Stroke length & durability

[Nastytls]

After looking at the new S&S 131" kit and it's required 4 5/8 crank, I was wondering, at what point are you going to run it to issues with durability with such a long stroke? 

[Don D]

Like most situations,  it depends on a lot of factors.  If the pistons retain decent skirts and don't pop out the bottom past the gauge point 4.625 should be fine. That said as I stated in the other thread the 114/117 4.5" flywheels take you right to 143". Now even with a bulletproof motor you can tear up just about every other connected part on the motorcycle.

[Nastytls]

Would you know, based on a given stroke, how far the skirts will protrude? If so, where is an acceptable limit to this?

I guess I'm not thinking absolutes, more along the lines of a rule of thumb... e.g. I won't build a combo past ( X ) because it's pushing the limits too far for a street ridden engine.

[Don D]

Its not possible.  This one gets solved by measuring or gathering data on the specific pieces, cylinder length piston skirt length and gauge point are the heavy hitters. You just don't want the gauge point uncovered. In twin cams this is why I build a lot more 117" than 124" engines, the cost is the same. The 117" is rock solid and lasts as long or longer than stock with forged 4032 pistons.

[Nastytls]

OK, so essentially, there is no real rule of thumb.

[turboprop]

After looking at the new S&S 131" kit and it's required 4 5/8 crank, I was wondering, at what point are you going to run it to issues with durability with such a long stroke?

Keep in mind, S&S sells thousands of engines, of all types with 4-⅝" stroke, and warranties them.

[rbabos]

OK, so essentially, there is no real rule of thumb.

Basically the longer the stroke, the lower the max rpm should be to keep piston speeds in check. Let's face it. You don't see any 120+CI engines revving to 9100, like the short stroke vrods and have any life span to it. Also , the longer the stroke, the piston skirt gets shorter, reducing piston stability in the bore. Two things going against you for life span.
Ron

[PC_Hater]

For about 100 years the maximum piston speed for a reliable engine was set at 4000 feet per minute. It applied to high powered aircraft engines and racing cars. Your engine for everyday use never got close to 4000 feet per minute. When quality materials and oils were used in ordinary engines they too began to have 4000 feet per minute as the maximum piston speed. Formula 1 racing cars finally got beyond 4000 feet per minute in the late 90s early 2000s. (or thereabouts)

So, your 4" stroke Twin Cam hits 4000 feet per minute at 6000rpm. Mine has been beyond that for probably 15 minutes in 15 years = safe and reliable.
Your 4 5/8 stroker = 4.625" hits 4000 feet per minute at 5,189rpm.

As for the short piston problem, that is nothing that special Carillo longer rods and then longer barrels and longer pushrods, oh, and some serious frame mods for the taller motor won't cure!

Question, has anybody tried using oval flywheels the way BSA did in the Goldstar? It should let you use normal pistons...