Daniel5691

So I now have a kitchen table full of gear and a few crumbs of money remaining.

Here's what I've done to set up for my first-time-ever cylinder bore measurement.

Please critique.

1) Zeroed and the measured digital caliper against a known 2.0 inch standard.
Digital caliper reads average of 1.995 after 10 attempts.
(1.997(1) 1.996(2) 1.995(6) 1.994(1) range on 10 measurements)

2) Math time: Ideal bore from factory is 96.960 through 96.980
96.96 mm = 3.82 inches.

3) Digital caliper set to 3.820 inches and snugged down.
Fowler cylinder bore gage assembled and dial zeroed on digital caliper at 3.820 inches.

Ready to take some measurements on my cylinder?
Or, am I missing a step in set-up?
Thanks for helping a truly ignorant noob.

Dan

jcorenman

Sure, critique is easy...

Let me preface my comments by saying that I am not an engine-builder. I've built a few engines but not 928s, and I left ours with GB-- there is no substitute for experience. But you are on the right path-- dive in and ask for help.

Fail. At least in the absolute sense, but it doesn't matter.

A caliper is not the right tool for absolute measurements, the basic accuracy is on the order of 0.001" (one thousandth), which isn't good enough-- that's about what the piston-cylinder clearance is. Some calipers read to half a thou (0.0005") but not repeatably.

The most common error with a caliper is using the thumb-wheel. Metal is elastic, think of the caliper fingers as rubber. When you tighten the calipers like a crescent-wrench, using the thumb wheel, the fingers bend. Instead, pinch your fingers around the part being measured, index-finger and thumb on the caliper fingers directly in line with the part being measured. Close with a measured force, the same each time.

And clean. A hair is 0.003 to 0.005", the finest hair known to man (the RPH) is around 0.002". Every measurement, wipe the part and the caliper faces with your (clean) fingers.

If you zeroed the calipers at zero (wiped clean), then your 2" standard should measure 2.000". Every. single. time.

But for bore masurements, that is just a reference point. So set it somewhere close for the purpose of zero'ing the bore gauge, and don't worry about the exact number.

No, 96.960 mm is 3.8173 inches. You gotta learn to respect the decimal points. If the factory meant 96.96 (i.e anything between 96.955 and 96.965) they would have written that. What they wrote was 96.960, the third decimal point being 0.00004". Really small. The best that can be measured with normal shop tools is one or two ten-thousandths.

OK, close enough.

OK, so you've got the bore-gauge zeroed to some arbitrary number, the next step is to measure the bore. You don't care about the actual number, because you know that your reference measurement is only within a couple thou, and there are parts of the cylinder bore that will not be very worn that will make a better reference.

So pick the best-looking bore, run the gauge around and find the smallest diameter. That will be the least-worn part, and probably near the bottom (below the rings), fore-and-aft (parallel to the crank axis). Now go find the worst (most worn, largest measurement), that difference will tell you how worn the bore is. Repeat that for each bore.

You know that piston fit that bore previously, what you are looking for in terms of an engine refresh is whether the same pistons can be reused in the same bores. The amount of wear (from smallest to largest measurements, for a given cylinder) will tell you that. But if the bores are worn, or the piston coating is gone, then it is new-piston time and a new ball game. In that case you will need a 3-4" micrometer that reads in tenths, and the next level of measurement technique. Or get the new pistons and give pistons and block to a (Alusil-experienced) machine shop and let them do the boring/lapping to fit the pistons.

Post what you find, and more experienced guys than me can tell you if that is good or bad. This is where experience counts. Measuring stuff is easy, knowing what it means in the context of these particular engines is the tough part.

Daniel5691

Hi Jim,
Thanks so much for the pointers.
I went ahead and ordered a Mitutoyo micrometer last night, and in the meantime I borrowed a couple of digital calipers from a very patient friend.

With both sets of his calipers, the zeroed bore gage reads 3.8255 inches, time after time.
Also, I had neglected last night that I had stored my bore gage and old caliper in a chilly garage, and they had all night to sit in the house at about 70 degrees...

I'm in no hurry, I am here to learn, have fun, and not destroy my car. It will be fun to learn to use the outside micrometer when it is delivered, and compare the measurements from both.

And yes, the go-ahead for the Mitutoyo micrometer WAS my last-minute Valentine's gift from my wife !!! LOL !!!! What a romantic ! ! ! ! !

Thanks again ! ! ! !

Dan

Imo000

Find someone that can SHOW YOU how to use these tools. You are not going to lean it from us trying to type it up. Consistency is very important, averaging measurements means you are unable to repeat the measurement so might as well throw out all the measurements otherwise, you'll be working with a guessed dataset. Why did you buy an imperial set of tools when what you need is metric. Constantly converting your measurements is a good way to introduce a screw up point.