How to tell if engine has been bored out.

like Larry C said ...look inside thru the plug threads...imo...in a well lighted area or with a good little beam light ...pull all four plugs -use a 17 mm end wrench and carefully turn the large timing side nut.. and get different anlges of observation.....then look at some stock piston sets and some domed overbore kits...a good quick way is to search each piston type by stock or big bore ect... on e-bay...look at the enlarged pics of stock pistons and you can see factories type of symbol arrows they will differ slightly in style from aftermarket pistons arrows..imo... and stockers will have granular to smooth- flatter domes ...sometimes you can even see the piston stamped numbers on the top...ect...a dentists mirror will help to see more angles.....most bore kits will be a different animal and can be spotted through the plugs hole.....imo

If you know the previous owner, he/she may remember what shop did any work on it, and that shop may still have records. It's a long shot, but worked for me once.

larrycavan wrote:

Increased cylinder volume squeezed into the unchanged combustion chamber size and unaltered cam timing should raise the static a hair...not a lot. You won't be able to tell from the PSI number with any certainty unless it's really up there.

Larry, wouldnt a stocker have a comp # less than 160psi, and a 10:1 have a comp # about 200psi new?

cam sizing and timing makes a differance also

PLUMMEN wrote:

cam sizing and timing makes a differance also

Exactly That's why it's hard to tell from compression guage alone.

You have 3 ratios

1 - calculated static [measured or calculated volumes]
CR = Cyl.Vol + Comb.Cham.Vol / Comb.Cham.Vol

2 - measured static
What your guage tells you. 2 guages can give 2 different readings.

3 - dynamic
Now were into cam timing & lobe profiles, rod lengths.....

Use a good LED flashlight to peek in through the plug hole. If it's got a dome with a relativly flat section on the top between the valve pockets, it's probably an aftermarket piston kit.

Stock pistons don't have that.

With my big lift/duration cams, i still have 200 psi. with stock cams u should have even more.

As I said in post 4, look into the cylinder with a bore scope and look for the telltale signs. Look for the lip on the side of the dome because a 900 will not have one, and look for cut valve pockets instead of the almost non existent cast ones on a stock ART piston. The piston on the left is a stock 1000 piston so it has a slight lip at the edge over the 900 dome.