Duel plug VLD

[Steve Slocombe]

Dear Lee, thanks for the nice words on the VL book. I don't see any compression code on your heads, but by eye they look like 8.2 heads as are the aluminium heads. I can feel the difference when kicking over the 80 inch VLDD with ally 8.2 heads compared to a regular VLD 74 with 9.3 high compression iron heads. I also have a pair on my 34VDR racer, where the extra compression must give a tad more power, and the weight saving is maybe 8 pounds on the bike, which is worth having for quarter miling. I had one other similar iron head to yours, where the area over the piston top had been milled out but no hole fitted for a second plug. I thought this might be for pop-up domed pistons, as otherwise the 'squish' effect of the Ricardo heads is somewhat compromised.

[Lee.c]

Thanks for all the help with solving mystery.

[Steve Slocombe]

As a speculation as to when these heads were available, I'm guessing 1935-40. From memory there are speed runs from the late 1930s with classes such as 74 cubic inches and unlimited/modified, where speeds of 118 mph were recorded. I'm thinking these were hot VLs rather than early knuckleheads. The special cylinder heads were available, as were the hook cams which seem to date to 1935, and the Tom Sifton gas flowing tuning tips for sidevalves must have been out there. A 1933 VLD set the AMA production bike speed record on Muroc Dry Lake at 104 mph in calendar year 1932, so these speeds seem possible for tuned and lightened VLs with tall gearing doing one way runs a few years later. After 1945 it would have been very different, but we still see VL forks used on post WW2 dragster bikes because they are so strong.

[Lee.c]

https://m.youtube.com/watch?v=Xwx6niByVKs

I finally got to hear it run! I didn’t have all the exhaust clamps on so it blew the front pipe out of the head almost immediately. I did end up running 2 coils on it so all 4 plugs work. Hopefully next summer It will be able to go down the road again. Can’t thank the members enough for all the knowledge shared on this site

[kitabel]

The number appearing on BT flat heads is the chamber volume in inches, not the compression ratio: high compression is a smaller number than 9.6 (45 heads are marked with the CR).

From the (great!) photo, that head is designed for use with a pop-up and domed (large radius) piston. A flat top (conventional VL/UL) piston will have no quench and suffer knocking unless you retard the spark. I've never seen a domed BTSV piston, but the common bore size (3.421875", .0156" short of FL) may allow use of a BTOHV piston.

[Steve Slocombe]

Dear Lee, thanks for the great video. On domed pistons for the flatheads, these were available as options in aluminum for the hot 1928/29 Harley twin cams so, as the 3.422" standard bore is the same as the VLs, it is conceivable that some hotrodders tried these out on the flathead VLs in the late 1930s.

[kitabel]

Several possible correction methods.
1. fill the chamber and central plug, keep the std. VL/UL piston (not practical, ruins a historic piece)
2. custom piston, perhaps $200 each, difficult to describe to mfg.
3. modify something, but the dimensions that must be very close:
compression distance (exact, or too tall, short will not work)
dome radius, if it's a regular geometric shape, or if there's enough material in the head, make it so
pin diameter
skirt length (not critical, but as long as clears the flywheel OD)

First thing to do: define the chamber recess, use clay, plaster etc.

[Lee.c]

You have me thinking about this quite a bit now. When I took it apart it had I believe.020 over flat tops in it. One of the pistons did have a little damage on top at the edge. (Going to have to dig them up and look at them again). Didn’t think much about it then but now I’m guessing it was detention. I’m going to have to look into this a little further. I’m not apposed to different pistons and a rebalance. Thanks for the info.

[kitabel]

If you're up for custom, some dims are already there from the VL base: pin diameter, skirt length, ring stack, bore (+.030" to clean up?), and compression distance (to the flat dome area, make it slightly taller to correct with lathe or shim to get .035" final P-H safety margin).
The tough one: maybe they can use your model of the recess to design the dome. If it's too tall, bad shape etc. you can trial fit it many times and remove aluminum with a Dremel until you're satisfied (much time, little money). The dome should have a small depression directly under the plug to allow the flame kernel to expand w/o restriction.

[Steve Slocombe]

Remember gas flow is as important as compression ratio on the flatheads, so don't go too crazy. Harley didn't seem to go over 7 to 1 compression ratio on the WR/KR racing flatheads.

[Lee.c]

Good info. Thanks