Needing help with .20 piston.
- Thread starter Hammbone1971
- Start date
- Tags needing piston.
Rupp Blackwidow minibike hs40 piston & ring kit
Thats the only .020 over piston I can find for a 4hp but its HS40.
I don't think you are boring it .2 over lol
Thats the only .020 over piston I can find for a 4hp but its HS40.
I don't think you are boring it .2 over lol
the block should have been bored and finished honed to the piston to get an accurate piston to wall clearance...
What about going with an h50 piston and have the block bored to 2 5/8"? The h40 and H50 share the same crank and rod so an H50 piston will work if there is enough meat to bore it that big...
What about going with an h50 piston and have the block bored to 2 5/8"? The h40 and H50 share the same crank and rod so an H50 piston will work if there is enough meat to bore it that big...
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My old piston is 2.620.i had it bored .020 and it came out to 2.645? Whats going on here?Will I be able to find a piston to fit this old tecumseh.I was told it was a h40 and it does have the bolt on plate for mounting and intake is on right side.I need some help.
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Randi, apparently he has it bored larger than the bore spec on the H50.
Bored to 2.645, but H50 Max is 2.626.
However max ring end gap is .020, so maybe and H50 std piston with ten over rings would work?
Right at the end of spec, or .0001 out of spec. if all measurements stated are accurate.
Edit: The only reason I know any of this is due to Randi's earlier help with my engine issues.
Bored to 2.645, but H50 Max is 2.626.
However max ring end gap is .020, so maybe and H50 std piston with ten over rings would work?
Right at the end of spec, or .0001 out of spec. if all measurements stated are accurate.
Edit: The only reason I know any of this is due to Randi's earlier help with my engine issues.
Dave your correct on the block being being than a stock H50, if the bore is 2.645 then it's .020 (20 thousandths) larger than a stock H50 so only a .020 piston will fill that hole....If the block was an H40 it was bored .164 since a stock h40 is 2.500"... I'm guessing it was actually an H50.... if the machinist didn't allow for fitting the piston to the bore the next step would be an H70 piston (2 3/4") 2.750 which would require the block being bored another .105....I don't know if it can be bored that far without a new sleeve.. here are some links for a .020 piston http://www.partstree.com/parts/?pn=33314b
http://search.cartserver.com/search...umseh_Parts&maxhits=20&keywords=33314b&go=GO!
http://search.cartserver.com/search...umseh_Parts&maxhits=20&keywords=33314b&go=GO!
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sure it's possible to wear that much.... the block wasn't an H40 though..it may have had a cover off an H40....whenever I have anything bored I have the piston there so the block can be bored and honed to fit since pistons can vary in size a few thousandths either way... Hopefully whatever piston you buy he left enough room to hone it to spec so that you have the right piston to wall clearance..
Boring your cylinder...
I concour. Hard for me to believe somone bored a cyl without the piston in hand that's a no no... When you ask to have a bore job done a good machinist sould give you a blank stare without having a piston to go by so he can give you the proper skirt clearance in the bore job the H40 which is .0055 minium clearance to .0070 maximum skirt clearance. His goal is to get the dia. of that piston and subrtact a little less than .0055 say a couple thousands or so less with his bore and finish with the extra couple thousands bigger with a cross hatch hone to that minium skirt clearance. What your dealing with is 5.5 thousands minium clearance and 7 thousands maxium clearance thats not much lead way that's only 1.5 thousands before it becomes critical. My H35 has 4.5 to 6 thousands skirt clearance so it's not much different 1 1/2 thousands in either direction...
I'm going to use a H35 as a example when my bore job happened. I used a telescoping inside bore measuring device I measured in 3 levels of the bore top middle and bottom and wrote all of that down and rechecked everything again at all three levels just to see if my measurements were the same then I rotated my measurements 180 degrees in the bore and did the 3 levels of measurements and rechecked those measurements again. When I say 180 degrees I'm measuring from say the wrist pin side to side and rotating 180 degrees to study the way piston is reacting to the bore rotating on the wrist pin. This will show the pattern of a wedge shaped wear pattern down the bore or a egg shaped wear pattern or a combination of the both. The highest measurent plus minimum skirt clearance added together shoud reveal how big the oversize piston you should go to. However if there is gouges and rough looking side walls in the cyl then it may take a few extra thousands to clear that up...
