Stuffing a big twin into a VL frame dates
back to the days when the Knucklehead was a
brand new, modern OHV engine, 1936.
There were a lot of advantages to a
conversion like this. VL frames were good for
a 25 pound frame-weight reduction, versus its
Knuck counterpart. The shallower VL rake
made for a quicker handling bike. In addition,
the physical look of a big twin in that compact,
single downtube frame layout, was not to be
denied. It's still the stylish case today.
While not extremely difficult to accomplish,
installing any of the pre-EVO big twins into VL
frames is definitely not a drop-in affair. How
much frame surgery required depends on
which version of the Harley V-twin you're
considering.
A few notes before we begin.
First, the newest VL frame you're likely to
come across is now approaching 68 years
old. These frames were manufactured by
combining forged frame junctions, virgin steel
rails, and furnace brazing. Herein lies two
important things to watch for.
A lot of VL frames around today have
rotted from the inside out. What looks like
pristine frame rails, may look like Swiss
cheese after sand blasting. Look for these
symptoms where the frame tubes enter, or run
through the forgings.
Next, VL frames were assembled using
the furnace braze method. This entailed
heating the assembled forgings/rails until
brass rod would melt, and flow into the
unions, similar to "sweating" copper plumbing
tubing together. When modifying a VL frame,
EXTREME CARE must be exercised in making
sure that ALL TRACES OF BRASS are
removed from modified areas. Electric
welding over brass will result in,
DANGEROUS CRACKS FORMING in both
your frame, and your weld beads. No good for
you at 80MPH!
Additionally, over its production run, the VL
forgings changed from year to year. While, at
first glance, they will all look the same, you
may experience varied forgings. You may
need to adjust your mods to suit the shape
and size of your particular frame forgings.
Shot 1
Also you are liable to run across already
modified VL frames. Some are well done,
others (more likely) are out and out butcher
jobs (pictures #1&2, This one was butchered
so poorly, that the engine snapped the
seatpost tube.) While this is not an all
encompassing frame repair article, a lot of
what you'll see here, can be used as a
guideline to double check, or correct frame
mods.
Shot 2
One last comment. The rake on these VL
frames is right in the area of 25 to 26 degrees,
in stock form. Consider this, the rake on a FXR
frame is 28 degrees, and the rake of a VL
frame is approximately 4-5 degrees LESS
than a later big twin rigid frame. Raking is
definitely in your future if you plan on running
at high speeds with your finished cycle.
Don't say I didn't warn
you!
With these warnings out of the way,
let's get to it. We're going to modify
four areas of the frame:
1. The lower tank frame rail (picture #3)
2. The upper seatpost/backbone forging
(picture #4)
3. The front , and rear lower engine
mount pads (picture #5, and #6 respectively)
4. The brake mounting area of the left
rear frame rails (picture #7)
Shot 3
Shot 4
Shot 5
Shot 6
Shot 7
Let's start our modifications at the
rear seatpost/backbone area (picture #3
again), to fit a Panhead, or early/late
Shovelhead. You are going to make a cut that
starts between the upper and lower tank tube
lugs, through the forging web, just short of the
upper lug for the seatpost.
Shot 3
Next, make a cut along the upper seatpost
lug, intersecting your previous cut*. (picture
#8)
Shot 8
*NOTE: If your intended
engine is a Knuckle, or a UL-series Flattie,
your surgery here is not as extensive. Both
engines will fit (if they aren't strokers), by
cutting just between the tank tube lugs and
stopping. Mark a cut line approximately from
the seatpost tube gusset (with the two holes)
on the lower I-beam section, to the upper tank
tube lug. A simple 1/8-inch thick gusset is cut
and shaped from cold rolled plate, to restore
the I-beam structure. The lower I beam
webbing with the two holes is removed, and
the forging is slightly shaped to flow nicely.
This is all the clearancing you need for these
two engines. If you own a stroker, you need to
make the full relief cut, as done for the
Pan/Shovel.(see drawing)
Shot 6
Before mounting your engine, to check
your rear rockerbox clearance, remove the lug
under the rear lower engine mount, that held
the rear footboard crossbrace. The engine will
not slide into place with this intact. It'll hit
the back engine casebolt tunnel. (picture #6)
Shot 9
Cut the loop right at it's upper arc,
and right in front of the lower seatpost casting.
Remove no more of the loop than is
absolutely necessary, to preserve the integrity
of the mount. (picture # 9)
Shot 10
Shot 11
Install your engine of choice (in this case,
a Panhead), using just the rear mounting
bolts (torqued to 35 lbs./ft.), and check your
clearances. (pictures #10 and #11)
Shot 12
As you can see, even with the forging
reduced to half it's original thickness, is
still a tight fit! Your gap here, over the rear
rockerbox, will be approximately 3/16-1/4-inch,
if you made a nice straight cut. Remove no
more material than absolutely needed to
adjust the gap! (picture #12)
Shot 13
We need to restore the structural integrity
of the forging next. A posterboard template is
made to fit the contours. (picture #13)
Shot 14
Most times, it is necessary to make a
different pattern for each side. (picture #14)
The patterns are transferred to cold rolled
plate, cut to shape, and a slight bevel is
ground on the edge of the plates, to improve
weld penetration. Then the plates are tacked
in place.(picture #15 and #16)
Shot 15
Shot 16
The engine is mounted once more as a
final check, removed, and the plates are
final-welded in place. Here's a tip:
When welding on any forgings, take an
oxy/acetylene torch with a "rosebud" tip and
preheat the forging before welding. This will
improve weld penetration immensely. After
you finish your welding passes, cover the
welded area with a fireproof covering (such as
a welding blanket - shown here), and allow
the forging to cool at a slower rate. Harley
forgings have a tendency to crack, if they are
exposed to cold air during cooling. NEVER
accelerate the cooling-off process. It will
surely crack as a result. Let welded forgings
take up to an hour to cool to the touch. Take
your time! (picture #17)
Shot 17
Your modified seat post forging, fully
gusseted, should look something like this
when you're finished. (picture #18, and
#19 respectively)
Shot 18
Shot 19
Shot 20
Before we can continue on with the full
engine mounting, the lower tank bar needs to
be fabricated, and welded into place. In the old
days, most guys cut the original lower tank
bar, shortened it , heated it at the front forging,
and bent it straight up till it hit the upper tank
tube, and called it good. It worked, but it
looked like hell! Watch how we handle
it.(picture #20)
Shot 21
The lug for the lower tank tube is cut off
flush, along the back angle of the neck forging.
