general question about overboring a cylinder
Thread Starter
Racer
Joined: Apr 2003
Posts: 342
Likes: 0
From: Kalamazoo MI
general question about overboring a cylinder
At what point when oversizing a cylinder bore must oversized pistons be used? My understanding is that there is a point when you only need to use oversize rings and same size pistons can be used. What about the standard .030" overbore on a 350ci to make a 355ci. Is the standard procedure to use oversize pistons or reuse pistons with oversize rings?
The particular project I am thinking about is oversizing a 3.0L Ford Duratec block (no it's not for the vette). The stock bore is 3.5" I would like to use cheaper off-the-shelf pistons for a Mustang 4.6L which feature a 3.552" bore. What do I tell my machinist? Simply bore the block out to 3.552" and use the Mustang pistons or do I need to account for rings in the bore number? Or can I simply bore the block say .020 over and reuse the stock pistons with 0.20" oversize rings? Thoughts please.
The particular project I am thinking about is oversizing a 3.0L Ford Duratec block (no it's not for the vette). The stock bore is 3.5" I would like to use cheaper off-the-shelf pistons for a Mustang 4.6L which feature a 3.552" bore. What do I tell my machinist? Simply bore the block out to 3.552" and use the Mustang pistons or do I need to account for rings in the bore number? Or can I simply bore the block say .020 over and reuse the stock pistons with 0.20" oversize rings? Thoughts please.
Burning Brakes
Joined: Sep 2006
Posts: 1,202
Likes: 3
From: MD
Except, before you do this you need to make sure the compression distance (distance from the wrist-pin to the top of the piston) is the same. 50 thousands is a lot of overbore. You also need to make sure that the block can support this.
Le Mans Master
Joined: Feb 2005
Posts: 6,485
Likes: 566
From: Clearwater Florida
overbore
According to the Helms shop manual the max recommended service (used) piston clearance is .0025 this is for the 5.7 TPI motor. At .030 overbore a std piston would be flopping around in the cylinder . And as mentioned earlier there are potentially major differences in the compression heights of pistons from one engine to another . Unless these are identical , the connecting rod lengths would have to be adjusted...by changing rods , effectively making the less expensive pistons a VERY expensive way to clean up an old block. Don't go overboard (pun intended) with this one!
Le Mans Master
Joined: Sep 2006
Posts: 6,050
Likes: 147
From: Phoenix AZ
2021 C4 of the Year - Modified Finalist
pistons
anytime an engine is bored, a new piston for the new diameter is required.
boring is the answer for excessive cylinder wear whether it's for mileage, or some other reason.
rebore, and install a piston with the standard bore+the overbore.
That's why they commonly refer to them as thirty-over pistons, because that's how much you bored it out
example: diameter of the bore=4.0010....diameter of the piston=3.992
the difference is the clearance so the piston does not sieze when it warms up and expands.
the piston and cylinder dimensions are rounded off, calling it standard
(4.00" ) bore and standard 4.00" pistons.
oversize rings are used in 2 instances,
A. so you can custom fit rings to your cylinder incase you need a nonstandard ring gap.
B. incase your bore diameter is just a re-ring, and you are not boring it, just re-ringing to cure oil consumption and restore compression. If the ring gaps on your cylinder are excessive, to control blow-by and resolve oil usage, get oversize rings, check the gap, and grind the ends to fit your abnormal cylinder size.
If you do this, make sure the manufacturer's recommendation for clearance between the piston and the cylinder wall is not exceeded
boring is the answer for excessive cylinder wear whether it's for mileage, or some other reason.
rebore, and install a piston with the standard bore+the overbore.
That's why they commonly refer to them as thirty-over pistons, because that's how much you bored it out
example: diameter of the bore=4.0010....diameter of the piston=3.992
the difference is the clearance so the piston does not sieze when it warms up and expands.
the piston and cylinder dimensions are rounded off, calling it standard
(4.00" ) bore and standard 4.00" pistons.
oversize rings are used in 2 instances,
A. so you can custom fit rings to your cylinder incase you need a nonstandard ring gap.
B. incase your bore diameter is just a re-ring, and you are not boring it, just re-ringing to cure oil consumption and restore compression. If the ring gaps on your cylinder are excessive, to control blow-by and resolve oil usage, get oversize rings, check the gap, and grind the ends to fit your abnormal cylinder size.
If you do this, make sure the manufacturer's recommendation for clearance between the piston and the cylinder wall is not exceeded
Safety Car
Joined: May 1999
Posts: 3,651
Likes: 5
From: Atlantis
Cruise-In I Veteran
Here's a simple answer for ya.
At ANY point a cylinder is 'oversized', NEW PISTONS MUST BE USED.
Standard 'overbore' sizes are .020, .030, .040, .050., .060.
Example: A 'thirty over' (.030) 350 is a 355 CID.
Each cylinder 'should' be finish honed to match a particular piston.
The match is made to obtain the 'skirt clearance' for the respective piston.
Ring packs come in sizes to match the cylinder overbore.
Ring packs come in standard sizes and sizes that the engine builder can finish 'fitting'.
The ring 'end gap' may be adjusted by filing to obtain the desired 'end gap'.
There are also some 'no gap' rings available.
Ring packs MUST match the piston style being used.
IIRC, ther are TWO standard ring widths in use today.
I think 5/64" is standard and the other uses more narrow ring widths.
The rings must match the ring land width for the pistons being used.
Hope that helps you to understand piston/ring/bore basics.
At ANY point a cylinder is 'oversized', NEW PISTONS MUST BE USED.
Standard 'overbore' sizes are .020, .030, .040, .050., .060.
Example: A 'thirty over' (.030) 350 is a 355 CID.
Each cylinder 'should' be finish honed to match a particular piston.
The match is made to obtain the 'skirt clearance' for the respective piston.
Ring packs come in sizes to match the cylinder overbore.
Ring packs come in standard sizes and sizes that the engine builder can finish 'fitting'.
The ring 'end gap' may be adjusted by filing to obtain the desired 'end gap'.
There are also some 'no gap' rings available.
Ring packs MUST match the piston style being used.
IIRC, ther are TWO standard ring widths in use today.
I think 5/64" is standard and the other uses more narrow ring widths.
The rings must match the ring land width for the pistons being used.
Hope that helps you to understand piston/ring/bore basics.
