Correct engine displacement when using overbore pistons?
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Road Master
Joined: Aug 2013
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From: Wyoming
Correct engine displacement when using overbore pistons?
I am rebuilding my 95" engine to a 106" using the S&S stroker kit. For those who don't know, the S&S 106" stroker kit replaces the 4" stroke oem flywheel assembly with a 4 1/2" stroke flywheel assembly, and uses 3.875 bore pistons, which is the same piston diameter as the 95" big bore I already have.
So, this afternoon I was musing about the following: to get the best fit with the new pistons that are part of the 106" kit, I had my cylinders re-machined and used +.05" overbore pistons. Now, as we all know, the formula for computing engine displacement on a two cylinder engine is Bore x Bore x Stroke X 1.5708 = Displacement. Thus, without the overbore pistons, the equation is 3.875 x 3.875 x 4.5 x 1.5708 = 106.13944 cu. in. (aka 106). However, with the +.05" overbore pistons, the equation is 3.925 x 3.925 x 4.5 x 1.5708 = 108.8962, which is, after rounding, 109".
I'm guessing that using overbore pistons is fairly common, especially on bikes that have had previous work. However, you never hear such builds being referred to by the larger displacement figure... or do you...?
There's no real point to this post, just interested in thoughts or comments any of you might have.
So, this afternoon I was musing about the following: to get the best fit with the new pistons that are part of the 106" kit, I had my cylinders re-machined and used +.05" overbore pistons. Now, as we all know, the formula for computing engine displacement on a two cylinder engine is Bore x Bore x Stroke X 1.5708 = Displacement. Thus, without the overbore pistons, the equation is 3.875 x 3.875 x 4.5 x 1.5708 = 106.13944 cu. in. (aka 106). However, with the +.05" overbore pistons, the equation is 3.925 x 3.925 x 4.5 x 1.5708 = 108.8962, which is, after rounding, 109".
I'm guessing that using overbore pistons is fairly common, especially on bikes that have had previous work. However, you never hear such builds being referred to by the larger displacement figure... or do you...?
There's no real point to this post, just interested in thoughts or comments any of you might have.
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Road Master
Joined: Aug 2013
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Likes: 8
From: Wyoming
Taking an 88" to a 98" is pretty common and that requires a 3.938" bore. So, yes, .05" overbore on a nominal 95" build is fine.
Road Master
Joined: Aug 2012
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From: CT
Do they even make pistons in that 3.925 bore for the 4-1/2 stroke? You very rarely hear of people running that oddball long stroke 106 kit which is probably why you don't hear about people making them a 108 or 109. Why not just go 4-3/8 107"?
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Road Master
Joined: Aug 2013
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From: Wyoming
Yes, S&S sells the 4.5" stroke flywheel assembly and the matching 3.875" pistons for a 106" stroker build. The S&S pistons are available in 3.875" as well as .05", .10" and .20" overbore.
http://www.sscycle.com/product/1999-...it-p24019.html
http://www.sscycle.com/product/1999-...8B-p24042.html
The reason I didn't go 107" (i.e. 4 3/8" stroke plus 98" pistons) is that I will probably add a turbo in the spring and don't want to go too thin on the cylinder walls as that will require the use of head gaskets that don't have a lot of material between the stud hole and cylinder bore.
Last edited by jmb79; Dec 8, 2014 at 08:14 AM.
Seasoned HDF Member
Joined: Mar 2010
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From: Poolville
another decimal............ .005
Dude you need another decimal point it's 5 thousandths or .005, 10 thousandths or .010 etc. .10 would be 10 hundredths and no one makes pistons over in those sizes....
Yes, S&S sells the 4.5" stroke flywheel assembly and the matching 3.875" pistons for a 106" stroker build. The S&S pistons are available in 3.875" as well as .05", .10" and .20" overbore.
http://www.sscycle.com/product/1999-...it-p24019.html
http://www.sscycle.com/product/1999-...8B-p24042.html
The reason I didn't go 107" (i.e. 4 3/8" stroke plus 98" pistons) is that I will probably add a turbo in the spring and don't want to go too thin on the cylinder walls as that will require the use of head gaskets that don't have a lot of material between the stud hole and cylinder bore.
http://www.sscycle.com/product/1999-...it-p24019.html
http://www.sscycle.com/product/1999-...8B-p24042.html
The reason I didn't go 107" (i.e. 4 3/8" stroke plus 98" pistons) is that I will probably add a turbo in the spring and don't want to go too thin on the cylinder walls as that will require the use of head gaskets that don't have a lot of material between the stud hole and cylinder bore.
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Road Master
Joined: Aug 2013
Posts: 837
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From: Wyoming
And my apologies to ottawaair. You were correct also. I swear I read and re-read the numbers several times and still saw .05 instead of .005.
As a side note, I really need to get some reading glasses. I've been putting it off for over a year now but this mistake proves how much I need them. Thanks for setting me straight.
Last edited by jmb79; Dec 8, 2014 at 12:12 PM.
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