Restoring a 1-3/4 HP Monarch, Part 3

Reclaiming its crown

| August/September 2010

The following is Part 3 of a four-part series documenting Peter Rooke’s restoration of a 1-3/4 HP Monarch antique gas engine badged by Nelson Bros.  Read Part 2 .

Cylinder head
Apart from the broken rocker arm at the point of an early welded repair and a split pushrod guide, the cylinder head appeared in good condition and the valves appeared new. However, the valve seats showed signs of rust.

After stripping the cylinder head by removing the valves, mixer, rocker arm and pushrod guide, the old paint was removed revealing bare metal. It was also necessary to scrape a thick layer of gasket sealant from the inside bearing surface.

The valve seats had to be polished to remove a thin film of rust. The contact surface of the valve was coated with fine grinding paste before using a grinding stick, which was rolled backward and forward between the hands to grind the seat. The valve was frequently lifted up and the paste re-distributed to ensure continued cutting. This was done until there was a clearly defined ring of polished metal around the seat. At this point, the valve was removed, cleaned of all traces of grinding paste and put in a plastic bag, marked with the type of valve – inlet or exhaust – so that it could be fitted to the correct seat later.

After both valves had been fitted, the head was carefully cleaned to remove all traces of the abrasive paste. A straight edge was held across the cylinder head and it appeared true. I later found that there was a compression leak from the bottom of the head so it was checked again by spreading some engineer’s blue on the surface plate. As can be seen from the photo on the opposite page in the middle, there was an area where no contact was made with the surface plate between 3 and 6 o’clock.

The quickest way to make level the cylinder head is to set it on the lathe and take some skim cuts. However, there was not a lot of metal on this head, and when skimming you could loose a few thousandths of an inch in setting it up. As the straight edge had shown no major defects I decided to scrape it. The area of contact on the head, shown by the blue, was scraped off and the cylinder head marked again.