HarleyTechTalk

2016 Fatboy S 124" S&S spigots, pistons are S&S/SA 4.135 Part# 92-1581. So... I recieved my crank from Darkhorse and the ARP center case bolt. Wanted to be sure it would clear the spigots so I mocked up and its good BUT I noticed that the front and rear piston skirts where very close to the case itself. Th rear has less than .003 at full rock BDC. The front has .018 at full rock BDC. Reading through S&S 51-116 instructions it states no callout for clearance except .060 on Rods, Lobes valves. What clearance should I have and should I mod the case or the piston? The cases have supported other setups of 124" but this being another .010 oversized piston obviously puts me larger..

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I'm in the process of hand sanding those effected areas with 400 Grit a few thousands at a time but only to the Rear effected area. At my kids track meet but will check later tonight for input. Thanks

First thing I would do is measure the machined cylinder counter bores in the case. Below is what's used for S&S Twin Cam 4.125" bore (and over bores)

I assume that you have S&S cylinders?

Also make sure that you have the crankshaft supported on the left side and level. .

The dimensions are spot on. I belive that when I'm checking the cylinders are sitting at an angle with the hold down. I only have 2 and will order more jims hold downs.i can physically line up the cylindersto square and it has the plenty of room that way. You ever work on something for days and just need to take a break for a fresh look. Thats what tonight was. Ready to seal the case halves.

4.162 realistically gives me .013 per side divide that by 2 and thats .0065 per edge. I lightly sanded each effected area .010 additional for peace of mind. Would l9ve to see a 131"

It's a pity the cylinder could not be longer as the short skirt piston being out of the cylinder ain't the best idea. I wonder why they don't offer a fix for the oil hole to use a longer cylinder. 

I wondered the same, My guess is the the oil galley and S&S making a safe decision. They can't control the quality of tooling being used or person doing the case bore. Collapsed oil passages are bad for a product image.

  Machine a 2" or more perpendicular oval (or round offset hole) across between the 2 case halves in the oil holes to increase the volume. Then sleeve the machined passages with another oval (or larger round) liner and take back the increase in volume (added from the oval expansion) to what you started with.  Now you can cut your clearance as deep as the new passage liner sleeve.  Machine the cases to the desired cylinder spigot length (and width), then re-sleeve the cylinders with longer liners and add the longer cylinder barrels along along with with coated pistons to further abate piston rocking.     

Perfect ^^^^

Seems like a tighter fit would be wanted to keep side rock of skirts to a minimum at BDC.  With in reason but it seems anything more than piston to cylinder wall clearance would work.

Quote from: JSD on April 14, 2025, 07:45:32 PMIt's a pity the cylinder could not be longer as the short skirt piston being out of the cylinder ain't the best idea. I wonder why they don't offer a fix for the oil hole to use a longer cylinder.

IMO Stuff like this is the difference between which way people should go in builds.  Or can it be built or should it and for what purpose.  This isn't so bad.  Look at the 4.5 bore m8's with longer strokes.  The S&S case will allow a bit longer spigots but then people just take them to 4.6.

Quote from: hrdtail78 on April 21, 2025, 11:07:15 AMSeems like a tighter fit would be wanted to keep side rock of skirts to a minimum at BDC.  With in reason but it seems anything more than piston to cylinder wall clearance would work.

I don't know by personal experience but I would be suspect that there would be a real possibility of those pistons becoming a tapping risk before or after warm-up.  The rod angle would be somewhat ideal at BDC but change in a hurry with the stroke direction. It would be interesting to know if it is a common fit and if there is any unintended consequences from the close design. 

Well, we are certainly gonna find out! It's definitely fitting a round peg in a square hole but I'm hoping that the angled squish band of the B2 pistons creates a more stable ignition and there are no issues. Gotta try it to know. Most would not waste there time but half the fun and therapy is building for me. The hope is to be able to run increased timing that otherwise wouldn't be safe. The reality however is that likely I could have used the last configuration again and achieve the same result this one will. If it works great! If not no big deal as I'm working on the second set off cases or motor. Love just trying to pushthr envelope and seeing what works. I spent most of this morning unshrouding the valve reliefs to hopefully promote more low lift flow. I've been trying to find a place to buy 2 sets of total seal top rings in the Gas ported design VS drilling the tops. I have an email out to see if they sell singles.

Quote from: Wookie3011 on April 21, 2025, 11:55:36 PM..........Most would not waste their time but half the fun and therapy is building for me...........

I love your attention to detail