tecumseh 2-13/16 bore piston

delray

Well-Known Member
#2
I don't have a new piston in front of me,but I believe it would be in the pall park of couple thousands smaller then the factory bore size and that should be about 2.8125 so just add .020 to that and you will have your oversize piston dem's and yes only the big bore hs-50's have the thick rings.
 
#4
delray. factory bore 2.8125 plus 0.020 = 2.8325. i measured the bore its perfect. i bought to piston on line. tecumseh new old stock. the part number was correct so i took it and the block to a shop. nice place two day turnaround. i never measured the piston skirt to wall clearance. i ran the motor about an hour to seat the rings.but their was still a puff of smoke when you first hit the throttle. so i took it apart again to address the valve guides which should have ben done in the first place. now looking at the piston at tdc i see a large space between the piston and the cylinder wall. i can actualy see part of the ring. so i grabbed the feeler gauge and fit an 0.018 blade in and touched the top compression ring. i know you measure skirt to wall clearance at the bottom witch I'm gonna do when i get the long feeler gauge blades i ordered. should there be that much space at the top off the bore? I'm thinking its the wrong piston. i can't fined the piston diameter in the factory spec sheets. i guess i will have to wait till the long feeler blades come and check it against the factory clearance spec witch according to the book 0.002 to 0.004. thanks for the reply.
 

delray

Well-Known Member
#5
delray. factory bore 2.8125 plus 0.020 = 2.8325. i measured the bore its perfect. i bought to piston on line. tecumseh new old stock. the part number was correct so i took it and the block to a shop. nice place two day turnaround. i never measured the piston skirt to wall clearance. i ran the motor about an hour to seat the rings.but their was still a puff of smoke when you first hit the throttle. so i took it apart again to address the valve guides which should have ben done in the first place. now looking at the piston at tdc i see a large space between the piston and the cylinder wall. i can actualy see part of the ring. so i grabbed the feeler gauge and fit an 0.018 blade in and touched the top compression ring. i know you measure skirt to wall clearance at the bottom witch I'm gonna do when i get the long feeler gauge blades i ordered. should there be that much space at the top off the bore? I'm thinking its the wrong piston. i can't fined the piston diameter in the factory spec sheets. i guess i will have to wait till the long feeler blades come and check it against the factory clearance spec witch according to the book 0.002 to 0.004. thanks for the reply.
yes, the piston tapers smaller at the top.
 

delray

Well-Known Member
#6
also if you got a problem with your valve guides you can still find valves with over size valve stems. that might fix your problem if your stems are not in to bad of condition.
 
#7
delray. i started this project way back in 2010. bike sat a long time. put it together 2015. moved again it sat 4 more years. only has a hour on it. saw the puff of smoke so i ripped it apart again. the valve guides are shot. i found both intake and exhaust valves 1/32 over a couple of days ago. they will be hear shortly. i also found the factory piloted hand ream. this is a lighted hs50 1972 according to the tag number. i just spent $505.00 on eBay for a lighted 1970 rupp hs40 for the aluminum flywheel and small rounded blower cover. i like the h35 look. now i found a new old stock hs50 block for $87.00 plus shipping. i will fix the old block at some point but I'm gonna start with a brand new block new valves new piston and transfer everything from old motor over. basically i will have a new lighted hs50 with the small aluminum flywheel and blower cover. I'm new on this forum there are some great people that know their stuff. this guy markus guided me to the right aluminum flywheel and blower cover. ben trying to figure that one out for years. i will keep posting as this thing goes together. I'm sure i will need more guidance along the way. and thanks for the reply. anthony.
 

delray

Well-Known Member
#8
markus is the guy when it comes to numbers for sure. if you can't figure out correct part number he will for you.
when you get it back together give it some time to break in and run and don't be afraid to change that oil after only running for 10 mins or so
won't believe how much metal will be floating around in that engine. i have seen too where the carb is not spot on and it looks like oil smoke puffing out(gray not black) fun little motors to play with when you get the bugs work out of them.
 
#9
I did the same thing you are doing and was extremely happy with the results. NOS Hs50 shortblock and put Hs40 lighted flywheel and coil, H35 shroud. Ran like a top.

Edit: +1 on Markus, I owe him one for helping me with part numbers for sure!
 
