1st thing to say is I’m doing this on a very limited budget & will be doing all work myself except machine work. I will soon start winter project of 95” upgrade to wife’s ’04 Fatboy, carb, SE 6200 ign. Module, V&H Long Shots & Thunder Monster baffles, tuned by me. It already has SE203 cams w/ upgraded SE camplate, hyd. tensioners, & hi-vol oil pump. I will be using ported heads with following specs: 87cc chambers, 1.94 intakes, 1.61 exhaust, The “Porter “ called them 510 heads because they were made for 510 lift cams (high velocity at lower lift) & he liked JE 10.5:1 with S&S 510 cams. The S&S cams are longer duration than SE so they would have lower corrected compression.
My question is, what pistons should I get? I already have cylinders that came off 103” motor with less than 800 mi. I don’t want to have ping problems or hard starting since this is wife’s bike, & don’t want her to deal with comp. releases. I’m thinking about SE cast 10.25:1 since they calc to 9.38:1 corrected & 195 cold crank comp. w/ .045 hd. Gasket. SE Forged 10.5:1 push the CC comp over 200. My wife don’t twist it to high rpm often so I’m thinking the cast 10.25 would be better. Or should I stick with flat tops? Any suggestions or opinions are welcomed.

 

JMHO but I think you are on the cusp of creating the conditions conducive to detonation. The .045 head gasket opens up the squish band and the A/F mixture doesn't disperse and/or combust as evenly as when the squish is tightened up. If the bike were EFI and could be "fine tuned" I would be less concerned about being able to adjust timing to address detonation without degrading performance but can't be done with a carbed bike.

I think you have to go with the 10.25 pistons (I am a fan of cast over forged) but would go with a .030" head gasket and shave the dome to achieve a corrected compressin of 9.3, or less and a CCR of 190psi or less and I would install compression releases in spite of you not wanting the wife to "deal" with them. A heat soaked restart kickback can be nasty and damaging. Pushing two button before startup gets to be a habit must like flippin the "Run" switch.

 

Thanks for input djl. I haven't found anyone around here to shave the domes but Wiseco makes a 4.6cc domed piston. With .030 gasket that calcs @ 9.11:1 corrected & 188 ccc. Wiseco's (Aren't they forged) are about same $$ as SE cast. I did get the heads back today from getting them tapped for C/R. I'm doing all my calculations on Big Boyz site.

 

Wiseco are forged, just set them up on the tight side of the fitment spec.

 

So do you like the comp numbers these make? I want as much power as I can get without ping issues. A local builder here says he likes cast H-D pistons best & they might wear a little longer than the looser fitting forged, but his main reason was noisier. His recommendation was flat tops but that would only be 8.39:1/170 ccc. Going to forged would probably get the 700 mile cyls honed a few thou & that would be good.

 

Even with the 4.6 cc dome, compression is on the low side.

I can only tell you what I would do in your situation bearing in mind that this build is for your wife's bike. I would not try to hone and fit a new set of standard size pistons in the 103" cylinders (3.875" bore). I would get a set of .010" OS SE cast flat tops and deck the heads to 82cc chambers and use a .030" head gasket. That will set static about corrected CR about 8.9 anc CCR about 184psi. It willl run good, get decent mileage, no ping issues and no compression realeases required. Not what I would do if rebuilding for myself; I would reduce chamgers to 78cc and install compression releases. But, this is your wife's bike.

If you do go with SE flat tops, the top end needs to be mocked up and valve to piston clearance checked because you have the 1.94" intake valve. The intake valve relief in the SE cast flat tops is deep enough but sometimes it must be opened up and moved outboard to provide clearance for the larger valve.

 

I am currently running just about exactly the setup you describe: 95" forged domed pistons, ~6cc, S&S 510 gear drive cams, ad a .030 head gasket. I haven't cc'ed my heads, but assume that they are ~87cc based on the results of compression test, which yields ~183-184 psi (comp calculator results in 185 psi for this setup). I can vouch that it's a pretty decent running package (mine's in a Road King). I do have compression releases, though I don't think they're really necessary at this psi; in fact, sometimes one of them doesn't close without blipping the throttle a few times.

I have a suggestion for pistons. I'm running a set of Keith Black forged pistons that have had the domes machined. I recently bought a set from RPM Machine in Utah for $165, delivered to my door! These are forged, low expansion pistons with coated skirts. These are part # 906C, with 11.3cc dome, 10.5:1 with 85cc heads. KB also has a part # 905C which has a 6cc dome and is 10.0:1 with 85cc chambers.

I'll PM you with the ph number of RPM Machine, in case you're interested. I'm also going to be going to a different setup; if you're interested in a used set of 510G cams, let me know, mine will be available.

 

djl, I bought std. KB 905c w/ 6.7cc domes. With my 87cc heads. SE203 & .030 gasket that calcs @ 9.29:1 corrected & 189 ccc @ 720' altitude using Big Boyz calc. Are my numbers right & does this sound ok to you? Do I enter dome vol for 1 piston or double it? I had the cr's machined in the heads, the machinist called today & said my 103 cyl were no good to hone & use so I must bore my originals or he has a set I can exchange with. I'd like your input about this setup before boring my cyl (or his).

 

[quote=djl;9105744]JMHO but I think you are on the cusp of creating the conditions conducive to detonation. The .045 head gasket opens up the squicsh band and the A/F mixture doesn't disperse and/or combust as evenly as when the squish is tightened up. If the bike were EFI and could be "fine tuned" I would be less concerned about being able to adjust timing to address detonation without degrading performance but can't be done with a carbed bike.

I think you have to go with the 10.25 pistons (I am a fan of cast over forged) but would go with a .030" head gasket and shave the dome to achieve a corrected compressin of 9.3, or less and a CCR of 190psi or less and I would install compression releases in spite of you not wanting the wife to "deal" with them. A heat soaked restart kickback can be nasty and damaging. Pushing two button before startup gets to be a habit must like flippin the "Run" switch.[/quote


I agree wholeheartedly in regards to the .030" head gasket, as the closer you can put the piston to the head, without contact during operating temps, the MUCH less chance for detonation you'll witness.
Releases are also a very good idea.
Scott

 

I don't see how you came up with those numbers; they look way low to me for a 36* intake close in a 103" with 87cc heads. FYI, you only input the dome volume for one piston. Manufacturer's dome volumes can be misleading, some don't include the valve reliefs. So, you input the 6cc dome volume that doesn't include the -1.5cc valve reliefs and your figures are off because the "net" dome volume is 4.5cc. You need to talk to KB and verify the net dome volume you should be using to calculate compression and get the KB tech guys to confirm. I have heard of TC head chambers being hogged out to 92cc by dropping valves in the seats and laying back the sides of the chambers but don't know how practical that would be. 92cc chambers and a .040" head gasket hit 9.4 corrected and 192; could be ping city with the 203 and a not so good tune. 9.29 and 189 are friendlier numbers but I think you are going to have to shave the domes to get the compression where it needs to be. Shaving the dome is trial and error; time consuming; cut and measure. That's why I prefer flat tops and decking the heads for compression; no guess work.

Is the machinist saying that it would take more than .010" to true up your 103" cylinders? That's a lot and unusual.