djl,

What I'm thinking is get a new set of 10.5:1 pistons, same as I started with, and leave the domes alone. That's probably the least expensive/simplest way to get where I want to be. I have a small, home machine shop, so I was thinking of milling the heads myself, but my lathe only has a 10" swing, so the heads won't fit, I don't think. I do have a vertical mill, but it does not have power feed, so by the time I buy a shell mill large enough, I might as well just buy a new set of pistons. When I first started looking at this I thought I'd be able to bolt my heads to my mill table and with a fly cutter reduce the chamber volume for the cost of a set of gaskets and a couple of days of my own labor. When I looked at Cometic's requirements for surface finish (50Ra) I figured there's no way I'll get there with a single point tool and manual feed on the mill, and I have no way of measuring Ra. More tooling needed. So I started looking at the lathe, which of course has power feed, but is too small to do the job. So now I have to pay someone to machine the heads, which leads me back to buying a new set of pistons. For simplicity's sake, as well as $ and effort expended, not to mention the fact that if I use the same pistons I already know piston-to valve clearance is ok. I suppose it could be argued that I may want to mahcine the heads a little bit anyhow to equalize chamber volume if they're unequal, but for what I'm trying to build, which is an affordable, fun, daily rider (I ride rouhgly 10,000 miles a year, most of the time; last year was cut short when I got off this bike too fast and ended up in a wheelchair for a little while), I think I'll get there with a new set of pistons.

I may decide to go with a different cam at some point, but I figure I should get the compression back up there first, maybe advance the timing on the cam to mover the power down a little lower in the RPM range (Scott's suggestion) and see how it runs. Besides which, since this thing has so few miles on it since the cylinders were bored & honed, as long as the new pistons measure up within a thou or 2 of the ones that are in there, I'll just whip the old ones out and put the new ones in. Maybe not textbook practice (I think I hear a few gasps already as I type this), but reality suggests it's been done before.

Those are my thoughts at the moment, at any rate.

One thing I forgot to ask the vairous people I spoke with about this is, what is the target corrected compression ratio to shoot for when looking at this sort of thing? When I first got the idea of hot rodding this thing, I wanted to increase the compression ratio to 10.5:1 and leave my S&S 510 cams in there. I fiured that would increase the power immensely without moving the torque way up the scale. Everyone told me I couldn't do it, it would ping. Well, Bob Wood suggests his TW8 or TW6 cam, and says these can be used in stock 8.5:1 all the way up to 11:1 "if you have the starter to turn it". Valve timing on these two cams are so close to S&S's 510 I can't see why it's OK to use the Wood TW6 (also .510 lift) but not the S&S 510. They aren't identical, to be sure, as the S&S has more exhaust duration, but intake timing is nearly identical. Can anyone 'splain this to me?

 

The BigBoyz calculator shows the the dome volume for the H-D forged 95" domed piston to be 11.5-13. Even at 13, the best you can do is about 10.2 unless you shave some off the heads; still way better than you have now. As Scott suggested, you also have the ption of advancning the cams which will not change static CR but will increase corrected CR whic will move the torque curve to the left.

In my neck of the woods, head milling would run about $75; much less than a new set of pistons. You are assuming piston to valve clearance is OK because there has not been contact. Unless you mock up and measure, you don't know. Installing a piston with a larger dome and/or installing the cams 4* advanced would be cause enough to mock up and measure.

Consider that possibility when working out a solution. As chamber volume decreases, so do cam choices.


I have used 9.3 corrected in the past as the target for a street build that is easy to tune, can handle 100* ambient temps OK (south Texas) and won't choke on corn fuel but still hit hard. IIRC, Scott sets his target higher, like 9.6

Installing the 510 isn't going to help much; corrected CR will increase but only a couple of points and won't make much difference. If you advance the 510s 4*, corrected CR will be back to where it would be with stock cams. In your situation, I think I would bite the bullet and move ahead with a permanent solution, whatever that is.

The 510 intake closes at 38*; Woods 6 at 40 and Woods 8* at 47*. There is no reason why you can't run any of the above but in your situation, the earlier close would probably be the best choice regardless of lift or duration.

