Big Twin Evo Head quandry...
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Novice
Joined: Jun 2025
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From: Cheyenne, WY
Big Twin Evo Head quandry...
Background:
I have an '86 FXR. My carb decided to puke fuel and wash out my oil. I changed the oil for new, rode for a bit, and it started smoking, With a borescope, it looked like the rings took a dive so I parked it, and finally got around to getting parts. I decided to go with the V-Twin Mfg. "83" kit, since it would bump up my comp to work better with my EV3 cam. After I pulled the jugs, I realized they look excellent. My dad had previously owned the bike, I'm the 3rd owner. He had the heads sent off for porting. Before I ordered the kit, I looked at the stamping on the heads: 16721-84A.
Here's where the quandry is... I got the 83 kit, and tore the top end down. I loose installed the pistons (no clips or rings) and dropped the jugs. I brought the pistons to top and tried to set the heads on, but they were hitting. I finally realized that these heads have bathtub chambers. I asked my dad where he sent the heads for mods, and he sent them down to Fort Collins HD just for porting. He's going to see if he can find the paperwork tomorrow. As far as I know, the big twin evo's never came with bathtub chambers, just D style?!? Is this correct? There are hand stamped numbers on the cylinder side of the head as follows:
XX
63
Do you think that's the CC size of the chamber? If so, with flat top pistons, that should put me at about 10.5:1 comp, which is the range I was originally going for with the 83 kit. I always seem to run into weird crap like this, which isn't really bad, other than I'm out $750 for the new jugs & slugs (already painted the jugs, can't send them back).
Should I just do a quick hone/deglaze and re-ring it? No lip on the old cylinders, smooth as a baby's butt. Still has crosshatch.
I have an '86 FXR. My carb decided to puke fuel and wash out my oil. I changed the oil for new, rode for a bit, and it started smoking, With a borescope, it looked like the rings took a dive so I parked it, and finally got around to getting parts. I decided to go with the V-Twin Mfg. "83" kit, since it would bump up my comp to work better with my EV3 cam. After I pulled the jugs, I realized they look excellent. My dad had previously owned the bike, I'm the 3rd owner. He had the heads sent off for porting. Before I ordered the kit, I looked at the stamping on the heads: 16721-84A.
Here's where the quandry is... I got the 83 kit, and tore the top end down. I loose installed the pistons (no clips or rings) and dropped the jugs. I brought the pistons to top and tried to set the heads on, but they were hitting. I finally realized that these heads have bathtub chambers. I asked my dad where he sent the heads for mods, and he sent them down to Fort Collins HD just for porting. He's going to see if he can find the paperwork tomorrow. As far as I know, the big twin evo's never came with bathtub chambers, just D style?!? Is this correct? There are hand stamped numbers on the cylinder side of the head as follows:
XX
63
Do you think that's the CC size of the chamber? If so, with flat top pistons, that should put me at about 10.5:1 comp, which is the range I was originally going for with the 83 kit. I always seem to run into weird crap like this, which isn't really bad, other than I'm out $750 for the new jugs & slugs (already painted the jugs, can't send them back).
Should I just do a quick hone/deglaze and re-ring it? No lip on the old cylinders, smooth as a baby's butt. Still has crosshatch.
Seasoned HDF Member
Joined: Mar 2010
Posts: 21,117
Likes: 7,643
From: Poolville
Post up a pic of the old pistons.. It looks as if some one opened up the combustion chambers for after market pistons..
If you want to use the new pistons and cylinders, you will have to get a stock set of Evo cylinder heads with the D combustion chambers..
If you want to use the new pistons and cylinders, you will have to get a stock set of Evo cylinder heads with the D combustion chambers..
Seasoned HDF Member
Joined: Dec 2005
Posts: 19,939
Likes: 8,036
From: poway
Heck, it looks like flattops. If you got the thickness in the pistons, whack the tops off. You'll need to CC the heads to figure compression but I'll bet you are close or over CR advertised for the kit with those heads and flattops.
Seasoned HDF Member
Joined: Mar 2010
Posts: 21,117
Likes: 7,643
From: Poolville
Aren't those Branch Heads?? You will need to cc the heads before you do anything.. I would call the shop they just came back from and get more information so you can figure where to go from here.. I don't know of anyone that will make custom pistons to fit the chambers.. I do know Hillside motorcycle and machine in Munnsville NY can mill pistons but not sure if your pistons can be milled to fit those combustion chambers..
I would most definitely think they are designed/machined for flat tops and 9.5:1 comp..
I would most definitely think they are designed/machined for flat tops and 9.5:1 comp..
Last edited by 98hotrodfatboy; Jun 20, 2025 at 03:44 PM.
Thread Starter
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Novice
Joined: Jun 2025
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From: Cheyenne, WY
My dad had the heads done years ago. If the "XX 63" is the cc's of the chamber, then that would set me at about 10.5:1 with flat-tops, which is the range I was originally shooting for. When I bought the bike from my dad 9 years ago, he said they had only ported and shaved the heads. Stock heads that are only shaved would still have a D shaped chamber. I'll have to CC the heads I guess, the best I can. I don't have a plate, but I can at least get a rough estimate, within a CC or two.
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