Topic: Stroking

[rbmgf7]

So, as i am up in the air with ideas for my 915cc engine rebuild, i come across a guy who has a cb900F crankshaft for sale. according to satanicmechanic, the 900f crank drops right into the cb cases with some alterations. what's nice is it bumps my displacement over 1000cc 

but how about performance? from what i understand, stroking is really only good for low end torque which is good for drag. i'm really not interested in 1/4 mile times since i would like thise hoss of an engine to last a while. it's going to be quite and investement to get this engine going so i'm kinda going to baby it until the time is right.

he's asking $80 plus shipping but now i have to struggle with a hy-vo camchain and sprocket. i see moonpie overame that adversity already. also finding shorter conrods but i'll be needing higher strength ones so i'll seek a custom set.

toss in some opinions

[754]

If you are not interested in 1/4 mile, then what do you have against increased  torque?

[Terry in Australia]

I'd like to do a crank conversion too, but haven't looked too hard for bits, it's a fair way off. There's a bit of work involved in the CB900/1100F conversion though, including swapping your ignition and alternator from one end of the crank to the other, Goldwing clutch assembly, making up special HiVo chains to fit, pistons with different sized wrist pins to match the conrods, etc. I think that APE offer a stroking service which might well prove cheaper in the long run.

Torque is good, torque produces accelleration, BHP produces top end speed. In real world conditions, I'd rather have lots of torque, than lots of BHP. Cheers, Terry.

[rbmgf7]

ok, cool. i don't want to get ahead of myself. i also don't know much about engine behaviour from different perspectives of modification. i understand the effects of boring, porting, balancing, etc. but never hear much about stroking. i just thought stroking killed top end since you were sacrificing for torque. i'm trying to remember dynamics. i thought the difference in crank radii had some effect of angular velocity which is proportional to piston speed....? i'm probably getting things mixed up...gah, shoulda paid more attention in class...

anyone here have a stroked CB? where's all the guys on satanic's site that have these awesome bikes!?! they need to be on this forum sharing their awesomeness!

[754]

I found a real bolt in stroker crank for a CB750. I turned 1080 on to it but he has not yet used it.

If he used it on his big-block it would have around 1200 cc,s.. pretty big jump from a 736cc.

[KB02]

I say get the crank.   

[eldar]

I suppose when you stroke it, you will drop your redline some. Maybe another option is to do a cyclex or wiseco kit with high cr pistons. I guess I just wonder how reliable the stroker mod of that style will be. I suppose it all depends on how you get on the gas and how well the job is done.

[bistromath]

Stroking increases piston velocity, which directly reduces redline. There are plenty of calculators out there to estimate redlines in car motors that apply for motorcycles, too. But if your cam stops breathing before redline anyway.... 

[rbmgf7]

how well this work for a naturally aspirated engine? that thing would have to be sucking in a lot of air/fuel. i'm sure MRieks port job will handle the head flow, i'm using an F2 head so the valves are larger, and i have 33mm CR's, but what i find funny is that i'm still restricted to the intake rubber manifolds ID.

[Terry in Australia]

I wouldn't be worried about reducing your available RPM, Rex Wolfendon in Oz is the most successful CB750 based race bike builder and racer here, he bores and strokes his race bikes out to as much as 1200cc's, and 150+ reliable rear wheel BHP. A large proportion of the "post classic" racers are Rex's bikes (he builds them to order) and they howl.

I don't know if I'd use an F2 head though, while they do have larger valves they were the F2's "achilles heel"  with a pretty horrible combustion chamber design, you'd be better off with a K head, with bigger valves. Talk to Mike Rieck about that, he does excellent work. Cheers, Terry.

[754]

You would still be safe to at least 10,000 rpm..

Piston speed is not that high on an oversquare motor, not like a really long arm on a v-twin.

Stroking however is very expensive, with only the size increase an 836 kit gives... at about a third of the cost..

[paulages]

i noticed that APE offers a 4 cylinder stroking service, up to 5mm. that's not too bad. anyone actually stroked an SOHC4?

[754]

I found a stroker crank for a CB 750 buddy?.. He has not yet used it..

Someone posted on here that they roadraced one for a while...

 They would be real expensive to make today..

[paulages]

I found a stroker crank for a CB 750 buddy?.. He has not yet used it..

Someone posted on here that they roadraced one for a while...

 They would be real expensive to make today..

i believe the APE price was upwards of $900.

[MRieck]

. I've seen several RC stroker cranks. They were used in a hillclimber. A buddy of mine had one magnafluxed years ago and it was junk with cracks everywhere. APE or Marine Crankshaft could stroke a crank 5mm easily and the quality would be infinitely better. Order a set of short rods and you are good to go. 

[754]

R.C. cranks in the catalog were listed as new cranks, no mention of welding.

 The one I found appeared to be a new forging.

[MRieck]

R.C. cranks in the catalog were listed as new cranks, no mention of welding.

 The one I found appeared to be a new forging.

I believe they were new stock cranks with welded throws. If they were billet cranks they would have cost a lot more money and I don't think anybody was making billet cranks for bikes back in the mid 70's. I could be wrong but the price tag for that RC crank gives it away IMO.