Contray to belief just trying to bore with a spring loaded hone does not get it simply because if the bore is egg shapped or wedge shaped it will not make a even cut because the hone just bounces around because it's spring loaded. Spring loaded hones have one purpose and that's for cross hatch finishes only...
This is why there is boring bars with a bit that cuts metal or using a ammco 500 series hone that has a set screw to lock in the stone bars and does not flex thus begins to cut a perfect circle and clearing up gouges and uneven wear patterns.
In my H35 the stock piston dia is 2.495 if you add the say 5 thousands skirt clearance then it becomes 2.500 bore. so that goes to show the piston is made roughly 5 thousands smaller so when the factory bores to the 2 1/2 bore then it fit's perfectly (roughly plus or minus 1 th.) so they are in the ball park.
When the motors wears down some then that's a horse of a different color as the bore becomes out of shape so to speak.
Long story short I never bore without piston in hand a few thousands to much skirt clearance and it spells disaster with excess piston slap and a waste of time. And just a small warning folks that run too tight of btc ignition timing specs and kick the piston to hard because it fired to early will give that bore and piston skirt to much wear because of the kick back violently in one direction and it kicks again to begin to go down from tdc and the piston will naturally kick again when it begins to go back up from the bottom of the stroke. This means it's kicking or rocking the piston 3 times when it should only kick twice in each stroke thus premature wear and tear on the piston and bore.:thumbsup:
I concour. Hard for me to believe somone bored a cyl without the piston in hand that's a no no... When you ask to have a bore job done a good machinist sould give you a blank stare without having a piston to go by so he can give you the proper skirt clearance in the bore job the H40 which is .0055 minium clearance to .0070 maximum skirt clearance. His goal is to get the dia. of that piston and subrtact a little less than .0055 say a couple thousands or so less with his bore and finish with the extra couple thousands bigger with a cross hatch hone to that minium skirt clearance. What your dealing with is 5.5 thousands minium clearance and 7 thousands maxium clearance thats not much lead way that's only 1.5 thousands before it becomes critical. My H35 has 4.5 to 6 thousands skirt clearance so it's not much different 1 1/2 thousands in either direction...
I'm going to use a H35 as a example when my bore job happened. I used a telescoping inside bore measuring device I measured in 3 levels of the bore top middle and bottom and wrote all of that down and rechecked everything again at all three levels just to see if my measurements were the same then I rotated my measurements 180 degrees in the bore and did the 3 levels of measurements and rechecked those measurements again. When I say 180 degrees I'm measuring from say the wrist pin side to side and rotating 180 degrees to study the way piston is reacting to the bore rotating on the wrist pin. This will show the pattern of a wedge shaped wear pattern down the bore or a egg shaped wear pattern or a combination of the both. The highest measurent plus minimum skirt clearance added together shoud reveal how big the oversize piston you should go to. However if there is gouges and rough looking side walls in the cyl then it may take a few extra thousands to clear that up...
Contray to belief just trying to bore with a spring loaded hone does not get it simply because if the bore is egg shapped or wedge shaped it will not make a even cut because the hone just bounces around because it's spring loaded. Spring loaded hones have one purpose and that's for cross hatch finishes only...
This is why there is boring bars with a bit that cuts metal or using a ammco 500 series hone that has a set screw to lock in the stone bars and does not flex thus begins to cut a perfect circle and clearing up gouges and uneven wear patterns.
In my H35 the stock piston dia is 2.495 if you add the say 5 thousands skirt clearance then it becomes 2.500 bore. so that goes to show the piston is made roughly 5 thousands smaller so when the factory bores to the 2 1/2 bore then it fit's perfectly (roughly plus or minus 1 th.) so they are in the ball park.
When the motors wears down some then that's a horse of a different color as the bore becomes out of shape so to speak.
Long story short I never bore without piston in hand a few thousands to much skirt clearance and it spells disaster with excess piston slap and a waste of time. And just a small warning folks that run too tight of btc ignition timing specs and kick the piston to hard because it fired to early will give that bore and piston skirt to much wear because of the kick back violently in one direction and it kicks again to begin to go down from tdc and the piston will naturally kick again when it begins to go back up from the bottom of the stroke. This means it's kicking or rocking the piston 3 times when it should only kick twice in each stroke thus premature wear and tear on the piston and bore.:thumbsup:
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