Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

The H74 is a LOT of machinery for the money.  Those of you who have made engine parts will understand.

A few things should be noted if you decide to invest in one.

With my engine, during the first test run the cooling system didn't seem to be keeping up with combustion heat.  It was noticed that the water pump would intermittently stop turning.  A flip with fingers got it spinning, again, but something inside wasn’t right.  Upon opening the pump I found some sand granules and tiny particles of brass debris, stuff not unusual in manufacturing.  Flushing the cooling system fixed it.

Another observation with the pump apart was that the impeller vanes were curved the wrong way for the shaft rotation.  I could not see any functional reason for Microcosm to do that. So, after reassembly, the drive o-ring got a half-twist to reverse the pump’s spin.  Cooling improved.

Cooling system water needs some sort of surfactant to eliminate the air bubbles that cling inside the hoses and passages.  Liquid dishwashing soap will work, and only a minuscule amount is necessary.  I add food coloring, too, for esthetics.

As received, the engine had varying valve clearances.  Also, the two spark plug gaps were visibly different.  Adjusting all for uniformity noticeably improved low rpm idling.

It has been said in other postings that Microcosm carburetors are difficult to fine tune.  The carb on the H74 is adequate, not exceptional.  As I see it, finer threads on the needle valves would be an improvement.  Also, eliminating some of the linkage slack would be worthwhile.

As with the M90 and M17B I’ve also experienced, the H74 starts fast on gasoline but seems to run cooler on naphtha.  I mix fuel 35:1 with Marvel oil.  That blend is a bit ‘rich’ but it keeps piston o-rings happy.

The H74 is not a wet-sump engine.  There is no oil pump nor splash scoops on the crank, and the crankcase side covers do not seal.  Prior to running, both ends of the rods and the camshaft can be lubed via the side access.  And, of course, oil every part that slides or pivots against another part.


Microcosm engines tend to have a huge grin factor.  Thanks, Jin.