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Thread: UL cylinders on a ULH

  1. UL cylinders on a ULH

    If i have to buy a set of cylinders anyway is there any advantage to using UL cylinders instead of ULH? I using a set on VT cycle Aluminium heads from Sweden. Any chances that the 74" will run cooler? Maybe kickstart easier with the lower compression? Meaning less volume in the same size head. This bike will be a solo cruiser. No side car.

  2. #2

    Re: UL cylinders on a ULH

    UL = 3 5/16" starting bore, ULH = 3 7/16" bore to start out.

    Modern repop cylinders that I know are patterned like the 120-37 13-fin cylinders. Building an 80 right off cuts out half the rebuildable "life" of the brand-new cylinder.

    Ya build a 74 with an M51 carburetor to start out. Forty years and ten or a dozen rebuilds (0.125") later, you replace the 1 1/16" venturi in your M51 with a 1 1/8" one (to M51L spec) and keep going through successive 80 o.s. pistons.
    Last edited by Sarge; 10-12-2019 at 04:24 PM.
    Sarge, Gerry Lyons, Fla.
    AGENT ORRNGE Survivor. So far.
    The Friendly Fire that Keeps on Burnin'.
    F.O.G. member: http://flatheadownersgroup.com/
    www.37UL.com
    "It's a 1937, and the downside is, it's out of warranty"

  3. #3

    Re: UL cylinders on a ULH

    The are the supplied sizes of replica cylinders and pistons. The first actual 3-7/16" piston is 1941 FL.
    Be careful with NOS original pistons (and examining a rebuilt engine by the number stamped on the dome), the UH sizes (inherited from 1930-36 VL) are based on 3-27/64" (3.421875") as "std.", so replica std. is actually +.015".

  4. #4

    Re: UL cylinders on a ULH

    Quote Originally Posted by Holtvintage View Post
    If i have to buy a set of cylinders anyway is there any advantage to using UL cylinders instead of ULH? I using a set on VT cycle Aluminium heads from Sweden. Any chances that the 74" will run cooler? Maybe kickstart easier with the lower compression? Meaning less volume in the same size head. This bike will be a solo cruiser. No side car.
    I see no advantage either way unless you are matching cylinders to cases/VINs. 13 fin belongs on a UH/ULH although I think they have repopped the 13 fin in 74". I have several BTSV engines that are STD 80" cylinders w/stroker cranks and they don't run any hotter than my 74" UL. In fact, the BTSVs I have run as cool as any HD engine I have owned. I wouldn't pass on the extra cubes for the same money. They are very hard to get high compression out of and that should not be an issue at all. Unless you are a youngster and live to be 100 and ride it every day, there will be more than enough rebuilds in a STD 80" cylinder to last a lifetime. Seadub

  5. Re: UL cylinders on a ULH

    I cannot speak about the repop cylinders but I can speak about OEM. It is my feeling that when Harley took their big 74" flathead motor to 80" they stretched the 'envelope' just a little too far. After looking at the remains of HD motors (side valve or overhead valve) over the last almost 50 years there is one thing that stands out. Almost every 80" motor that I have looked at, whether V or a U suffered a catastrophic failure usually related to heat. If you look at a set of UL cases they are usually original VIN but if you look at a set of H cases more often than not they are restamped - why - I think because of the destruction of the originals. Now the cases in any given year are the same as is every other component in the bottom end so the problem is almost assuredly somewhere else and that has to be the only thing that is left - the cylinders and pistons.

    My suggestion for cylinders - go with the UL's.

    Jerry



    Quote Originally Posted by Holtvintage View Post
    If i have to buy a set of cylinders anyway is there any advantage to using UL cylinders instead of ULH? I using a set on VT cycle Aluminium heads from Sweden. Any chances that the 74" will run cooler? Maybe kickstart easier with the lower compression? Meaning less volume in the same size head. This bike will be a solo cruiser. No side car.

