Big Twin Evo Head quandry...
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Novice
Joined: Jun 2025
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From: Cheyenne, WY
Poor-man CC'd my heads. They are about 85CC. That puts me in the 9:1 range, which sucks. Oh well, it is what it is. Crosshatched my original jugs and popped in the flat tops.
Any way to get this thread moved over to the EVO forum? I didn't realize it existed, or would've posted it there originally.
Any way to get this thread moved over to the EVO forum? I didn't realize it existed, or would've posted it there originally.
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From: Sydney Australia
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Joined: Jun 2025
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From: Cheyenne, WY
I think I've found a solution to getting compression... XR1200 pop ups, but I'm getting conflicting info. Since the heads I have use the same chamber as a 1200, I should be able to use high comp 1200 pistons IF the wrist pin height is the same, and the bore is the same. Everything says the bore is the same. Is the wrist pin height and offset the same, though? Winter project idea, since I just got the bike back together tonight.
Still confused about the numbers on the head. Should be a D-chamber, not bathtub. I had this thread moved to EVO section because... Evo nerds (no disrespect... I nerd out on **** all the time, like these damn heads! I have to know. It literally keeps me up at night).
Still confused about the numbers on the head. Should be a D-chamber, not bathtub. I had this thread moved to EVO section because... Evo nerds (no disrespect... I nerd out on **** all the time, like these damn heads! I have to know. It literally keeps me up at night).
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Joined: Aug 2008
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From: Ohio
Hotrod mentioned something getting ignored, those are Branch heads, Screaming Eagle copied the chamber design for Evo's except using a Twin Cam sized spark plug hole.
Branch-Okeefe is the modern day company taking over the Branch head design and still in operation, Branch made pistons for those. Like Hotrod mentioned 9.5-1 is a common compression with common Evo flat tops, Screamin Eagle made a 10.5-1 bathtub chamber piston also for it's Evo heads. Once you get a idea of chamber CC's believe you are going to be happy with flat tops and a thinner head gasket which learned from John Sachs is a more efficient engine instead of flame travel with hot spots in a dome piston but agree if 85cc high dome is needed, Branch Okeefe will have these since the company was based off Jerry
American Rider
1984 – Harley-Davidson introduced the engine that saved the company… the 1340cc Evolution Big Twin. Didn’t hurt Branch much either! Fact is, this was the heyday of Flowmetrics. Even during development, years earlier, Jerry had offered his notions of what the stock Evo heads should be. To deaf ears it turned out. So when the limitations of the standard “D-shaped” chamber (among other things) became known… he was ready. In the beginning, and for a while thereafter, modding the basic “D” was an avenue explored by many… not least Dick O’Brien himself. However, in the end, the clear superiority of so-called “Number 4”, bathtub-chambered heads by Branch won by a mile! H-D’s own Screamin’ Eagle heads for Evo Big Twins… although introduced almost 15 years later… strike many as a reluctant acknowledgment that the factory couldn’t do any better. Only more “affordable.”
Branch-Okeefe is the modern day company taking over the Branch head design and still in operation, Branch made pistons for those. Like Hotrod mentioned 9.5-1 is a common compression with common Evo flat tops, Screamin Eagle made a 10.5-1 bathtub chamber piston also for it's Evo heads. Once you get a idea of chamber CC's believe you are going to be happy with flat tops and a thinner head gasket which learned from John Sachs is a more efficient engine instead of flame travel with hot spots in a dome piston but agree if 85cc high dome is needed, Branch Okeefe will have these since the company was based off Jerry
American Rider
1984 – Harley-Davidson introduced the engine that saved the company… the 1340cc Evolution Big Twin. Didn’t hurt Branch much either! Fact is, this was the heyday of Flowmetrics. Even during development, years earlier, Jerry had offered his notions of what the stock Evo heads should be. To deaf ears it turned out. So when the limitations of the standard “D-shaped” chamber (among other things) became known… he was ready. In the beginning, and for a while thereafter, modding the basic “D” was an avenue explored by many… not least Dick O’Brien himself. However, in the end, the clear superiority of so-called “Number 4”, bathtub-chambered heads by Branch won by a mile! H-D’s own Screamin’ Eagle heads for Evo Big Twins… although introduced almost 15 years later… strike many as a reluctant acknowledgment that the factory couldn’t do any better. Only more “affordable.”
Last edited by 1997bagger; Jun 28, 2025 at 08:00 AM.
