Hi guys. I have a crazy question for y’all. I have a 1970 Rupp-spec Tecumseh HS40 with a worn-out bore, and since oversized pistons are difficult to find at best, I was wondering if one could substitute a standard-sized Tecumseh HS50 piston, since they’re a dime-a-dozen. Is there enough “meat” in an early HS40 block to accommodate such a large piston? Has anyone ever tried this before?
HS40 Piston Overbore question
- Thread starter ubernoob51
- Start date
Someone wrote in here in the past that they got into one of the threaded holes for either the the air deflector shroud on the side or the front for the shroud/lower tank mount trying that, cant remember which. You woud also only be able to use a late 1969-up wide bolt pattern HS40 block. Keep in mind that Tecumseh added weight/material to the crankshaft on the HS50 to compensate for the larger piston as well to keep it as balanced as they could. The HS50 Head gasket will need to be used If you have a go at it and maybe use a threaded aluminum insert machined in if you poke through the bore. Maybe use the late model piston since it is smaller by like .017" over the early versions
On late model HS50 and HSSK50 engines with the slightly smaller bore and thin rings and the smaller exhaust valve, i have bored those out to use an old style HS50 thick ring piston. It works out to be pretty much exactly .020 over. And like you said, standard size late model HS50/HSSK50 piston and rings are pretty easy to get.
i would try and bore to the thin ring HS50/HSSK50 standard piston. what the heck. if the bore is already wasted, it's not like you have a lot to loose. you can get a new crank on ebay.
HS50 = 2.812" bore
HS50 (newer HS50/HSSK50/LH195 starting 1990s) = 2.795"
HS40 = 2.625" (note the H50 also used this size, but had a longer stroke.)
i would try and bore to the thin ring HS50/HSSK50 standard piston. what the heck. if the bore is already wasted, it's not like you have a lot to loose. you can get a new crank on ebay.
HS50 = 2.812" bore
HS50 (newer HS50/HSSK50/LH195 starting 1990s) = 2.795"
HS40 = 2.625" (note the H50 also used this size, but had a longer stroke.)
don't forget too guys that Tecumseh change the hs-40 piston also in there later years with a thinner style ring. this was something that I use on my hs-40 build. so you do also have the option there too. .010 or .020 and had no problems balancing the engine. for some crazy reason you wanted to keep the factory bore you could go the expensive way. have it sleeve and then it will last forever. sleeving a engine not a bad thing. there are some benefits with it.
.020 piston and rings
stock early style hs-40 piston with thick rings.
hs-40 build
https://www.oldminibikes.com/forum/index.php?threads/hs-40-build.161335/
.020 piston and rings
stock early style hs-40 piston with thick rings.
hs-40 build
https://www.oldminibikes.com/forum/index.php?threads/hs-40-build.161335/