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Return to: LT500R - Engine
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#Post#: 21160--------------------------------------------------
Piston dome
By: stpltn250r Date: June 26, 2013, 8:49 pm
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Someone please tell me if I did this correctly.
I dont have any jigs nor do I work in or around a machine shop.
I went to measure the dome rise on a LT500 piston. There is what
seems to be a little flat spot smack dab in the center of the
piston. So what I did was laid a flat bar across the piston. I
measured from the underside of the bar to the bottom of the
piston or surface piston was sitting on. I than measured from
the timing edge of the piston to the bottom of the skirt. Than I
subtracted the two and assumed that to be the dome rise. I came
up with 6.4mm. Think it was little less than that like 6.397mm.
Does that sound correct for a 89.5mm piston? Do the dome rises
change with bore size? I wouldnt think they would.
#Post#: 21180--------------------------------------------------
Re: Piston dome
By: Motorgeek Date: June 27, 2013, 9:47 am
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What are you trying to do? Are you trying to calculate the dome
volume using procedures like most four stroke builders use. If
so there will be some volumes not accounted for and erroneous
compression ratios will be calculated The domes on most two
stroke are not a constant radius.
You need a $30,000 CMM to accurately measure the top of the
piston to determine its actual dome radius or how many different
radi it has.
Save your self a lot of time and calculation and just measure
the actual clearance volume a running engine "sees".
1. Place the piston about and one inch from TDC
2. .Put two little daps of grease on the bore where the end
gaps of the rings are located.
3. Rotate the crank so that the piston moves to TDC. This
should seal and keep the end gaps from leaking liquid.
4. Wipe off the excess grease
5. Install the head and head gasket .
6. Fill the volume on top of the piston with ATF or mineral
spirits from a Burett (graduated cylinder) to the top of the
spark plug hole.
7. Subtract 2.1cc from the volume number you measured in step 6
if you are using a 3/4" reach spark plug. I use 2.1cc because
it is a good average number for the volume that a B8ES spark
plug occupies in the spark plug thread. Your should now have
the actual volume needed to make a good compression ratio
calculation.
8. Now that we have an accurate compression ratio number what
do we do with it??????. What does it mean????
9. This procedure should take less than 5 minutes.
#Post#: 21194--------------------------------------------------
Re: Piston dome
By: stpltn250r Date: June 27, 2013, 12:12 pm
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Well i have the bimotion head program. I know jts just the guy
who made its opinion on what is what. I didnt think about the
inconsistent radius of the whole dome of the piston. It just
asks in the program a piston height. By using the bore of the of
the piston and the height of the dome in a spherical cap volume
calculator It seems to be around 18cc. Adding that to the dome I
was playing around with it adds up to be the dome cc uninstalled
would be a 57cc dome. And 14.2 CR.
#Post#: 21213--------------------------------------------------
Re: Piston dome
By: Motorgeek Date: June 27, 2013, 7:22 pm
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[quote author=Stpltn250r link=topic=2058.msg21194#msg21194
date=1372353150]
Well i have the bimotion head program. I know jts just the guy
who made its opinion on what is what. I didnt think about the
inconsistent radius of the whole dome of the piston. It just
asks in the program a piston height. By using the bore of the of
the piston and the height of the dome in a spherical cap volume
calculator It seems to be around 18cc. Adding that to the dome I
was playing around with it adds up to be the dome cc uninstalled
would be a 57cc dome. And 14.2 CR.
[/quote]
Most of the compression ratio calculators and formulas for head
geometry do not account for the volume that is above the top
ring between the piston and cylinder wall.
I could use Calculus and calculate the exact volumes, but it
takes much longer to accurately measure all of the dimensions
that is needed to put into the equations than it does to use the
procedure I gave in a former post
#Post#: 21219--------------------------------------------------
Re: Piston dome
By: stpltn250r Date: June 27, 2013, 10:30 pm
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well thanks for the response but dont think the question was
answered. "is that the proper way to find the height of a dome".
i also cant see how the area above the ring can play that much
in the displacement of the head. but what do i know.
#Post#: 21221--------------------------------------------------
Re: Piston dome
By: WestTexasKing Date: June 27, 2013, 11:06 pm
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So you basically measured from the edge of the piston (lowest
portion of dome) to the center of the piston (highest portion of
dome), height-wise.
You used two measurements, diametrically opposed, and averaged
the two accounting for an imperfect level on the bar.
If all you're looking for is the piston dome height from the
edge of the piston, it sounds reasonable, probably reasonable
enough to use in your program.
BTW, that tiny gap above the ring can easily be as much as the
small recess for the spark plug, which is a factor in
compression ratios.
The pistons I just replaced in my agcat had .4" between the
piston edge and first compression ring, then factor in the 1/8"
gap between the piston and cylinder...that's a lot of extra
volume!
#Post#: 21228--------------------------------------------------
Re: Piston dome
By: Motorgeek Date: June 27, 2013, 11:54 pm
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[quote author=Stpltn250r link=topic=2058.msg21219#msg21219
date=1372390218]
well thanks for the response but dont think the question was
answered. "is that the proper way to find the height of a dome".
i also cant see how the area above the ring can play that much
in the displacement of the head. but what do i know.
[/quote]
The diameter of a Wiseco piston above the ring is about 0.50 mm
smaller than the bore. The top ring is about 3mm from the
timing edge of the piston. I would "guestimate" the volume
above the ring between the piston and cylinder wall to be at
least 0.2cc.
#Post#: 26335--------------------------------------------------
Re: Piston dome
By: stpltn250r Date: September 13, 2013, 9:02 am
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got a buddy with a CMM said he will measure it for me!!
#Post#: 54691--------------------------------------------------
Re: Piston dome
By: Gaz Date: July 27, 2023, 9:04 am
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Lol 10 years later...Did you get a measurement of Piston dome
volume using the CCM?
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