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Author Topic: Piston dome  (Read 14431 times)

Offline Stpltn250r

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Piston dome
« on: June 26, 2013, 06:49:45 pm »
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.

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Offline MotorGeek - Jerry Hall

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Re: Piston dome
« Reply #1 on: June 27, 2013, 07:47:32 am »
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.


Offline Stpltn250r

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Re: Piston dome
« Reply #2 on: June 27, 2013, 10:12:30 am »
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.

Offline MotorGeek - Jerry Hall

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Re: Piston dome
« Reply #3 on: June 27, 2013, 05:22:47 pm »
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.

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

Offline Stpltn250r

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Re: Piston dome
« Reply #4 on: June 27, 2013, 08:30:18 pm »
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.

Offline WestTexasKing

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Re: Piston dome
« Reply #5 on: June 27, 2013, 09:06:22 pm »
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!

Offline MotorGeek - Jerry Hall

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Re: Piston dome
« Reply #6 on: June 27, 2013, 09:54:38 pm »
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.

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.

Offline Stpltn250r

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Re: Piston dome
« Reply #7 on: September 13, 2013, 07:02:00 am »
got a buddy with a CMM said he will measure it for me!! 

Offline Gaz

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Re: Piston dome
« Reply #8 on: July 27, 2023, 07:04:14 am »
Lol 10 years later...Did you get a measurement of Piston dome volume using the CCM?

 

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