Late model 40mm cranks

Street Ryderz

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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!
 
Grubee...I thought for sure to see an improvement not a loss!
Sounds like Grubee branded product is losing its way in the motorised bike engine world when something like a crank is being cheapened in such a fashion, which of course is going to ruin most folks balancing efforts to say the least.
 
Sounds like Grubee branded product is losing its way in the motorised bike engine world when something like a crank is being cheapened in such a fashion, which of course is going to ruin most folks balancing efforts to say the least.
All cranks are sourced from other manufactures, Grubee does casting of cases and covers and cylinders, all the rest of the parts are out sourced.
 
It's not just the Crank's I got from Grubee that are like this, most all I've seen of late are like this!
I'm going to add tungsten weights to cure these crank's, they are very well put together, with little side play in the rod compared to many, and well I just need that 40mm stroke for my ceramic jug build!
 
Did they at least stuff the case to keep the crank case compression up?
They have all way's had the tight cases, around 1.14 over the more common 1.12, doesn't sound like much but it does make a difference.
I did not use the Grubee case/bottom end yet, I used my 07 half breed tight case and put this crank and jug, I'm running it with stock port timings and even with my 11/30 gearing it pulls right out of the hole and gets up around 50 but just doesn't want to pull beyond that, where the previous one did no problem.
 
They have all way's had the tight cases, around 1.14 over the more common 1.12, doesn't sound like much but it does make a difference.
I did not use the Grubee case/bottom end yet, I used my 07 half breed tight case and put this crank and jug, I'm running it with stock port timings and even with my 11/30 gearing it pulls right out of the hole and gets up around 50 but just doesn't want to pull beyond that, where the previous one did no problem.
What rod do I need for the hs 660 I heard it's a husky 120 mm rod but from what
 
What rod do I need for the hs 660 I heard it's a husky 120 mm rod but from what
Most just use a 40mm stroke long rod but it's completely up to you if youu have a 12 ton press and a jig made up to change the rod go for it!
Just remember your altering the geometry when changes like this are made, going from an 89mm rod to a 120mm changes the angularity and dwell, piston speed and so on do your research for best results!
 
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