Not infrequently, I get messages/questions from novice Honda 250-305 Dream owners asking about the rings that should fit their bikes, during an engine overhaul. Of the three models, only the Dreams had two types of pistons/rings, which do not interchange.
Honda believed in long service lives for their bikes. In their early efforts with the 250-305 Dreams, they sought to keep piston and ring wear down to a minimum, so the first Dream engines came with piston rings with a 1.80mm cross-section thickness (vs. the 1.5mm rings used on the CB/CL editions). The little 250s actually had four piston rings in the early days, with the oil control ring located down beneath the piston pin location. For domestic usage, this specification was probably sufficient, however when the new CB72-77 Super Hawks were released in 1961, Honda found themselves making two types of rings for their engines. As production rates ramped up and more engineering analysis data came in, they chose to use the narrow 1.5mm ring sets on all engines and removed the 4th ring from the 250s, as well.
Honda made the changeover right at the beginning of 1962 for the 305s (CA77-210152) and the beginning of 1963 for the 205s (CA72-310102). With a small number of bikes built and sold in the 1960-61 years, you would think that the available rings left for sale in the world would be tiny, however a check of eBay auctions shows quite a few 259/266 code ring sets and most of them not giving out the additional information about what they actually fit. This is what prompts emails and phone calls to me, asking why the eBay ring sets don’t fit their 1962-later CA77s and the 1963-later CA72s. Usually the ring sets are quite inexpensive, so owner’s will jump on the chance to save a few dollars, especially as the value of the 268-275 ring sets are jumping upwards, dramatically. Because the vast majority of 250-305 models being restored now are Scramblers and Super Hawks, there are a lot of people vying for those parts. The total of Honda Super Hawks and Scramblers produced is something beyond 160,000 units, factoring in another 30,000+ Dreams yields a big demand for new engine parts now.
Honda’s “book price” on obsolete ring sets (and they don’t have them in stock now) ranges from $66 to $99 per box (2 sets, one set per piston) and piston prices are bouncing off the $100 figure EACH. The fallback option is to search out some late Dream pistons to use in Scramblers and Super Hawks. You still have to come up with the 275 code rings, but the pistons for Dreams are more commonly found or have been until recently. The piston crowns are machined differently between the CA and CB/CL versions, but generally yield about the same compression ratios when installed in either engine. A properly timed and machined 305 engine will yield up about 170-175 psi on my compression gauge, using either set of pistons.
Tim McDowell at ClassicHondaRestoration.com offers a whole piston/ring/ kit to bring a worn out motor back to factory specs using forged pistons and modern rings with a 3-piece oil rings, at prices ranging from $269 to almost $400 for pistons and new sleeves. Add in any machine work costs for boring, repairing loose liners, decking the cylinder block, etc. and you have a good chunk of change invested just in the middle part of the engine assembly.
Chances are that the great majority of the 250-305 engines out there to be rebuilt are going to use the later “thin ring” pistons and piston ring sets. Just be aware that if you jump on a “good deal” for Dream pistons/rings that they will actually fit your application. In most cases, that have come my way lately, that has not been the case and the mistake costs time and money lost in the overhaul process. Be sure to ask if the rings or pistons use the 1.8 or 1.5mm ring widths.
Bill “MrHonda” Silver