Miking Main Bearing Bores

Steam Driver · Steam Driver's Garage

What is the correct procedure for miking the main bearing bores? I know the area(s) of concern are at 45 degrees, but is only one measurement made in the center of the bore, or a couple of measurements moe towards the edges? Also what about micrometers; there doesn't seem to be a lot of room for manipulating a real inside mike, so are telescoping gages accurate enough?

The cap'n · 07-22-2014, 08:48 AM

A dial bore gauge is the proper tool for this. I can't seem to upload an image, so you'll have to Google it.

The Cap'n

Steam Driver · Steam Driver's Garage

Just playing devil's advocate here, I don't see why this (I looked it up) would be any better or different than an inside mike or the telescoping gauges and an outside mike, except maybe it combines functions. To me the issue would be getting any of them square with the bore.

The cap'n · 07-22-2014, 01:03 PM

This tool is pretty easy to use, and you can use it way down inside the engine with relative ease. Getting it "square" isn't really an issue.

The Cap'n

Walt Fricke · 07-23-2014, 08:23 PM

Yes, I think the shops routinely just use dial bore gauges for all this - mains, rods, cylinders. Quick, easy, and accurate.

Inclines one to take measurements at several points to check for ovality. I've got one, and snap gauges. Neither were expensive. I don't trust my snap gauge measurements as much as the dial gauge, which you can rock to see what the true minimum reading (as in completely square to the bore) is. Doing that by feel with a snap gauge can't be as accurate. I think manipulating an inside mike in any of these situations would be painfully slow, especially as you'd want to take multiple readings to be sure you had one of them square.

Jon B · 07-23-2014, 10:25 PM

Quote:

Originally Posted by The cap'n

A dial bore gauge is the proper tool for this.

I agree with Walt and the Cap'n, a dial bore gauge.
It is the instrument recommended by Porsche for measuring crankcase bores.

Steam Driver · Steam Driver's Garage

OK, thanks for the advice

KTL · 07-25-2014, 09:36 AM

Think about the telescoping gauges and how they work. You set them in position, then measure the length of it. How do you know it's the correct dimension? You don't. So you put it in, rock it around a bit trying to get the smallest length/dimension and measure again. Very tedious and not all that accurate.

Like the guys said, dial bore gauge is the way to go. A solid quality bore gauge kit from Mitutoyo is not a ton of money and very accurate. Jon's got a very nice Matra set there. Nowadays, quality like that is BIG bucks....

Fowler makes a decent one for occasional use. It gives repeatable results and is only $125 for a 1.4 in. to 6 in. measuring range. The only gripe I have is the 0.0005" increment dial gauge is not quite as accurate as we'd like it to be for bearing clearance checks. 0.0001" increment is the way to go IMO.

Also note that if you're measuring case main bores, you want to have the No. 8 main bearing insert installed. It aligns the case. W/out the No. 8 insert installed, your bores will be misaligned. If you're measuring the bores with the bearing shells installed, that's a different story because you're likely to have the No. 8 installed too.

Good luck

Steam Driver · Steam Driver's Garage

That's interesting about installing the #8 main, as one would think the dowels, etc., would do that fine. It's not in the manual I have (which actually shows the #8 main bore being measured, no bearing). Is this some learned by experience lore not in the books?

KTL · 07-25-2014, 07:47 PM

Yep it's learned. Take a look at the case and you'll see the case has no means of precisely aligning the bores towards that side of the case w/out that bearing insert in place.

Walt Fricke · 07-25-2014, 09:32 PM

I think Steam Driver (like me at first) might have been thinking of the #1 bearing, which is where the dowels are (unless it is a race motor and pinned in all the main journals). Nothing like that on the other end, and the dowel holding the #8 in place doesn't do anything without the #8.

Jon's picture must be from the same manual, and it shows the #8 being measured, but you can't measure it with the #8 in place.

Jon B · 07-26-2014, 12:40 AM

Kevin is correct, the rear case halves must be aligned for accurate measurements, as there are no dowels or sleeves in the rear for self-alignment (at least through the 70's; did Porsche dowel the rear on later engines?).

The top photo I posted is from the factory Workshop Manual, Volume III, in section titled Measuring and Reconditioning Crankcase. Before the case hardware is tightened to proper specs, the manual says "Align both crankcase halves with the aid of a plastic mallet. The joint in Bearing 8 must not be offset in relation to each half. Using the inside micrometer, cross-check bore of Bearing 8. Realign bore if necessary." The photo might be showing this alignment check, before measuring the other bearing bores. Tap with mallet as necessary, check again until aligned, then tighten.

You can insert the #8 bearing a short distance into the rear bore to align it, then remove before tightening the case hardware, or machine an aluminum plug for easier removal. If you leave the #8 bearing in the case, you cannot pass the bore gauge through it.

If the case is going to a machine shop for other work, they can check the bores for you.