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Fork and blade conrods

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IanH26/03/2019 23:07:16
76 forum posts
57 photos

Various V twin engines use fork and blade style conrods.


Without worrying about the pros and cons of this style of conrod, does anyone know of a rationale that links the orientation of the rods with the direction of rotation of the engine? There are proponents for both solutions, but is this just custom and practice, or is there an engineering rationale to support one solution or the other?


Neil A27/03/2019 10:39:00
59 forum posts

I don't think that there was a particular convention for which way round the rods were fitted, although manufacturers would have their own preference. It is probably more to do with how much clearance there was inside the crankcase for the big end bolts. This would have been fixed in the design stage.

I can't remember ever swapping the rods around when a reverse-rotation engine was built, that would have caused confusion when you have a standard rotation engine sitting beside it in a boat. The only thing I remember is checking the timing of the oil feeds to the bearings and up the rods to make sure they worked both ways.

Reverse-rotation engines are a bit special as it is normally easier to install a reverse reduction gearbox, keeps both engines the same on a boat.


Howard Lewis27/03/2019 21:34:34
3370 forum posts
2 photos

Possibly, Fork and Blade rods were used to keep pin lengths, and hence crankshaft lengths, as short as possible, to maximise torsional stiffness. They increase the parts count, having six bearing shells, instead of four, and at least two more big end bolts.

Reverse rotation engines were a bit of speciality, since they needed all sorts of things to be handed for them, lip seals, gear thrust bearings etc.

Far easier to use a gearbox with an idler.

The only thing that you might have to watch was the correct orientation of each rod, if had an offset drilling for an oil spray onto the bore. Less vital if there was a spray through the little end of the rod for under crown cooling.


IanH28/03/2019 08:31:40
76 forum posts
57 photos

Thanks for your thoughts.

I am looking specifically at pre war JAP engines (and Harley Davidson practice up to the current day) where a built up crank avoids the need for conrods with split big ends. I am concluding that unless the original design makes specific provision for oiling based on the design and direction of rotation of the con rods, then it probably doesn’t matter.

Those who have a preference maintain that the fork rod (which is rather more asymmetric than that shown in the drawing in the original post) contributes to the oiling of the blade rod big end bearing by scooping oil laden air from the crank case that then finds its way to the bearing. In the case of JAP engines the blade rod runs, or rather rocks, on a bronze bush on the outside of the hardened steel bearing shell of the fork rod. Whilst there may be some truth in this, I am not sure that this can have been considered a principal/critical source of lubrication for the bearing.


Perko729/03/2019 07:57:01
337 forum posts
24 photos

I bit off-topic but I've seen this design once before, it was on a trophy presented to my father for his Morris Minor at a Morris Car Club rally in Queensland. The trophy was a polished timber base with a set of conrods mounted on it just like those in the OP photo. The conrods were clearly marked as coming from a Rolls Royce, presumably a V8. They were pretty solid. My guess it was designed that way to minimise vibration from offset conrods when the engine was running?

Ian S C29/03/2019 09:38:09
7468 forum posts
230 photos

Good chance the RR con rods came from a Merlin engine. The Merlin had its cylinders opposite each other (instead of staggered), this keeps the engine a bit shorter, and lighter.

Ian S C

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