If you build a 69 stroke engine with A pistons, the length of the cylinders and the pin height works out such that your deck height is right around where you want it to be for a street engine with about 7-8.5:1. Small adjustments can be made with thin shims.
If you build a 78 stroke engine with B pistons, exactly the same thing happens. The shorter pin height compensates for the increased stroke.
If you pick a 74 stroke, you are in \"piston limbo\". Neither A nor B pistons work.
Using A pistons will make the engine wider than a 2275, because you will have to use at least a 2.5mm spacer under the cylinders. Spacers like this are usually custom made.
Using B pistons will give you so much deck height that your compression will be like your lawnmower, around 5:1. To fix this you have to cut the cylinders shorter.
Then with either pistons, the following fit problems may or may not occur.
Using A pistons means your engine is wider. Your head studs may not be long enough. Your cylinder tin will be pushed out creating a gap between the end of the tin and the case. The fan shroud will appear to be too narrow for the cylinder tins. Available cross bar linkage may be too short, the hex may not reach the ball studs in the air cleaner bases. Using a slip joint exhaust means that the slip joints will have less overlap, and more likely to leak. Stock pushrod will be too short. Custom PRs will have to be made even for a W series cam with 1.1 rockers. The whole thing will be a tight fit, especially in an oval window.
Using B pistons will make the engine narrower than stock. You may have to shorten the head studs. And the cylinder tins. Then the fan shroud won't fit into the cylinder tins. Push rods will be too long. Your cross bar may have to be chopped shorter.
There are just so many extra things you have to do when building one of these bastard stroke engines, there's absolutely no reason to ever build one. 74 stroke engines take way more work. Some guys think they are saving time and money by buying a 74 stroke crank because they think the case doesn't have to be clearanced. This is \"pound foolish\" Its way easier to do a bit of grinding to fit a 78 stroke than it is to do all the other crap to build one of those engines. I believe Darren charges extra to build a 74 stroke engine vs a 78 stroke.
