Finished drawing of the sleeve drive by the talented Mr. Bob. As it turns out having the sleeve drive shaft rotating in the same direction as the main crankshaft really helped us. I’m sure that Sir Roy Fedden didn’t imagine we’d be doing this to his state of the art Aero engine some 70 odd years later! But even so, he no doubt faced the same design hurdles (and some more!).
Chain drive will be easy to achieve and adjust. The oil feed will be modified however as it is much easier to oil the sleeve ball joint by taking a drilling down the centre of the shaft. This means we won’t be able to run a points cam on the outside, so an alternative ignition system will need to be found. I think a hall sensor located close to the sleeve drive sprocket is probably the best solution, but I need to do some research.
Great progress though, I just need to get my hands on some parts first. Fingers crossed!
Here’s a photo of the nuts holding the barrels on. No doubt that the right tool is required to remove them without damaging them. Hopefully the threads are a standard size, as our design doesn’t warrant using original nuts. Finding them would be tricky for starters!
Today I managed to get my hands on a Bristol Hercules engine! The contact I made a few weeks ago at the Newark aero jumble came good, and true to his word Graham had a few engines around. The one in the picture didn’t look too bad on first inspection, but it had stood with no exhaust manifolds for some time and the sleeves were quite rusty. Some of them looked pretty bad, I doubt that the barrels could be removed without causing damage. (and that’s the last thing anyone wants!).
However, there’s another engine stored elsewhere that is apparently in much better condition. I will need to go back again to see it, as access to that part of the airfield needs to be pre-arranged. Graham is kindly sorting this out, along with (hopefully) the loan of some special spanners. The nuts holding the barrels on are almost splined and will certainly require care in removal to avoid damage.
Looking at the amount of room to swing a spanner, particularly on the rear bank of cylinders, an ordinary hex nut would be no use. You couldn’t swing a spanner through the 60 degrees required to get on the next flat, so Bristol came up with a solution.
Graham seems a nice chap and is willing to help. With any luck, this time next week I might have some actual parts!
Just a quick scribble as he puts it! But really its a fair amount of work and very clever. With Google and computers at our disposal its all too easy to take things like this for granted. Given the size of two sprockets and length of chain, work out the distance between the centres of the sprockets. Easy!
But this is proper old school engineering. No computers, no Google. Not even a calculator! You need to know the formula and do the maths.
Be like Bob!
Talking to Bob the other day we discussed the means to drive the sleeve. As the sleeve drive shaft needs to rotate in the same direction as the main crankshaft, chain drive seems to be the easiest method.
Bob had dug out a triumph boneville manual from his library and showed me the primary drive arrangement. The drive ratio is perfect at 2:1, and the chain tensioning method would suit us too. The sprockets are physically too large, so we’d go down to 22:44 teeth from the original 29:58.
Bob showed me some calculations he’d done which are quite impressive. The distance between the shafts on the Boneville isn’t stated, but knowing the chain length and sprocket sizes he was able to calculate this. Transposing this to our sprocket sizes means that the chain tensioner will still fit. That does make life easier.
Not in situ yet, but here’s my new workbench round at my friend’s workshop. It’s solid as a rock and very heavy. To get it in the van (including lifting), we opted to make the top removable. A lot more work drilling and tapping, but worth it as it gives me options for fitting it in. I could make it L-shaped instead of having a straight run.
When I start looking for a decent lathe I’ll have an idea of whether the sizes I need can be bench mounted. I expect most will come on their own stands given their size, but that will add additional cost. That comes a little further down the list though. For now my next job is painting the base and rebuilding it in situ.
Up at the crack of dawn, I sped up the M1 to Newark this morning to catch the aerojumble at the air museum. I was convinced that someone would have a dusty square cardboard box with a new old stock Hercules piston! The ball joint from the sleeve I’d find in a box marked "miscellaneous – any item £5". I’d be driving home with all the parts I needed by half 10 🙂
It didn’t quite go down that way! I had a few detours on the way so I arrived later than I wanted. I was cursing my luck, imagining the bloke on the piston stall telling me he’d just sold a pair! As it turned out I needn’t have worried.
The jumble was interesting but mostly aircraft instruments and memorabilia. Some great stuff there and a friendly atmosphere, but it was fairly obvious that I wasn’t going to find anything on my list. I was all set to leave when I decided to make one last circuit and ask everyone, just in case.
Most of the stall owners had nothing, as I expected. But I did track down a gentleman who has three Hercules engines in various states! He was quite busy so I took his card and said I’d be in touch. It might come to nothing, but it’s always worth asking!