Yes, I was also mentally debating the 585 vs 640 question... My current build is 10.5:1 which is kind of between the two 124 options...

 

Here's why I've been thinking about it: I basically have to pull the heads and barrels off sometime soon. I'm the second owner of my bike, and I've had it for several years. The first owner had the engine built around '02/'03 by an excellent local shop that unfortunately doesn't exist anymore, Simon's Performance. Strong drag bike pedigree, excellent builders. It's 95 inches, Stage Two heads with big valves, 10:5 Wisecos, Andrews TW60 cams, compression relief valves, SE selectable ignition module, etc. It was last dyno'ed in '03, and made 106 lbs/ft and 108 hp, which I think is a pretty healthy 95" build. It's been reliable since then, but has recently started burning some oil. My belief is that it's rings more than guides, as oil smoke only seems to be visible under acceleration. But it could be both. I pull the plugs periodically to look at them, and they now have a lot of oil on them. I looked inside with a smartphone boroscope (which didn't work very well), and there appears to be significant oil residue caked on the piston tops. My first clue that there was an issue came when I started getting some pinging that was never before present. I took a degree out of the ignition timing and it went away. Had to do it again a couple of thousand miles later. That's when I looked inside. My theory is that the deposit buildup has raised the compression ratio.

My first thought was that I would have the barrels honed and fit new pistons, while also bringing the heads in for deposit removal and possible guides/seals if necessary. Then I started thinking, why not just drop on a set of S&S 4" (100 CI) barrels and pistons instead of messing with my barrels? Anytime stuff has to come apart, I like to find a gain..

But that would still leave the possibility of valves needing attention, and I've been wanting to upgrade my cam chest which I don't think has ever been done...

Obviously the cost is not equivalent, but I started thinking... If I have to pull the heads off, rebuild the rocker boxes, fit new pistons and barrels, open up the cam chest... Would I rather spend the time on an engine swap that ends up with well engineered matching components, a healthy bump in power, a two year warranty...

I'm not dying to spend the money, but I could...

Just pondering for now, but I need to determine a course of action and start moving... I'll start with a compression check over the weekend.

I probably should've noticed things going awry sooner, but the bike runs pretty well, oil smoke hasn't been very noticeable until just recently, and oil consumption isn't noticeably different... Maybe I'll open it all up and then decide how best to proceed.

 

If money is an issue (you brought it up a few times), why not just rebuild your current motor? Usually guys say "money's not a problem", or "I've got the cash", but can't get their head around putting down the 10 plus grand when all is said and done to make a reliable 150 plus HP.
You could go bigger cubes and use your current heads-Andy Simon is an amazing HP guy and if he did the original heads, they should be fine with a refresh (springs, valves, guides) in a slightly bigger cube package. Then, 120 HP or so is easily within reach and it won't hit your wallet as hard...just remember, if you have a stock lower end...well, do you feel lucky?
I have had an 80, 88, 96, two 124's (stock and built) a 126 ( my favorite and current motor in both stock and currently built forms), and a 131", and can tell you, it gets pricey fast, real fast. The Mik 42 ain't gonna cut it for a 124 (but based on your riding style and goals I would put a 126" S&S in there), so bye bye Mik-way too small. Need new pipe, full clutch (not just a pack upgrade), induction and prepped carb, chain drive, and tune for starters.
So, no, another grand on top of the motor ain't cutting it if you want to do it right-but, when done, it will be one nasty Dyna...
Depends on how committed you are, both with your wallet and your goals.

 

do it!!!!!!!! ultimate fxdx...

 

Thanks for your thoughts, just the kind of input I was looking for (and from someone who knows). Of course I have to think about money, but I do have it to spend right now (fortunately)... It's more a matter of feeling that I'm using in the way that makes the most sense for me. Yes, Andy Simon's work is outstanding... It's a bummer that he isn't in my area any longer. I may email him to see if he has a recommendation for a local engine shop... I was thinking that a complete S&S motor may be the best blend of power/reliability/cost effectiveness if I want to end up in the 120-130HP range. Short of that, I was thinking about rehabbing my heads and dropping on the S&S 4" pistons and barrels. But... I would still have a stock bottom end and a cam chest in need of an upgrade... That logic is how I started thinking about a whole engine. I guess I didn't really consider the full rebuild route because I don't know a local builder (I don't consider myself competent to do the bottom end, although I feel fine with the top and cams).

Maybe I'll try to find someone in the area to consult with...

Thanks again!

 

640's are horrible in town, but they don't really come on until 3500 + rpm. I run them in my 120" (11.8:1) on my Softail. I will say they are choppy, but it's tolerable at lower rpms.

 

In my life I have paid for all the lessons I have learned. I thought this was a good lesson... It is cheaper to do it right the first time.
A lot of folks have also said "For what it cost me to build my XXX I could have bought the S&S 124 crate motor.

: Mike

 

42 Mikuni seems small for a 124. Get the 45mm and a pingle petcock. You will need to upgrade your clutch. I'd get the evolution clutch basket as well.

 

I haven't used anything but a Pingel since my '79 Sporty!

 

Thanks, Mike... I know this to be true...