HS40 Rupp

#23
dalpan, if you ever want to get into hot rodding a hs-40 here is my hot rod hs-40 . both me and dave have built a hot rod hs-40 engines and has been a lot of fun playing around with these little guys. there no ohv engine when it comes to that kind of power,but still fun to playing around with.....
here is my crazy build.
https://www.oldminibikes.com/forum/index.php?threads/hs-40-build.161335/
Sounds like. Symco, never heard of it, looked it up on the internet, looks like a good time. I'm in....
 

delray

Well-Known Member
#25
cool, I should be getting a call from him this week on my stroker crank.
so have you figure out what you want to do to the bore yet. I am assuming if the crank was that bad the bore is not good either.
 
#26
I have an engine off a 71 snowblower , 4HP, that is excellent shape, still can see the hatch marks in the cylinder. My original block has a nice hole in it. Putting my original side cover w/bearing, crankshaft, camshaft and flywheel in the 71 HS40 block. I think I'll go with the billet connecting rod w/the piston from the 71 engine. My ring gap measures .017. Will verify the bore with ID mic tonight.
 

delray

Well-Known Member
#28
I have an engine off a 71 snowblower , 4HP, that is excellent shape, still can see the hatch marks in the cylinder. My original block has a nice hole in it. Putting my original side cover w/bearing, crankshaft, camshaft and flywheel in the 71 HS40 block. I think I'll go with the billet connecting rod w/the piston from the 71 engine. My ring gap measures .017. Will verify the bore with ID mic tonight.
was that with new rings for your end gap? if so that will still work,but that is little on the high side. finish hone really should be a 400 grit. seen them still work with less...320. kind of a forgiving engine when it comes to rebuilding them.

dalpan there be some minibike racing on grass this weekend in Gleason, WI. thats in your back door area too.
https://www.northernroundup.com/
 
#29
The ring gap is with the original rings from the 71 HS40, cylinder has not been honed. Maybe a new set of rings?
Thanks for the heads up on the Gleason weekend.
 

cfh

Well-Known Member
#31
I find if you have a ring gap between 010 and 020 it works out fine. For a used cylinder, not re bore, 010 is ideal. Tecumseh recommends 006 but that would be with a fresh bore. Ive pulled used hs50/hs40 apart with 030 ring gap and they were actually running fine.
 
#32
The ring gap is with the original rings from the 71 HS40, cylinder has not been honed. Maybe a new set of rings?
Yes, I agree with Delray, always replace the rings on these.

The correct ring end gap for YOUR ENGINE is .007 to .017. Not .010 and .020, as that is for the newer solid state engines that use the thinner rings.

It can be argued that your wider gap is fine, as it is on the very end of the allowable spectrum. I don't know any craftsman who would not install fresh rings during a rebuild of a small gas engine. ;)
 
Last edited:

delray

Well-Known Member
#36
Checked the bore 2.627, haven't honed yet, planning on 320 grit. @delray got a call , my crankshaft is done will pickup tomorrow. .
Surprised that quick... View attachment 246203
2.627 little on the high side , but if it hones out good....quickly(under 10 seconds)and the new rings fit good. it will run just fine. if you do go with a billet rod? you might as well set the governor at 4700 rpm's. it will pull strong up to that with a rupp header on it too. maybe clean up the casting little in the bowl area too and have fun with it.
i still haven't got my crank back yet. i hope your not getting my crank or your getting stroker motor....LOL
 

delray

Well-Known Member
#38
cool dalpan, I think that work out good for bean doing both cranks at the same time. little bit of setup you need to do first before grinding the crank. first you have to mount the stone on this big/lathe and then true up the stone and then cut the radius on the edge of the stone, same as the crank journal radius....etc....
sounds like he got yours right in too because everything was setup to go. good deal...….


I should see mine tomorrow...getting ship to me.
 
#39
2.627 little on the high side , but if it hones out good....quickly(under 10 seconds)and the new rings fit good. it will run just fine. if you do go with a billet rod? you might as well set the governor at 4700 rpm's. it will pull strong up to that with a rupp header on it too. maybe clean up the casting little in the bowl area too and have fun with it.
i still haven't got my crank back yet. i hope your not getting my crank or your getting stroker motor....LOL
@delray , "Clean castings the bowl area"? do you mean the intake/exhaust ports? Sorry, I'm still learning all the "hotrod terms :).
 
#40
Yes. Not Delray. LOL. You can open them up quite a bit. Just be wary of punching though the back side. I use a dremel.

Bearing in mind that I am still not Delray, here is an instruction sheet I did up a few years ago for maxing out your HS governor setting. :D

Loosen up the 1/4" hex screw that holds the 2 pieces of the governor arm together.
Rotate the governor rod coming out of the block clockwise with some pliers or a small crescent wrench. DON'T force it.
Keeping the rod rotated, push the top of the governor arm to the right to cause the throttle to open to “wide open throttle. “
Holding all of that, re-tighten the ¼” hex screw.
Basically the governor rod coming out of the engine is all the way to the right when the throttle is all the way to the right.
Naturally this needs to be done with all links/springs in place and if you change anything you need to re-adjust as described.
 
Top