Case inspection, align bore or not Options

[930cabman]

Recently I had my local good guy machinist make a steel inspection shaft to pickup the main bearing bores leaving a thou or so clearance. Seems to make sense, similar to installing the crank with new bearings and making sure the crank spins smoothly (easy). The next part of the plan would involve fabricating another gauge to pickup the cylinder mounting surfaces. Sure this can all be done with indicators, but I am trying another tack with this process. Thoughts?

I also use Plastigage to check actual (after crush) clearances

Working a 2.7 911 cases I have found the main bearing bores become elongated due to many heat/loading cycles. This is very common to this magnesium case and keeps guys like Ollies in business. Is this elongation also common to our VW/914 cases

TIA

[Montreal914]

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[r_towle]

First, find the bearings you can actually purchase.

[930cabman]

First, find the bearings you can actually purchase.

Got Silverline on the shelf

[barefoot]

First, find the bearings you can actually purchase.

Got Silverline on the shelf

Now, see if the main brgs. are a tight fit in the case, if so line boring probably not needed

[930cabman]

First, find the bearings you can actually purchase.

Got Silverline on the shelf

Now, see if the main brgs. are a tight fit in the case, if so line boring probably not needed

The testing gauge fits very good, turns by hand with a thou or so clearance at all four case bores

[Superhawk996]

First, find the bearings you can actually purchase.

Got Silverline on the shelf

Now, see if the main brgs. are a tight fit in the case, if so line boring probably not needed

The testing gauge fits very good, turns by hand with a thou or so clearance at all four case bores

FYI - your looking for elongation of the bearing saddles. The middle saddle is particularly prone to be hammered out by crank whip. A test bar that crosses all three with 0.001” clearance really isn’t going to tell you what you you need to know.

Measure with bore gauge is best. 2nd choice would be telescoping bore gauge (snap gauge) and measured with a micrometer. Should be same diameter across two axis perpendicular to each other.

[930cabman]

First, find the bearings you can actually purchase.

Got Silverline on the shelf

Now, see if the main brgs. are a tight fit in the case, if so line boring probably not needed

The testing gauge fits very good, turns by hand with a thou or so clearance at all four case bores

FYI - your looking for elongation of the bearing saddles. The middle saddle is particularly prone to be hammered out by crank whip. A test bar that crosses all three with 0.001” clearance really isn’t going to tell you what you you need to know.

Measure with bore gauge is best. 2nd choice would be telescoping bore gauge (snap gauge) and measured with a micrometer. Should be same diameter across two axis perpendicular to each other.

A bore gauge is probably best and will give the size and roundness of the bores, but will not result in knowing of the alignment of the bores. this gauge is new to me, prussian blue will provide additional information. So far I am pleased with the results.

Anyone have an extra 78mm stroker crank laying around?

My 2056 has given great results (including 30+ mpg highway), but taking a different tack this one.

[Superhawk996]

More info use as you see fit.

Align boring is usually done to address elongation of the saddles not to fix “alignment” per se.

The case 1/2’s will have a bit of material taken off the mating surface. This forces the main bearing bores to become oval with the major diameter being on vertical plane of the case seam. The horizontal plane is the minor diameter.

So the 1st step in align boring is to actually make the situation we originally didn’t want (oval bearing saddles) - even worse.

Once that is done, the main bearing bore is recut to be perfectly round to fit the oversize bearing. By default, the bearing bores are then also “aligned” though the primary purpose was to make them round again and to fit the new oversize bearings.

If you haven’t already found this post on your own it’s worth reading and Jake Raby chimes in as well
http://www.914world.com/bbs2/index.php?sho...p;#entry1384278

Note: the OEM machine work is done with more precision than any aftermarket / machine shop align bore will be. Most shops use a pretty hokey boring tool that pilots off the nose bearing / crankshaft seal bores and won’t be as rigid as the OEM tooling was that cut the bores the 1st time. In addition, align bore changes the dimensions between crank center line and the cylinder registers and has cascading implications to compression ratio and valve geometry.

Bottom line - don’t align bore if you don’t have to

[Superhawk996]

Regarding crush: the other post you posted to is 12 years old. Not sure you’ll get the answers you seek there.

I couldn’t find a good Mahle document quickly but this covers the basics. Crush is normally taken into account when you specify oversize bearings and the machine work is done to OEM oversize bore spec.

Usually will have to contact bearing supplier if going Outside the normal specs. Likewise, engine builders develop their own preferences based on application.

https://www.kingbearings.com/wp-content/upl...ce-bearings.pdf

[930cabman]

thank you for the information from King Bearings, great reading material.