  6. #6

    Re: UL cylinders on a ULH

    If you had looked at the cylinders that belonged to the repaired cases, you most likely would find thin walls that were at the extreme limits overbored barring no other builder oversights or failures not related to the cylinder wall integrity. A STD bore ULH has way more than enough meat to hold up under all of the riding pressure you can inflict. Not an issue. Seadub

  7. #7

    Re: UL cylinders on a ULH

    U heat problems:
    1. failure to grasp the baffle purpose (like boring holes in the front baffle "to let the oil drain faster")
    2. 1/8" piston to head clearance*
    3. exhaust seats worn, corroded, sunk well below deck height**
    4. leaving the spark retarded
    5. ignoring manufacturer's skirt clearance and ring end gap
    6. intake air leaks
    7. bad high speed adjustment

    * very few U engines I've examined had anything like the factory's recco for deck clearance, most are well below the deck @ TDC and running a 1/16" gasket
    ** If the high side of the exhaust seat is below the deck it should be fixed - that's what the factory relief cutting tool is for (read the service manual), if you don't have one this can be done very well with a Dremel

  8. #8

    Re: UL cylinders on a ULH

    FYI, the repop cylinders have been re-engineered to have more meat in critical areas that caused failure in the OEM cylinders. To the point that one set of FHP cylinders I bought had to be relieved around the base of the cylinder to get a base nut wrench on. I have a SnapOn that works like a charm on those tight areas and it would not clear. There were specs put into the repop design that Paul from ACF required before he commisioned a batch. Sorry to say he has gone off the radar, but he got his specs in on the ones he ordered. As far as I know, they have been good to me. Not sure if they are anything parallel w/V-Twin. I do not know their source and if it was US made, I think it would be easy to figure out who made them. Doubt it. Pretty sure Paul's were from elsewhere, not US made. He did get a batch of large intake port, KHK size made. I have a set on an engine that has yet to go down the road but was way cheaper than a set he had to do by hand that is the cat's meow. The larger porting make the huge intakes and .425 lift cams wake up a flattie. High compression attempts are always a reliabilty issue on these. Larger displacement is about the only safe way to raise compression. I am not an engine builder, but have always researched design possibilities to pass on to the ones that are. I miss Paul as his knowledge on the BTSV is amazing and dead nuts reliable. Hope he makes it through the tunnel and comes out the other side. He has taught me a lot. Seadub

  9. #9

    Re: UL cylinders on a ULH

    More stroke is the easiest answer to more compression. All older H-D S V engines have the same compression ratio problem: the valves are too far away from the bore (laterally), which adds chamber volume with no benefit. This is partially addressed by the 1940-52 WLDR Special/WR and K/KH/KR.
    The universal "tilt" of all SV valves away from their bore axis (fore & aft) adds more chamber volume, but more importantly mis-directs intake charge away from the bore. This was a necessary compromise dictated by the original 1928 choice of what the cam chest will look like (common to all H-D 4-cam engines). The cam pitch distance is equal to the valve stem pitch (in each cylinder), but not located directly under the valve seats, but offset at an angle.

    Indian made a different mistake: their valves are much closer to the bore, and parallel to the cylinder axis in both planes. The small Chief bore (3.25" vs. ULH 3.422") also shortened the seat-to-bore distance. Result: smaller chamber volume = higher compression (6.75:1) without reducing transfer area. The Chief Blackhawk IIRC was rated (estimated?) at 50 hp despite tiny intake ports and a 1-1/4" Linkert. The Max Bubeck interview w/r/t the "Chout" (twin carburetor) engine claimed 65 hp @ 4,400 RPM.
    No, I can't think of any way to use the Chief top end on any H-D engine.
    Last edited by panic; 10-14-2019 at 04:59 PM.

  10. #10

    Re: UL cylinders on a ULH

    I bought a very interesting set of heads a while back that are alloy cast from an iron set of OEM heads w/short fins. It has a major reduced chamber above the bore w/generous cutouts for the valves. Haven't mocked it up w/pistons but imagine it will provide more compression as well as presenting more challenges. You are dead on w/increasing the displacement to automatically increasing the CR. Setting a perfect/proper squish band is where a lot of HP is lost or gained. Usually isn't easy, but sometimes can be very difficult. Most builders don't even try unless it is a hard request from the client. Setting a squish band perfect at the lowest possible CR is even harder. I went through that decades ago w/4-T trials bikes. I have found that to be one of the biggest little things on all 4_strokes where performance matters. Seadub
    Last edited by bianese; 10-14-2019 at 06:08 PM.

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