“Save the Decaf!” — rebuilding and renewing the engine of 1978 CB400A Honda motorcycle (named Decaf)

Saving my 1978 Honda CB400A “Decaf” bike from a near fatal blow — lower gasket blowout and huge oil leak! (like, dumped a quart+ of oil in minutes while riding fast on highway at high temps). It was inadvertently overfilled with oil (my bad) and driven too hard (twin brother’s not so good) combination that killed the Beast!

Work with my twin brother (a very gifted self-and-internets-taught car and motorcycle moonlighting mechanic!), I as assistant especially for the entire reassembly and much of the cleaning work.

We had to take the top 3 layers off the engine stack (AKA “complete top-end rebuild”),
remove the pistons, and strip all the way down to the gearbox.
Layers: [valve cover] gasket [valves] gasket [cylinders and pistons] gasket [gearbox and crankcase]
Very neatly Honda makes them all stacked and connected via 8 super long screws that compress, when properly torqued, the entire set under the valve cover.

We:

  • replaced the piston rings (very thin split rings circling the pistons that make them “float” up/down the cylinders),
  • re-honed the cylinders (you use a “flexible hone tool” to clean, slightly resurface, polish and add a light “cross-hatching pattern” to them),
  • cleaned the pistons completely (lots of carbon buildup after decades!) and installed 3 new gaskets — especially paying attention to the lowest gasket that tore and blew oil using some “gasgacinch” sticky product around the oil jets).

Then we did a full valve job (gets the valves ideally aligned to the 4-stroke engine stages),
timing belt adjustment, oil change, and more.

In addition, the air filter was trashed, so replaced that and fashioned an air filter custom holder to keep it from slipping back into the air box.

Bonus points: exhaust and carbeurator gasket replacements.

About $150 in parts (piston rings alone were OEM Honda and $75+), but probably the equivalent of $800-1000+ in labor.

A challenging and very educational labor of love!