tools from the best UK suppliers would be very much more expensive.
I had occasion to inspect the threads on a set of Roger Moss heavy duty cranks. Those big-end and centre screw threads were superbly cut – clean as a whistle, they screwed in very smoothly with an almost imperceptible waggle. Roger admits to being considered expensive but you do get the precision that you pay for.
I must be lucky, as about 25 years ago I bought an unused, ex-WD, wooden-boxed set of HSS Cycle thread taps and dies, that even include some oddities like 9/16″ left-hand thread, and some of the 20 TPI variations from the more usual 26 TPI. When I see the prices being charged for single HSS taps and dies I think that my old set is worth its weight in gold. These sets used to turn up quite frequently at sales of ex-WD equipment, when WW2 kit was still being sold off, but I haven’t seen another since I bought mine.
Re. lubricant for cutting threads in high tensile and stainless steel.
I have always had excellent results with Lard, just raid the fridge. Bacon fat is almost as good.
Whatever you do, don’t use engine oil.
On mw 1910 round bed Drummond that is not fitted with a back gear for low speed work Ihave made a cranking handle with a expanding shaft ala cycle handlebar stem it fits in the mandrel good for thread citting. Ihave used Trefolex? paste for cutting threads on steel if you can get it . [ its good ]shoot me down folks am just an old metal butcher. Regards D F.
David Brierley informs me that the Swift engines had socket-head cap screws as cylinder holding-down bolts, and years ago he got some from Aerco. He will enquire if there are any still available. (Matt Holder would have had to order a large quantity to get a factory to get a batch of special bolts made).
I agree with Brian about the problem of securing the traditional cylinder holding down bolts with hexagonal heads. Many cases bear the battle scars of past slipped spanners. Personally I much prefer socket cap screws, but am conscious that many owners wish to retain original style fittings externally. If I make a new “Replica” engine, then I always use A2 Stainless cap screws to secure the barrel. I also use stainless cap screws to hold the head down. I had some stainless BSF cap screws specially made for Richard’s racer as he uses a modified Shipley iron barrel. Threads. I confess to cutting threads to near depth then finishing with a good ground form HSS die lubricated with, guess what? Castrol R! Roger