Street Ryderz
Well-Known Member
Late model 40mm cranks
So a sad discovery was made this past month in testing the zl 40mm stroke long rod cranks being used of late, I got a few from the Grubee gt5b 69cc engines.
These cranks have thinner flywheels than the older models being around 15.5 mm thick where as previous cranks were 17mm thick, this reduction is thickness greatly effect’s there ability to push the piston up through higher compression builds, due to the loss of rotational mass, the new 40 mm stroke long rod crank after being balanced weighed 2.8lbs where my previous balanced 38mm short rod crank weighed 3.2lbs, now that may not seem like much but trust it’s huge!
Like I said above this really affects the ability to push the piston up through higher compression, and harder to get any high rpm out of!
The first engines to come to N.A. had 19mm thick flywheels where the weight’s were bolted on with no balance holes whatsoever, they had crowded bearing big ends and were almost 4 lbs, I still have a few usable ones that I will balance and give a go again to see if that’s what’s needed for my basically stock timings in the gt5b/g4 jug to get over 50, the gt5b crank gets close but just can’t handle my gearing the way my previous 38mm short rod crank(zae50) did.
Port area/volume’s are the same, compression and squish are the same, same pipe, head, carb and with the added displacement of the 40mm crank I thought for sure to see an improvement not a loss!
So a sad discovery was made this past month in testing the zl 40mm stroke long rod cranks being used of late, I got a few from the Grubee gt5b 69cc engines.
These cranks have thinner flywheels than the older models being around 15.5 mm thick where as previous cranks were 17mm thick, this reduction is thickness greatly effect’s there ability to push the piston up through higher compression builds, due to the loss of rotational mass, the new 40 mm stroke long rod crank after being balanced weighed 2.8lbs where my previous balanced 38mm short rod crank weighed 3.2lbs, now that may not seem like much but trust it’s huge!
Like I said above this really affects the ability to push the piston up through higher compression, and harder to get any high rpm out of!
The first engines to come to N.A. had 19mm thick flywheels where the weight’s were bolted on with no balance holes whatsoever, they had crowded bearing big ends and were almost 4 lbs, I still have a few usable ones that I will balance and give a go again to see if that’s what’s needed for my basically stock timings in the gt5b/g4 jug to get over 50, the gt5b crank gets close but just can’t handle my gearing the way my previous 38mm short rod crank(zae50) did.
Port area/volume’s are the same, compression and squish are the same, same pipe, head, carb and with the added displacement of the 40mm crank I thought for sure to see an improvement not a loss!
