Thursday, February 7, 2019

The 250-305 Engine overview (Part 2)… the other stuff to know…

The engines have the same architecture among the three versions. All of the engine cases are basically the same, apart from the early ones not having a primary chain tensioner mounted. They are not “machine matched” at the factory. The 1960-61 cases and cylinders had a different oil supply feed for the top end, thus there are two different base gaskets. The 1960-61 engines had a “rear breather” system cast into the back of the top case, which allowed the crankcase to be relieved of pressure during operation. 1962 and later engines used a breather plate and drain tube coming from the top cylinder head cover.

1960-61 cylinder heads used a small 10mm spark plug, changed to 12mm in 1962. Pre-1965 heads had an inverted U-shape design in the forward fins, which was changed to a sharper V shape from 1965 onwards.

The die-cast cylinders, introduced in 1966, used a narrower camchain tensioner to allow better air flow between the cylinders. The cylinders will all interchange between the various engines, apart from the previously mentioned 1960-61 oiling passage differences.

Honda Dream intake valves are smaller in diameter than those used on the CB/CL models. They do share the same exhaust valves, after 1962.

The camshaft sprockets and locking nuts were changed in 1962 from right hand threads to left hand threads. There are about four different camshaft/camsprocket spline patterns which precludes any attempts to mix and match camshafts and camsprockets from other years.

Early transmissions had straight cut gear dogs, finally upgraded to back cut dogs which offer better dog engagement and fewer missed shifts.

The shift forks, shift drum, kickstarter shaft, kickstarter pawl/spring/plunger are universal parts fitting any and all 250-305 engines, except Dreams with rotary gearboxes. There are two different lengths of shift shafts, changed around 1962.

There was a change to “shallow spline” shafts in 1967 which included the crankshaft and primary drive sprocket as well as the transmission input and output shafts. You can use the later shallow spline sprockets on early deep spline shafts, but not the other way around.

Honda used both Nippon Denso and Kokusan ignition and charging system components. Using mismatched branded points is a no-go with these ignition systems. Use of aftermarket point sets from Daiichi on a Dream is also a no-go, as they do not allow proper timing adjustments. For best results, used ND points on ND point plates. ND parts have a -004 suffix, while Kokusan have -005 parts suffixes. Either brand might be found on a Dream engine, so pay attention before you order your parts.

The point cam/advancer shafts which run through the inside of the right camshafts come in 2 outside diameters which match the ID of the holes on matching camshafts. They don’t interchange! Check the return action of the camshaft sprocket return springs before assembly. The springs should return the weights back to resting position. Remove excess slop by pinching the ends of the springs very slightly. Check the camsprocket for any looseness in the rivets which hold it together.

The ends of the intake and exhaust rocker arm pins are 2 different diameters. They don’t interchange either! There are two types of rocker arms, but all will interchange.

I use Honda’s GN4 motorcycle oil in 10-30wt for most applications. Honda originally specified non-detergent oils, but that technology has been far advanced by current motorcycle-rated oil products on the market now. Full synthetic oils are really not necessary unless you are involved with racing activities and even those may not require synthetic oils. Normally, these engines are pretty oil tight, but synthetic oil molecules are so small that they can work their way past old seals and gaskets.

Speaking of gaskets, most of the original gasket materials were all or partially made from asbestos, so be very careful with scraping and grinding away on stuck-on gaskets from the past 60 years.

These fine folks have come up with replacement chains for the 250-305s, including primary chains, oil filter drive chains, camchains and starter chains. They are in the UK but get parts shipped out pretty quickly, most of the time…
Sprockets Unlimited
Telephone/Fax: 00 44 (0) 1386 831341
E Mail :

Last time I checked, Honda’s “suggested retail price” for ONE 250-305 piston was $186... for ONE piston. Tim McDowell has replacement WISCO forged pistons available with and without sleeves for less money.

You can interchange CA pistons with CB pistons and the other way around. Just find a matched set of whatever you can find and it will be fine. OEM Honda pistons have ART cast into the sides and CB/CL77 pistons have CB77 marked on the sides as well.  There are a fair number of aftermarket replacements, made back in the 1960s, which may not be quite right. I have found some that have too small piston pin holes. The piston crowns on early and late pistons are quite different. Whichever ones you use, get a matching set for installation. Aftermarket pistons might need a smidge more clearance than OEM pistons, which I usually have clearance at .0015” (one and a half thousandths).

MrHonda has both the center camchain guide rollers in stock as well as the NLA 266 code clutch adjusters available which fit all 250-305s, the early CB500 Four and all of the 450-500T twins. Contact me through my site:

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