This story is just a short mention of the various valve tappet covers used on the 250-305 Honda twins, starting with the first ones made back in 1957. The product code for the first dry-sump Dream engines was -250 for 250s or -251 for the 305s. Honda’s wet-sump engines, beginning in 1960 carried over some of the dry-sump model’s parts, including the -250 code tappet covers. These -250-code covers are domed-shaped overall, but have a distinctive flat edge around the bottom, just above the threads. This tappet cover is seen on all dry-sump Dreams from 1957 through 1960.
When the first wet-sump engines were produced, they carried the early tappet covers over into the first year or two of production. My 1961 CB77 has those covers, as did my 1962 CL72 #39. It is kind of odd that they would use the -250 code covers on 1961-62 models, when the original C72 Dream, which carries the -259 code parts was introduced in 1960.
The actual part number issued to the wet-sump engines was a -259 code part, which is a full-domed cover with no flat edges. Most of the 63-later CL72s and all of the CL77s through 1966 had “flat top” tappet covers, which were noticeably shorter in height, perhaps to increase the clearance between the bottom of the fuel tank and the top of the cover. Super Hawks shown in various road tests and in the setup manuals also had these flat top covers. Honda showed a -268 code tappet cover number in their part number system for awhile, but then superseded them back to the -259 parts. It is believed that the -268 caps were the flat-style covers.
In 1967, both CBs and CLs had a new, revised tappet cover which was a partial dome, which was smaller in diameter sitting down atop the flat top base. There is no designated part number for this last year model cover, however. It is not the first time that Honda changed a part, physically, but failed to change the part number associated with the item. Often, if there is a way to make the part stronger or more easily at the factory and it doesn’t change the function or the way it interacts with other parts, then the part number remains, even when the part doesn’t look the same as it was in the beginning. Another example was the rear swing arm on the CB72-77 Super Hawks. The originals had brazed-on, cast iron lugs for the rear axle mounting, secured to the ends of tubular steel legs on both sides. By 1965, Honda substituted this type with a new “flat plate” type which was welded directly to the slotted ends of the tubing.
No one has discovered the how and why of their design changes, but all four types have been seen on the various bikes, through the years. From what I have observed, all 250-305 Dreams from 1962- onwards had the -259 code domed caps.
As parts have dwindled, it seems that the -259 tappet covers have been the most produced and sold as replacement parts for engine builders and restorers. Still, for those in the know, the tappet cover style could either make or break a closely-judged restoration with “points-off” for the wrong type.
Bill “MrHonda” Silver