crank rebuild

[MossboysRacing]

I have an extra 85/86 short rod crank that I am having rebuilt. The motor is coming out this weekend and is due for some work, which equals money :( Anyway, Can i have the short rod crank rebuilt with the 87-89 long rod, to basically make it the later crank? I know i will need a spacer, but i would prefer to have the extra rebuilt, than buying a different one. Also, which company? I was going to go Stock Honda, or Pro-X ?

[destey]

Won't the longer crank not only give the piston upwards height at TDC, but increased downwards depth at BDC? Thus throwing off the port timing and possibly having the piston skirt hit the crank? The spacer will fix the issue about the piston on its upstroke, but what about the downstroke?

Im new to this and no expert by any means, just want to throw some questions out there.

I've got a 86 I have split apart right now, just got back my OEM crank from labaron's power sports where they put on a pro-x connecting rod.

[hondamancbr03]

Originally posted by destey
Won't the longer crank not only give the piston upwards height at TDC, but increased downwards depth at BDC? Thus throwing off the port timing and possibly having the piston skirt hit the crank? The spacer will fix the issue about the piston on its upstroke, but what about the downstroke?



The longer rod does not increase the stroke and a spacer plate will be required....What the longer rod does is take off side force of the piston against the cylinder wall which gives longer life for the piston and some believe a faster rev-ing engine. As for brand of rod, very few companies make products as good as Honda.

Side note: The only way to increase stroke is a stroker crank.

[wilkin250r]

Originally posted by hondamancbr03
The longer rod does not increase the stroke and a spacer plate will be required....What the longer rod does is take off side force of the piston against the cylinder wall which gives longer life for the piston and some believe a faster rev-ing engine.

Almost.

You are correct, it does not increase piston travel, and yes, the longer rod decrease side-load on the piston and gives longer piston life.

However, as a general rule, the short-rod cranks are the ones that rev faster. And it's not just a matter of weight, the longer rod gives a longer dwell time around TDC, and gives slightly different (smoother) power delivery.

[mxduner]

true the honda parts are hard to beat. also run the later piston if you go with the long rod, and the spacer plate and you be set.

[hondamancbr03]

Originally posted by wilkin250r
Almost.

You are correct, it does not increase piston travel, and yes, the longer rod decrease side-load on the piston and gives longer piston life.

However, as a general rule, the short-rod cranks are the ones that rev faster. And it's not just a matter of weight, the longer rod gives a longer dwell time around TDC, and gives slightly different (smoother) power delivery.

I stopped by Led Peformance today to pick up some parts and asked Arlen if he tested the long rod vrs. short set up, he used the same barrel on both cranks and said there were no difference in HP, we did not discuss power delivery and curve on the graph's. I'm the type that will not disput your statement until I do the reseach so i'll buy what you're selling:) Nice to see that some one knows and appreciates dwell time and how it affects the transers.

[MossboysRacing]

thanks for the help guys

[rustyATV]

Originally posted by hondamancbr03
I stopped by Led Peformance today to pick up some parts and asked Arlen if he tested the long rod vrs. short set up, he used the same barrel on both cranks and said there were no difference in HP, we did not discuss power delivery and curve on the graph's. I'm the type that will not disput your statement until I do the reseach so i'll buy what you're selling:) Nice to see that some one knows and appreciates dwell time and how it affects the transers.

I think (thinking theoretically, not with practical experience) what's actually affected is the upper RPM limit. A longer rod with higher dwell times also results in higher piston acceleration rates for the same RPM's, since the piston has less time to travel the same distance.

[All250R]

Originally posted by rustyATV
I think (thinking theoretically, not with practical experience) what's actually affected is the upper RPM limit. A longer rod with higher dwell times also results in higher piston acceleration rates for the same RPM's, since the piston has less time to travel the same distance. I think the more practical concern for most general usage engines is the port timing changes/increases a little with the shorter rod. So another consideration is if your cylinder is ported for the short rod and you like how that engine feels, putting a long rod in will change/mellow the port timing a little. You may or may not notice the difference. It's not usually a single design variable that makes a big difference in rider feel. For most engines in garages, it's often the combined health and setup of the entire engine that could stand some synergy help.

[hondamancbr03]

Originally posted by All250R
I think the more practical concern for most general usage engines is the port timing changes/increases a little with the shorter rod.
If he uses a shim for the long rod his port timing will remain the same.... Has anyone ever checked the port timing on a 86 barrel with a long rod without the shim? Obviously the head would need to be cut to recieve the piston with this configuration.

[All250R]

Originally posted by hondamancbr03
If he uses a shim for the long rod his port timing will remain the same.... Has anyone ever checked the port timing on a 86 barrel with a long rod without the shim? Obviously the head would need to be cut to recieve the piston with this configuration.
Where does the shim go? Are you talking about the spacer plate?

[hondamancbr03]

Yes, your spacer plate.....I refer to it as a shim being you shim the barrel up or down depending on your desired port timing. I'm sure i'm odd man out refering to it as a shim.

[All250R]

Originally posted by hondamancbr03
Yes, your spacer plate.....I refer to it as a shim being you shim the barrel up or down depending on your desired port timing. I'm sure i'm odd man out refering to it as a shim.
Thanks for the reply. It may not be obvious at first glance, but the rod length has an impact on how much time the piston will spend at the top of the cylinder and conversely when it will uncover ports for a given crank angle.

The spacer plate doesn't account for this. The plate simply accounts for the height the barrel needs to be for a piston with a wrist pin in a different location.

[hondamancbr03]

Originally posted by All250R
Thanks for the reply. It may not be obvious at first glance, but the rod length has an impact on how much time the piston will spend at the top of the cylinder and conversely when it will uncover ports for a given crank angle.

If i remember correctly it's 1.2 degrees to the + from the short rod to the long rod.....I may have to double check but that's what i seem to remember. Does that sound right to you?

[All250R]

It depends on the crank angle but 1.2* is high. Yes though, the short rod uncovers the ports a little quicker.

[derby]

Originally posted by All250R
It depends on the crank angle but 1.2* is high. Yes though, the short rod uncovers the ports a little quicker.

The magic # is .663 degrees difference and as all250r said the short rod uncovers quicker.

[hondamancbr03]

Originally posted by derby
The magic # is .663 degrees difference and as all250r said the short rod uncovers quicker.

Is your number the complete cycle of a port opening AND closing? Or just the amount of degree added from TDC to BDC?