*The original valves, guides and valve springs
Many types of inlet valve springs were used. With the years the spring force increased, in particular after that the inlet lifter arms were provided with rollers in 1917. Common inlet valve springs for the 1917 to 1921 models have the size:
The sizes of the original valves and valve springs for the 61 cubic inch engine are:
Exhaust valve (about): 44x8x144 mm
Inlet valve (about): 46x7x49 mm
Exhaust valve spring: 23.5x3.15x65 mm (D x wire x L), 10 windings.
When compressed to 49 mm the spring force should be 13 to 16 Kgf.
18x2x65 mm (D x wire x l). When compressed to 38 mm the spring force should be between 10 and 13 Kgf. However, when rebuilding an engine having inlet lifter arms with rollers (after 1917) the inlet valve spring for the later models (after 1921?), which are conically wound can be used. These inlet valve springs are about 50 mm long, have 10 windings of 3.1 mm wire. The diameter at the top is 18 mm and at the bottom 27 mm. When compressed to 40 mm the spring force is between 10 and 12 Kgf, at 35 mm about 18 Kgf.
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.
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.
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