case inspection steps?

[dion914-6]

Folks,

I've just completed the teardown of a 75 2.7S (non-CA) engine. I've been pleasantly surprised by what I have found along the way. Everything about this engine seems to have been well cared for. The Mahle pistons/cylinders are in good shape, although I pulled the headstuds out, they weren't ready to come out and gave me a good fight while coming out clean. The crank bearing surfaces (both mains and rods) are in great shape, and the bearings themselves look very good showing very even, minimal wear. From what I could tell, this case has never been split and has lived its life in a cooler, northern climate.

This is my first 2.7 rebuild, so after reading BA's and Wayne's books I really expected the bearings to be a mess, or at least showing uneven wear due to case movement. I also expected to see ovalled through bolt holes, but those are good too. I've put a straight edge on the bearing webs and bearing journal surfaces themselves, and everything seems to be perfectly straight, level and as it should be.

Aside from checking the mains for roundness, what other case inspections could I do myself? I'm not suggesting skipping a formal inspection, but I have always done everything on my engines and would like to understand what I can do to give myself piece of mind.

This case appears to be in much better condition than the 2.0T engine I built last year.

Thanks,

Dion

[304065]

Dion,

In Bruce Anderson's book, he details the inspection steps. For example, one of his first steps is "measure crankcase line bore." The specifications are given in the little white spec book which is absolutely essential to a proper rebuild.

Here's the rub. Line bore measurements are properly performed with a "bore gage" which is typically a dial indicator attached to a measuring instrument which has two or three carbide points that press against the wall of the bore.

There are two issues with this method, however. The first relates to "gaging feel" by which you MUST practice with the instrument on a standard until you can get reliable readings. If the instrument is not aligned with the points at exactly 90 degrees to the bore, the measurement will be too high, by the pythagorean theorem. However, with time and practice a repeatable measurement can be obtained.

The other relates to the instruments themselves. Dial bore gages are NOT direct-reading instruments: they must be "zeroed" on a "master" which is typically a steel donut that has been machined to a precise internal dimension. As you might expect, that level of precision is not inexpensive, so your cost to obtain a master ring that is exactly 57-and-change-mm, for example, can exceed the price of the bore gage.

Some suggest that a micrometer can be used to set the gage. I've tried this and it's not easy to accurately align the tips of the gage with the micrometer's anvils. Perhaps a micrometer stand would help.

Finally, there ARE direct-reading bore micrometers available: Mitutoyo's Holtest and the Browne and Sharp Intrimik come to mind. These are much more reliable insofar as they self-center in the bore, eliminating the pythagorean error, and will give you a direct measurement without having to be zeroed against a master ring. However, a master ring is required to calibrate them. The big drawback is that they are expensive, new ones being in the thousands of dollars.

The point of my treatise on inspection and gaging is that there ARE measurements you can make yourself, but they require an investment of dollars to acquire the right instruments and time to make them work. If that's cool with you, I say go for it, and see my other thread called, "Measurement Instruments and Metrology" Measurement, Instruments and Metrology
for a running discussion on what these gages are and how to use them. If that's not your cup of tea, most machine shops, not even Porsche specific ones, have this equipment and can make the measurements for you.

I personally like to have the ability to check the machine shop's work but I'm a slightly different breed of cat.

Measuring the crank journals is very possible at home. I recommend you purchase a micrometer that has resolution of 0.001 mm instead of the 0.01 that are commonly and inexpensively available. I just bought one made in the PRC for $50, with digital readout, which should be more than adequate for the amount of use I expect. Were one in a high-volume production environment the Mitutoyo or Mahr brand would be preferable not necessarily due to accuracy but to the ability to have it serviced when it wears out, which is not really an issue for me.

Here were my results with a basic mic. Crank Measurement, and a question about Fillets

[dion914-6]

Thanks for the feedback John. I had seen your other threads and read them with great interest. Although I have done my own crank and rod measurements in the past, I'm going to double check my work this time. A have a fellow Porschephile frind who is an ex-machinist and has much better equipment and know-how than I do. However, I do not have a bore gauge so that inspection step will most likely be farmed out.

Again, I appreciate your feedback and have learned a lot more by reading your response and attached links.

I'll go re-read BA's book now.

Dion