Alpine951

I am in the process of checking the bores on my motor. The haynes manual says take three measurements of the bore at the ridge, half way down the bore and at the bottom. I am assuming bottom means to the bottom of the piston top when its at the bottom of its travel. It also says take these measurements parralel to the crank and perpendicular to the crank. However it doesn't say how both crosswise measurements are used. Do you take the average of the crosswise measuremants for actual bore diameter? Is the cross wise for the purpose of determining out of round?. What is the relevance of the measurements at top, middle, and bottom. The porche manual says only do it at something like 62mm down the bore and it doesn't mention measuring it crosswise. WHich is correct? I have read on here about out of round and a factory spec mentioned but if the shop manual doesn't mention measuring crosswise then how do you determine out of round.

Alpine951

Correction. The Porsche manual only says measure crosswise. The terms used in the manual are transverse to the block....meaning crossing the engine or crank.....drivers side to passenger side. I know there are some of you who know this. I missed my car for a year and I need it back. If you can please explain the correct measurement process I would appreciate it very much.

m42racer

The measurement is done across the bore as this is the direction of the thrust. The Piston will rock in this direction. Usually the measurements are done in both directions so that when the bores are honed they can be honed round. The reason to measure in 3 places, top middle and bottom is to achieve a bore with no taper. Another reason the thrust direction is measured is that most Pistons today are not round. The dimension across the Pin centerline is sometimes shorther than across the skirts in the thrust direction.

You can measure as much as you wish. More information the better. Honing these blocks is difficuit as the bore is blind and the hone cannot get a full stroke. Keeping the bores straight and round is all about the operator and his skills I'm told. Too much pressure and too much material to remove will heat up the block and make the size bigger. It's also really important to use a honing plate with a Gasket and torque the Girdle on as well. I have never done this but I have watched it been done several times and seen the final sizes with 0.0001" top to bottom and in both directions. This I am sure takes skill.