Measuring Piston to Cylinder Bore Clearance

[paul25g]

Hi guys,

I'm starting my first ever car engine rebuild (1966 Mustang 200CID) and have a newbie question.

I'm trying to calculate the piston to cylinder bore clearance. I know for the O.D. of the piston, the workshop manual says to "take the measurement at the centerline of the piston pin bore and at 90 degrees to the pin bore axis." No problem.

For the I.D. of the bore, the workshop manual says to "Measure the diameter of each cylinder bore at the top, middle and bottom with the gauge placed at right angles and parallel to the centerline of the engine. Use only the measurements obtained at 90 degrees to the engine centerline when calculating the piston to cylinder bore clearance."
My question is, which I.D. measurement for the bore do I use when calculating the piston to cylinder bore clearance? The measurement taken at the top, middle or bottom of the bore?

Also, is the cylinder at the front of the engine (nearest the water pump) considered to be the Number 1 cylinder and then Number 2 is next back and so forth to Number 6 nearest the flywheel? Just want to be sure of the correct terminology.

Thanks!

Paul

 

[CNC-Dude]

Well ideally, the bores need to be straight with no taper and not out of round, but you will need to check the bores at all the places you mentioned,ie Top, Middle and Bottom of the bores. This also requires the use of a dial bore gauge to be able to do this with any accuracy. A telescoping gauge would be a last resort method, but far less accurate if you don't know how to use them properly. Also make sure your pistons have some taper to the skirts and aren't collapsed. The difference of size between the piston miked at the wrist pin bore and the cylinder is the piston-to cylinder clearance. Mike all the pistons and don't assume they all are the same size.

 

[paul25g]

That clears it up, thanks!