A Gearheads View of The Forty Horse Engine
By Bob Donalds  

While reading an article on 36 hp assembly data and tips I was reminded of the steps I would take and things I noticed when I  built forty horse and formula vee engines. The fv engines were based on 40 hp engines and saw 6000 rpm. I thought I would share some of  them with you and the forty horse faithful. When I hang  rods on 36hp & 40hp cranks I pay extra attention to rod and rod cap aliegnment.  The 40 hp rod bolts go through the rod and screw into the rod cap leaving room for the bolts to move in the rod bolt hole.  I use my set of feeler gauges, putting the largest blade that will fit between the rod  and crank. This pushes both  rod halves to one side. When the bolts  have been lightly snugged, I install the end of the same size feeler gauge, twisting it just a bit as this will catch any edges that protrude, adjust as needed. Then after checking the rod for freedom of movement, and lightly  tapping if needed at the matching  numbers on the side of the rod as shown in the manual.   Next torque the rod and recheck. Remember 40hp rods don't have the self-aligning capability that later ('66 & up) rods have. My average time is one hour for four rods.

The rod to crank side clearance very, depending on the manual you are using; most do not cover this.  VOA gave their line mechanics a small book called: With Out Guess Work   In it  the clearance is listed two ways; New tolerance: 1) 007 to 0156 ths. and wear limit of.03 for the 36 hp and  new tolerance: 2) .004 to .016 ths with no wear limit listed,  for the 40 hp.  I have used up to 018 ths with no problems. I think its important to mention that the pistons get lubricated by the the oil  thats thrown off the con rods bearings.

The problems I've seen when reusing 40hp cases are the cam bore wear, this is noticeable just past the cam at the flywheel end, and the oil pressure light that can come on at warm idle. There is no replaceable cam bearing in the  36hp, and in most 40hp blocks. The camshaft sits right in the case itself. You can get heavy duty oil pumps for the 40hp that will keep the light out. Also Rimco, I'm told, bores cases for cam bearings in the forty horse.

Another tip, you can tell if your case did come with cam bearings without splitting the case. The boss, or the raised mating surface of the case halves, directly under the fan shroud, will be larger than  the rest. Were talking about only the part that can't be seen with the fan shroud in place.

One other thing that comes to mind is the up grade for the forty horse cases   The 1600 cases would make good replacment. The only dimensional difference is the cylinder opening. On the forty horse jug I would cut a groove in the cylinder with a lathe just below the base gasket surface.  This groove is for an O ring this allows the cylinder to be centered in the slightly larger cylinder opening. This way I was able to use the later cases. This was also a SCCA legal upgrade for formula vee.

Some of the other things I did to Formula Vee engines were to hone the piston pin hole making the rist pin free floating in both the rod and piston. I also increased the skirt clearance by honing the cylinder to .004 ths, this lets the piston get the oiling and room to grow (heat expansion) that it needs to survive the higher rpms and temps that go with them. Speaking of higher rpms, not only do I use new valve springs, but I also shim underneath them to increase the spring tension. Shims come in .030 and .060 thicknesses. I have used two .060 shims or more if the valve is deeper in the seat.  This increased spring tension allows the valve lifter to follow the cam through a broader (higher) rpm range without floating the valves.

The pistons rings are very thick in the old VW engines and when the top ring groove has worn it lets the rings flutter. Used pistons don't  make power over 4000 rpm even with new rings.

The available exhaust valves for the forty horse does not have the all important chrome stem.  In the past I've cut down the head of 1500-1600cc exhuast valves with the chrome stem for moderate to severe duty use, like a judson super charger, or any bus used on the highway. I suggest finding somebody, or a shop with a lathe for this job. Valve facing machines will not do this.

The next thing is the cam and the lifters. New lifters are different than the orginal;  the head is thicker and the lifter is one piece, not two. This thicker head is good. The problem is the lifter head needs more room. If  the clearance from the top of cam lobe to lifter is not at least .080ths., the litfer can, and does, hit the case. To correct this I cut the lifter bore with my die grinder making a raduis at the top allowing the lifter to sit lower. If This clerance is to small it can sound just like a rod rap, oh yes it can... 

 enhances the top 1/3 of the camshaft lobe  where the valve is open the widest, effectively taking a larger breath with each opening of the valve. Later 1.1to1 rockers can be identified by the double fordge marks just above the rocker shaft. 

on its edge, also increasing valve lift. I would also go so far as to grind the rocker arm surface that faced the valve. This way I could back the adjuster off even more, allowing me to use even longer pushrods. All this was a lot of work for numerous small gains that could make the difference in tight race class. I wouldn't even attempt to build an SCCA motor as such today, but as a point of reference I've seen them sell for $1500-3500, ready to race.

Most vee racers know who builds hot engines and what it really cost to build one up.

The cylinder heads are a topic that need an article by themself  Gene Bergs article on heads is well worth checking out.

This article is dedicated to the menory Bill Murdock who lost his fight with cancer this past summer.He spent many hours translating my thoughts to the finished product you have seen my name on. 

~Boston Bob

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