First, the Tech Soliloquy:
The post-1974 engines (F0/F1/K7/K8 and F2-3 series) used pistons with domes on top to increase compression. This change came about because a new hemi-style combustion chamber was introduced (for reasons discussed below) that was larger in volume than in the early engines. On the F0/1 and K7/8 engines with head casting “-392-“ the combustion chamber is one larger size, and on the F2/3 engines the casting “-410-“ type is an even larger chamber, to fit its larger intake valves. Accordingly, the “-410-“ piston domes are taller than the “-392-“ domes, but even with this additional height, the F2/3 engines have LESS compression than the F0/1 and K7/8 engines.
In 2012 the world’s supply of ART/Honda pistons for these engines dried up. In their place, using the earlier flat-topped (“-300-“) piston causes a low-compression situation that removes considerable horsepower and midrange torque, although it still provides a decently-running 750 engine overall. Calculations indicate the engine output drops about 10%-16%, depending on which cam your engine has inside. This isn’t a deal-breaker, as the engine can then run on the crummiest fuels you can find, but the bike does lose some spirit.
Here in the forums, MiGhost (thanks, MiGhost!) brought out the point that the DOHC750 pistons (1979-1982) have similar skirts and piston pin diameters to the SOHC4 type, but the shoulders are too tall to directly fit the stroke. After he kindly sent me a set of those pistons to play with, I got a tool made to let my shop cut back the shoulder so as to fit these engines’ stroke. Then I got some new DOHC rings (ouch!) and wrists pins (double ouch!) and a fellow SOHC4 member here in Colorado brought me his K7 engine as the Guinea pig test. We bored it to fit (this comes out to 4th oversize, if these were SOHC4 pistons, or 759.8cc) as we can still use the stock head gasket, and I checked all the valve-to-piston clearances with the tried-and-true “Clay Test” on top of the pistons. We get about 2mm of clearance, worst case, which should be far more than enough. I cut the shoulders back 1.5mm overall height, which made them flush with the top of the cylinders when the base gasket was installed.
The tall, flat domes on these pistons will not disturb the swirl pattern a lot in these later engines, as the swirl is already far less than in the earlier ones. The swirl will have already been ignited (at 5-35 degrees BTDC) before the crown causes interruption of this flow. In effect, it converts the chamber to a DOHC750 style chamber, but with some swirl (DOHC has none). During overlap cycle, the high flat dome should act just like in the DOHC engines, blocking the transfer of unburned fuel across the piston crown. This was done to reduce tailpipe emissions more than anything else, but this has an added benefit at lower engine speeds of not inducing a burn back up the intake tract, should an early-opening cam be introduced to the engine.
But, if the user wishes, a early-intake cam duration could be added (before even 10 BTC) for improved low-end torque as compared to the post-1974 cams. The post-1974 SOHC4 engines all had cams that opened (intake) at 0 BTC to stop this “back-burn” problem (as early opening valves introduced tailpipe emissions at 2500 RPM). This later opening sapped away the torque in the 3000 RPM range. The result was/is that the post-1974 bikes usually feel listless in city driving below 5000 RPM, if one is used to the earlier bike’s performance. While the higher compression and larger carbs of the later bikes introduced some more top-end power, most riders seldom use that power until 70 MPH or more, so it often goes unnoticed, or at least not used very much.
Next, as I assemble them, will come the picture posts!