(picture #21)
Shot 22
With the lower tube cut off, you will see
what remains of the tubing, and you will also
see the line of brass that held the tubing in the
lug. (picture #22)
Shot 23
The remaining tubing, along with all the
brass, in this area, needs to be completely
removed and the area contoured to blend with
the lines of the casting. (picture #23)
Shot 24
A piece of 1-inch diameter, 1/8-inch wall,
DOM tubing needs to be bent in an arch that
will allow it to meet the upper tank tube
approximately in the area that is directly over
the front of the rear rockerbox. In addition, a
section of 1 1/8-inch wall tubing is needed to
form a lug for the front of the lower tank tube in
the forging. (picture # 24)
Shot 25
The socket is cut approximately 3-inches
long, and a 30-degree cut is made on one
end. The angle cut is purely for esthetic
reasons. The lug is seam welded to the end
of the tubing, and a plug weld is used on each
side of the lug to secure it in place. (picture
#25)
Shot 26
Then, the assembly is placed in the neck
forging, and the curve of the upper tank tube is
marked and cupped in the tube. The lower
tank tube is ready to weld in place. (picture
#26)
Shot 27
After welding, the beads are blended into
the surrounding areas. (picture #27 and #28
respectively)
Shot 28
If you've done this right, it's
hard to tell that the lower tank tube
wasn't original! (photo #29, and #30
respectively)
Shot 29
Shot 30
We're almost there, only two
modifications remain.
With our lower tank tube in place, we can
bolt the engine back in the frame. Again,
mount the engine with the back engine case
bolts only, torquing them to 35 lbs./ft. Looking
at the profile of the engine/frame, you'll
notice that the engine tilts to the front of the
frame, and doesn't sit square on the
front lower engine mount pad. (picture #31)
Shot 31
Shot 32
In addition, if you look from above, in the
area of the front case mounting ears, that the
holes in your mount pad and your case ears
will not align. (picture #32)
Shot 33
To correct this, we need to add a 1/8-inch
thick, cold rolled plate to the frame mount pad
and relocate the mounting holes.
Remove the engine, cut your plate, and
tack weld this to your frame mount. Note the
plate is not quite as wide as the pad itself.
This allows us to run a good, solid bead.
Reinstall the engine, torquing it to spec again.
Using the case ears for a template, a layout
punch (or, a long 3/8-inch grade-5 bolt, with a
point ground on it) is dropped through each
ear, and the bolt hole locations are punched
into the plate. Do this as accurately as you
can. It will pay off when you bolt the engine
back into place.(picture #33)
Shot 34
Remove the engine, and weld the plate in
place. Flip your frame over, and plug weld the
original front mounting holes shut. When cool
drill your center-punched marks with an
1/8-inch pilot bit, followed by a 3/16-inch bit,
followed by a 3/8-inch bit. Doing the drilling for
the mounting holes in these steps will assure
you of very accurate hole alignment. When
finished, your mounting pad, and your case
ears, should align. Additional thin shims may
also be necessary here for a perfect bolt
down.(picture # 34)
Shot 35
A simple upper engine mount was
fabricated out of 1/4-inch thick cold rolled
plate, cut to shape. Using the top motor
mount, bolted to the cylinders, the upper
mount location is marked and welded to the
lower tank tube. After the welds cool use the
motor mount, bolted to the cylinder heads, as
a jig to drill the mounting bolt hole in the frame
mounting tab. (picture #35)
All we have left is fabricating the anchor
plate for the rear brake. This client opted for a
'36-up OEM mechanical brake. As you
can see, the original VL slot for the backing
plate locating tab isn't even close.
(picture #7 again)
Shot 7
First, the upper part of the brake tab was
removed and the base was draw-filed flat.
(picture #36)
Shot 36
Shot 37
Next, a 1/4-inch thick plate was cut from
cold rolled steel, with a relief that would allow
the backing plate locating tab to slide back
and forth during chain adjustments. (picture
#37)
Shot 38
After the plate was squarely tacked in
place, a small spacer was required to fill the
gap between the upper plate and the upper
frame rail.(picture #38)
Shot 39
One final check, and the plate was welded
into place. (picture #39)
Shot 40
The finished plate. (picture #40)
And here's two VL frames headed
back home to Cleveland. (picture #41)
Shot 41
I hope this gives you a good working
knowledge of what actually goes into fitting a
big twin engine into a classic VL frames.
It's not a difficult modification to do. But, it
does involve making your patterns and
replacement pieces accurately. It involves a lot
of measuring, cutting, measuring, cutting,
assembling, disassembling, welding,
assembling, checking and double checking.
Take your time, goddamnit. It saves
screaming later.
Later,
Irish Rich / Shamrock
Fabrication -
www.shamrockfabrication.com