74 cylinders on A 61 motor

[DANPATCH]

Can 74 cylinders be used with 61 flywheels ,using FLH pistons. Would the compression ratio be way off. I do not want to use 74 flywheels .

[Rubone]

Yes, they can, and yes, the compression ratio does not work! However there are ways around it and the resulting 68" engine is a great combo. I have built couple engines like that and loved how they performed. The way I did it was to shorten the cylinders to get the ratio I wanted. It is not the difference in the length of the 61 and 74 cylinders, but rather the location of the piston pin relative to the crown. The 61 pistons are taller.Depending on the pistons you use and what you want the ratio to be determines how much to remove. I believe i took off about .090 from the top of the cylinder. Do not take it off the bottom as it will weaken the base flange. And be sure to shorten the head bolts so they don't bottom and bind.
Robbie

[T. Cotten]

Dan and Robbie!

I had great luck with nothing more than high compression (10:1) 74" pistons when the cylinders were already worn enough to accept the overbore without sacrifice.

One did phenomenally well at .155" over, and I hope to inspect the cylinders again, soon.

Torque-plates for fitting to the pistons are manditory, of course.
And perhaps it was the lowered balance factor that made it jump like a scalded dog.,.. beyond the displacement itself.

...Cotten

Doh!

I forgot that 74" cylinders are taller! Nevermind, I've never tried that!

[cdndewey]

Me and a couple of buddy's built something like that as a race bike 20 plus years ago to a 39, only we used 61" barrels punched out 200 thou and run dual Linkerts. We took it to Bonneville when Easyriders was doing their thing to beat the Kawi record and had a chance to try it there. We ran it at a local drag strip and the local road race track on the vintage weekend. We blew out the rear barrel testing it one day, (I didn't find all the shrapnel, but I found some pieces with a 60 thou wall thickness) but for the most part it went pretty good for the two years we ran it. It's now a restored bike.

I'm building my 47EL with 61" wheels and 74" pan barrels. I have a set of BT flathead rods that I will probably use. I'm a couple of years away from it happening yet as I still have to gather parts.

[knuckleheadtim]

10.5-1 pistons will give you about 7-1 compression and run really well. Another great trick is to move the crank pin out like an 86" stroker using a 74" crank pin. This will give about a 3-5/8 stroke and run like the wind. Personally, I think they run even better with small port heads because you get more velocity in the ports.