#10
markus is the guy when it comes to numbers for sure. if you can't figure out correct part number he will for you.
when you get it back together give it some time to break in and run and don't be afraid to change that oil after only running for 10 mins or so
won't believe how much metal will be floating around in that engine. i have seen too where the carb is not spot on and it looks like oil smoke puffing out(gray not black) fun little motors to play with when you get the bugs work out of them.
about the oil. when i pulled it apart there was oil towards the back of the piston in the middle of both valves. the piston had no carbon buildup on it. then i yanked the valves. underneath the exhaust valve was a mess. a bunch of oil. the intake had nothing at all. there was a little play but nothing like the exhaust. i could actually move it a 16th back and fourth.
 
#12
I did the same thing you are doing and was extremely happy with the results. NOS Hs50 shortblock and put Hs40 lighted flywheel and coil, H35 shroud. Ran like a top.

Edit: +1 on Markus, I owe him one for helping me with part numbers for sure!
this one should run as good as my h70 back in 1976 when i was 16. i ported it had a cam in it shaved the ibrows they called it fire sloting back then. my frend tommy had the same motor and mods. the book we got the modifications from said it would make 15 hp if the porting came out right. if they weren't 15 hp they were close. anyway i called dynacams up. there going to weld and regrined my cam to i believe he said 245. he said they have two bigger grinds but i don't want to go much past 5000 rpm. I'm also getting an arc billet rod. i don't want to snap a rod after this kind of investment.so with a little port clean up it should run strong but not insanely strong. i will be posting as parts arive. anthony.
 
#13
245 cam is a great upgrade
Don’t forget springs
With Hs40 flywheel and 245 cam and proper valve springs, you will almost definitely exceed 5000 rpm
But with billet rod that’s no problem
 

delray

Well-Known Member
#14
the 245 is a very strong cam and set up correct with the right mods it will pull power way past 5000 rpm's I think you will like it once you get the mod's done too.
here is my hs-40 build with that cam. some of my mod's were little extreme. but man it really moves out for a little guy.
https://www.oldminibikes.com/forum/index.php?threads/hs-40-build.161335/
if you are looking to just pull 5000 rpm's with your motor. there is some tricks you can do make it work very good with a stock cam.
 
#16
the 245 is a very strong cam and set up correct with the right mods it will pull power way past 5000 rpm's I think you will like it once you get the mod's done too.
here is my hs-40 build with that cam. some of my mod's were little extreme. but man it really moves out for a little guy.
https://www.oldminibikes.com/forum/index.php?threads/hs-40-build.161335/
if you are looking to just pull 5000 rpm's with your motor. there is some tricks you can do make it work very good with a stock cam.
yes you are right. I'm getting 18 lb springs.
 
#18
what was the new hs-50 block? large bore? big valves? points, electronic ignition? any good pictures....
the new block is an exact big bore big valve point ignition hs50. it is on a parts list with my tag number. hs50 67045 i found it today accidentally. there closed on sunday. I'm gonna call as soon as the place opens in the morning. don't want it to slip thru my hands. its listed as in stock. if its not i will be crushed.
 
#19
delray. jesus dude i just looked at the hs 40 build. you took this to the next level. no no the level after that. i wonderd if there was ever a tecumseh with a real roller bearing on the mag side. did i see write? a caged needle bearing the mag side. thats frigin awesome. i see you have access to real machines. had two bridgeports and to lathes at my old job. owner died shop closed down. the new style piston is certainly the way to go. i was thinking the same thing. but the new style piston is for the small bore hs50. if there is something I'm missing please tell me. i also wanted to put nice brass or bronze valve guides in my block and briggs valves but no access to the machinery. you can get overwhelmed with this stuff real easy. seeing all this great stuff is getting me crazy. i have to go back to your post and reread it. great stuff. gettin cross eyed typing. i be up four hours trying to absorb all this. i keep you posted about the block. anthony.
 

delray

Well-Known Member
#20
yes it has a needle roller cage on the mag side.
if this makes you even more crazier, I got couple Tecumseh blocks that where made in Italy with full roller bearings on both sides of the crank.
that will be for a future build for sure.

I did find some pistons out of a different flathead motor that come with a thinner ring and lighter and stronger piston too. but then again it would take a ton of different little mod's to make them work. I will be posting more on this in the future with my OHH BUILD.
starts on page 6. I kind of like keeping all my ohh builds in the same thread. makes it little easyer to find.
https://www.oldminibikes.com/forum/index.php?threads/tecumseh-ohh-build.131688/page-6
 

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