I am sure you will figure it out; you seem to be one of the thinining ones.

 

Thanks for the input. The current pistons are KB 906C, which I would repeat. Forged, coated skirts and reasonably priced. Dome volume, according to KB, is 11.3cc. I like the idea of their hypereutectic (or however you spell it), and if it was a car engine with liquid cooling I'd not hesitate to use them. But the instructions for those pistons say to retard the timing 2-4 degrees because of their lower thermal conductivity... in other words, they keep the heat in the combustion chamber and the motor runs TOO HOT! And, I think it was your remark I saw in another thread about a good forged piston getting you through on a 100* day... we don't have a hell of a lot of those, but I ride all over, and we do get some. I was in Tennesee a couple of years back and it was 98-99* most of the time (of course, it was 100-101* up here that week!). I did do valve to piston clearance checking when putting this motor together, so the same piston sholen't have an issue; the valve reliefs are in the same place. But I hadn't thought about the effect advancing the timing would have. You're sure enough right about that.

As for the cost of milling the heads, I expected higher. I figured a couple hours by the time someone gets them set up on a face plate. Of course, it'd be about 10 minutes actual machining time, but that's the way that sort of thing goes. Shop time around here usually runs around $75-$85/hour, I think. Don't know for sure, as I do my own wrenching, so I don't pay all that close attention... though I do know the one shop charges $85/hr.

I don't know why I'm reluctant to take .040 or .050 off the heads... I've seen over and over that .060 is OK. But that seems like a lot to me, and I wonder about intake fit, valve shrouding, etc. I know the BB Chevy's with the larger combustion chambers flow better than the ones with the smaller chambers. Maybe partly because of bigger valves, but one factor there is valve shrouding in the smaller chambers. That's a fact. Maybe not an issue here, since the shape of the chamber doesn't change, but still seems like a higher dome is prefereable. Of course, that's contradictory too, because flat tops are best for flame travel... Yeah, I'm a thinking one, all right... too much thinking, usually...

In any case, thanks for all the input. I'm going away for the weekend, in the car, so this is out of sight and out of mind for the time being. And like I said, it does run, just not like I'd like it to run.

 

I am running a set of BigBoyz heads on one of my bike right now that had .065" removed, >89cc chambers to start. Ran the SE204s that fall and winter but when the Texas summer heated up in July, she started to ping. Replaced the 204s with TW44s, problem solved. Same peak numbers but the TQ curve shifted to the right a bit. Not a stump puller but a great open road cam; 60mph 5th gear roll on and you're looking an 100mph in a couple of heart beats.

Most will tell you that .080" is "safe" piston to valve clearance but .060" is OK if you don't over rev. However, an old "grey beard" with many years of H-D wrenching behind him told me I could go as close as .030"; I had .040" on the exhaust with the 204s. More now with the 44s as the TDC lift is lower but never had any issues with the 204s and hit the rev limiter at 6200 many times. Intake bolted right up but with the machining capability you have, if the flanges don't face up just right, you can take care of it

 

Just thought I'd update this since I got a fair amount of input. For the time being< i threw the 510 cams back in, since I had them on the shelf and could do it for the cost of a few gaskets. I threw canned PC map in, intended for a 95 incher with flat top pistons and SE 203 cams, which are somewhat similar to the 510's, and my exhaust, and it runs very well. Much flatter torque, according to the seat of the pants dyno. No more dirt bike like power band with nobody home down low.

It still isn't what I set out to put together, as I have said here, so when time and $ permit I am going to pull the heads and jugs. At that point I'll "cc" everything and set cr where I want it exactly. I think I've figured a setup on my milling machine that will let me mill the heads effectively and I'm pretty sure I can fit the cylinders on my lathe, so if I need to correct deck height I can do that, too. If the cylinders don't fit on the lathe there are a couple of capable machine shops around, including the guy I've been dealing with at the local hot rod bike shop, as well as a couple of others who can certainly take care of that if needed.

Looks like I'll be able to get where I want to without emptying my bank account altogether. Thanks to all for all the helpful input here.