I myself use replacement springs of conical form but wound
with 2.5 mm wire so as to avoid too much stress on the inlet valve mechanism.
Spring forces are about the same as for the helically wound 1917-1921 springs.
*Replacement
valves and guides
Lack of original oversize
valves made me think of replacement valves and guides that were more readily
available and easy to adapt to the HD engine. I found such valves in the
TRW (Federal Mogul) catalogue 1996.
Catalogue page copies:
Page 1: suitable exhaust valves
Page 2: suitable inlet valves and exhaust
valve guides.
Inlet valves can be used as exhaust valves because the material used
nowadays is of better quality than that of the original exhaust valves.
The following replacement valves were selected from the possible candidates
in the TRW catalogue and so far have given excellent results in my engines:
Exhaust: TRW 1637 - 44x9x140.5 mm (slightly too short but not problematic).
Inlet: TRW 151534 - 47x8x112.2 mm (has
to be shortened to 49 mm).
The advantage of using these valves is that the shafts are somewhat
thicker than the original valve shafts so that in most cases a worn out
valve guide can be simply rebored and reamed to fit the new valve shaft.
Reaming should be done after a rebore to an undersize of about 0.2 mm,
only then is the pressure on the reaming tool cutting edges such as to
achieve the correct clearance value (9H7 and 8H7).
If the exhaust valve guide bore is beyond repair, also valve guides
are in the catalogue that need only minor adaptations to fit the hole in
the cylinder. Mercedes 280 valve guides, (TRW 81-2696) size 15x9x54 mm
are recommended. These valve guides are made of B4 alloy, an alloy particularly
suitable for highly stressed exhaust valve guides (e.g. for the Porsche
911 Turbo) and very easy to machine.
*Machining
to be done
A vertical drilling machine and
small lathe are necessary to do the machining. So as to avoid any problems
of cracking, instead of the common interference fit of valve guide
and cylinder, a screw threaded fit is recommended (as also used on the
later introduced 74" models).
Figure 1 and Figure 2 show how the valve guide
and cylinder were adapted and how perfect alignment of the valve guide
is achieved.
Alignment of the new valve guide with respect to the centre-line of
the worn guide bore must be assured to avoid an out-of-centre of the valve
head, requiring extensively remachining of the valve seats, an area where
normally not much metal is left after 80 years of use. Therefore, all reboring
should be done with a special rebore drill. Such a (rather expensive) drill
has more than the usual two cutting edges and has therefore improved self-centering
properties. Try to line-up the drill and valve guide bore as well as possible
but allow corrective movement of the cylinder during drilling and use plenty
of cutting fluid. Only then is a perfectly centered bore achieved.
It can be seen from the drawing that the drilled hole is stepped. A
bore intended for the screw threaded part (take 12.7 mm for screw thread
14x1.25 mm; common spark plug thread) and a short extension of 12 mm diameter.
This extension is intended to provide a sliding fit with a "nose" provided
on the valve guide, machined to slightly less than 12 mm diameter. Such
a sliding fit at the end of the valve guide helps in achieving a perfect
alignment in the bore. The valve guide needs only minor machining to provide
the screw thread and the "nose". Flats are ground on the side to allow
a spanner to grip the guide for screwing it into the cylinder. A small
screw (5 mm, not shown in Figure 2) locks the guide on its thread.
I assume that the further machining steps, such as valve seat reaming,
are known, if not you can refer to the books mentioned in the HD engine
article.
The original inlet and exhaust valves have a valve collar with a
key traversing the valve stem for locking-up the valve spring. Such a construction
is not mechanically stable and conversion to now commonly used conical
cotter keys is recommended. I used the TRW types LK 1612 for the (Mercedes)
exhaust valves and RK-8H for the (Citroen) inlet valves. The necessary
adaptation of the valve spring collar is a straightforward job (enlarging
the valve stem hole to the conical configuration of the keys) on the lathe.
*Further
recommendations
If the exhaust valve seat is
badly worn the larger valves, such as the TRW 171019, can be recommended
as a suitable replacement. Because of the extensive machining required
for flattening out the area around the seat, which is of larger diameter
than the hole of access to it, this job should be left to the professionals
having the right machine tools (can be expensive though and often it is
cheaper to buy another cylinder!).
As is noted above, the replacement valve is slightly shorter than the
original one. To compensate for some of the missing length, valve stem
caps can be used, such as on the 1911 to 1914 HD motors. I used Alfa Romeo
caps which are available in different thicknesses for valve clearance adjustment
in those engines (the double knocker Alfa Romeo engines 1600, 1750 and
2000).
The guides on the inlet housings are quite easy to machine or to replace
and because the inlet housing can be machined on the lathe as a unit no
particular difficulties are encountered. Grey iron, B3 or B4 alloy
guides can be used here. If someone wants more information please contact
me.
*Back to home page
