Someone PM'ed me recently asking about differences between the HS series/4hp and 5hp engines crankshafts so they could figure out what they had. Since I had a bunch of shafts out the other day I snapped some pics and felt this might be helpful to others.
First off the HS40's and 50's DO NOT share the same crankshaft! There is one tiny difference between them, interchanging would problaby turn your engine into a paint shaker :laugh:
Start with Tecumseh literature, Here are the specs of the HS40 and HS50 crankshafts. These are both 1980-older spec sheets so they are the small taper cranks that would be the ones you would find in the 70's era engines.
The difference is in the counterbalance/flyweights and there is additional material on the HS50 to compensate for the larger piston they use (I underlined those measurements in the photos above). All other measurements are actually the same between them. They Look nearly identical next to each other so you want to be careful that you don't mistake them. This held true post 1980 as well, they just bumped up the the taper and thread/nut size for the flywheels at that point.
you can see in this photo of some used and NOS ones I have that they can be easily mistaken for each other:
(Top-standard bushing sidecover crankshafts, bottom-Ball bearing sidecover shafts, all "small taper" pre 1981 versions)
A few things that can help if your just looking at a loose shaft that you forgot to mark as what it fits-not like I have never done that :anon.sml:
Early small taper HS40 shafts will have a pretty severe cut out on the output side of the rod journal side as compared to the the HS50. I have the ruler pointing to it in this photo:
This was done becuase in the first few years of HS40 production (1968-1970) they had a mechanical compression release camshaft. It had a huge intake lobe on it and they compensated with a shorter lifter so they had to clearance the crankshaft for it to fit.
Standard "BCR" (bump compression release) on the left-crosses between HS40 and 50 engines BTW . Early "MCR" (mechanical compression release) on the right-HS40 only till 1970
From what I have encountered, all replacement shafts for the HS40 (small taper) have them cut in regardless of production date. That of course would be so there is not an installation problem and not have to make 2 different shafts. They didn't have to put that deep of a cut in the HS50 shaft because the HS50 was not produced till 2 years after the end of the line for the MCR cam as HS50's didn't hit the streets till sometime in 1972.
And of course the best "tell" of all is to actually measure. A quick and dirty way I do it is simply doing this on the input side counter balance :thumbsup:
Have fun playing with your shafts....I know I do :laugh:
First off the HS40's and 50's DO NOT share the same crankshaft! There is one tiny difference between them, interchanging would problaby turn your engine into a paint shaker :laugh:
Start with Tecumseh literature, Here are the specs of the HS40 and HS50 crankshafts. These are both 1980-older spec sheets so they are the small taper cranks that would be the ones you would find in the 70's era engines.
The difference is in the counterbalance/flyweights and there is additional material on the HS50 to compensate for the larger piston they use (I underlined those measurements in the photos above). All other measurements are actually the same between them. They Look nearly identical next to each other so you want to be careful that you don't mistake them. This held true post 1980 as well, they just bumped up the the taper and thread/nut size for the flywheels at that point.
you can see in this photo of some used and NOS ones I have that they can be easily mistaken for each other:
(Top-standard bushing sidecover crankshafts, bottom-Ball bearing sidecover shafts, all "small taper" pre 1981 versions)
A few things that can help if your just looking at a loose shaft that you forgot to mark as what it fits-not like I have never done that :anon.sml:
Early small taper HS40 shafts will have a pretty severe cut out on the output side of the rod journal side as compared to the the HS50. I have the ruler pointing to it in this photo:
This was done becuase in the first few years of HS40 production (1968-1970) they had a mechanical compression release camshaft. It had a huge intake lobe on it and they compensated with a shorter lifter so they had to clearance the crankshaft for it to fit.
Standard "BCR" (bump compression release) on the left-crosses between HS40 and 50 engines BTW . Early "MCR" (mechanical compression release) on the right-HS40 only till 1970
From what I have encountered, all replacement shafts for the HS40 (small taper) have them cut in regardless of production date. That of course would be so there is not an installation problem and not have to make 2 different shafts. They didn't have to put that deep of a cut in the HS50 shaft because the HS50 was not produced till 2 years after the end of the line for the MCR cam as HS50's didn't hit the streets till sometime in 1972.
And of course the best "tell" of all is to actually measure. A quick and dirty way I do it is simply doing this on the input side counter balance :thumbsup:
Have fun playing with your shafts....I know I do :laugh:
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