I know its not a Porsche, but I could really use some HELP!

[Hugh R]

I"m rebuilding my Aston Martin DB4 and I'm having a binding problem on the main bearings. I also own an 84 Targa, and I'm an active member of the board. Can some of you Gurus on engines please help a brother out? Here's the link to the Pelican off topic. Please look at page 4 Thanks very much. I don't get squat on the Aston website, which is why I'm turning to you.

Pulled the DB4 Motor Today

[eapcpa]

First pull the crank and mic the bearing journals with a regular micrometer. Plastiguage has limitations if things are not straight.
Next put only the bearing shells on each end of the block without caps. Set the crank in and use a dial indicator on each intermediate bearing journal while slowly rotating the crank to check for runout. Then torque all of the caps without the crank and bearings and use an inside micrometer to check the bore of the caps for size and runout (roundness). I assume you have the specifications for this engine. I would use a trusted local machine shop that has the equipment (Mics) to do this if you don't have the tools or don't have access to them. It should not be expensive for a shop to take the measurements. It sounds like you have mismatched the caps or have a bore alignment issue (align hone). Those of us with magnesium Porsche cases are very familiar with this problem. As far as the caps being mismatched, put the caps on without bearings or crank and torque them down. Then do a visual through the block for any obvious alignment mismatch at the mating seam of the cap and block. If you see something off then flip the cap 180 degrees and reinspect. Then you might try switching the caps. If you can find it easily like that you will be lucky. These type issues usually cost me a trip to the machine shop, but not always. If the crank is straight and you find a cap missmatched to the block and can correct it then assemble it and use plastiguage before final assembly. It should rotate pretty easily when properly assembled and torqued. This is a step you want to get right the first time before proceeding. I don't think I have left anything out, hope my explanation is clear.

[Hugh R]

Ed,

Thanks very much. I found that I had some slack in some bearing clearances, (optimum is 1.25 to 1.75 thou, as in inches. I mixed and matched some bearing shell halves and got something that works. I can now spin the crank around without any notice of drag or easy spin through the full 360 degrees. BTW, the caps are numbered and indicated which side they belong. With a 3 foot long crankshaft and striving for 1.5 thou., plus or minus, I was concerned about line bore, crank straightness, but right now I'm good.

[john walker's workshop]

kind of scary that some bearing shells are different than others in the same box.