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Joined: Mar 2010
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From: Poolville
Personally I think your moving in the wrong direction. Stick with the Wiseco pistons and give WFO Larry a call. I'm sure he has a set of heads that will allow you to use the pistons..
http://www.larrysmotorcyclemachine.com/
Or have a set of custom piston made. I would think Scott at Hillside Motorcycle and Machine can mill a set of Axtell 30* pistons to fit those heads..
https://www.hillsidecycle.com/
Remember, You wanna play, Your gonna pay....
http://www.larrysmotorcyclemachine.com/
Or have a set of custom piston made. I would think Scott at Hillside Motorcycle and Machine can mill a set of Axtell 30* pistons to fit those heads..
https://www.hillsidecycle.com/
Remember, You wanna play, Your gonna pay....
Last edited by 98hotrodfatboy; Jun 28, 2025 at 07:04 AM.
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From: Poolville
One more thing I would like to add is that without a Burette, I doubt you got an accurate measurement of those combustion chambers.. They absolutely (to me) do not look like 85 cc heads...
What Cam was in it when you took the motor apart..????
What Cam was in it when you took the motor apart..????
Last edited by 98hotrodfatboy; Jun 28, 2025 at 07:13 AM.
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From: Cheyenne, WY
It has an EV3. That's why I was wanting the higher comp. Heads, or pistons... either way, the block is getting naked again this winter, For now, I just want to ride. Getting ready to fire it up this morning and then throwing on some new rubber.
Seasoned HDF Member
Joined: Mar 2010
Posts: 21,117
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From: Poolville
So you put the flat tops back in and assembled everything including the EV-3 cam? If so, once warmed up a little do a compression test and report the numbers..
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From: Cheyenne, WY
Damn. I was at the shop mounting and balancing new rubber today. I could've done it then. I can't remember if I have a comp gauge at home or not (too many tools is not a problem, lol). I have about 5 miles on it so far. I forgot how much power this thing has already. Ill look at home and see if I have a gauge, otherwise I have a few small issues to fix first and then Ill take it to work and test it there.
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From: Florida
My stollen 90 had those heads, or very similar. Maybe not the same kidney shape, but Definatly not a D shape. I replaced the pistons with a Wiseco piston that was supposed to be 10:1 with the Screaming Eagle head. I’ll look to see if I have the part number. I didn’t cc mine because I didn’t care… I checked real quick for clearance using clay and assembled knowing I was going 127 in a few months. Mine had an EV46 cam that I replaced with the EV51. It was stollen with only about 4,000 miles on that top end. Mine also had springs for 180 seat, 10 degree titanium keepers and some port work done. The original owner told me it was the “branch kit” He said it was the 100 hp kit when I bought it… lol, more like the 75hp kit. Oddly enough it didn’t have adjustable push rods and the lifters were aftermarket but I couldn’t tell who’s. Not sure about the “Branch kit” I really didn’t care since the motor was temporary.
K1675 is what I put into the 1990 softail 80” motor. These were very mild dome and put cca from 175ish to 200ish. Woke the cam up nicely. In building engines I like the lowest dome possible with the highest mechanical compression. With Shovels it was my secret weapon. But with an economy quick top end on a temporary motor I didn’t care. And frankly that dome is so minimal I don’t think the motor cared. The spark plug is closer to the top than a shovel with a dome, not too worried about hot spots or flame propagation keeping the other side burning less efficiently.
Got me curious.. I just went to weisco site and the pistons I used are not for screaming Eagle heads… I know they cleared, I kept a good 35thou clearance everywhere in the chamber at minimum… that’s always my by thumb minimum. I try to keep 40 valve to valve/ valve to piston too. I didn’t blue print it, so I didn’t write stuff down,but the heads did look like yours.
K1675 is what I put into the 1990 softail 80” motor. These were very mild dome and put cca from 175ish to 200ish. Woke the cam up nicely. In building engines I like the lowest dome possible with the highest mechanical compression. With Shovels it was my secret weapon. But with an economy quick top end on a temporary motor I didn’t care. And frankly that dome is so minimal I don’t think the motor cared. The spark plug is closer to the top than a shovel with a dome, not too worried about hot spots or flame propagation keeping the other side burning less efficiently.
Got me curious.. I just went to weisco site and the pistons I used are not for screaming Eagle heads… I know they cleared, I kept a good 35thou clearance everywhere in the chamber at minimum… that’s always my by thumb minimum. I try to keep 40 valve to valve/ valve to piston too. I didn’t blue print it, so I didn’t write stuff down,but the heads did look like yours.
Last edited by Rains2much; Jun 29, 2025 at 06:02 AM.
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