Clearance Checks: Crank Mains to Main Bearings

[dannichols1474]

I am rebuilding my 75 2.7L engine and while the case, crank, rods and flywheel are still at Ollie's I am reading through Wayne's 911 engine rebuilding book. This is my first 911 engine rebuild, but I have done rebuilds of other engine types (V-8's, import I-4's, motorcycles) so I am familiar with many of the practices covered in the book.

In chapter 5, section 2 Crankshaft Assembly you describe using plastigage to check crankshaft to connecting rod bearing clearance.

My question is: In chapter 5, section 3 Crankcase Assembly I don't see a similar mock up procedure to use plastigage to check crankshaft to main bearing clearance.

Would checking this clearance using dry bearings, bare crankshaft (no con rods attached) and no case sealant be valid?

Sample procedure: Put the 7 bearing halves into the 4,5,6 case half, put a strip of plastigage across the bearing surfaces, place the crankshaft onto the bearings. Then install the 7 other bearing halves into the 1,2,3 case half and then lower the 1,2,3 case half onto the 4,5,6 case half. Install the case through bolts and torque to spec then disassemble and lift out the crankshaft and measure the plastigage.

Thank you.

[PeteKz]

Dan, your procedure is close, except do not place he Plastigage "under" the crank.

You should rotate one case half in the engine stand so that the inner edge of the case half is facing up and you can lay the crank down into it. Install the bearing shells and then lay the crank into them, but avoid rotating it. Then put pieces of plastigage on the top of the crank journals, then put the "top" half of the case on and torque it down with the through-bolts. That will squish the plastigage. Remove the "top" case half and use the paper that the plastigage came in to measure how far the plastigage flattened out, and read the clearance.

[dannichols1474]

PeteKz,

Thank you!

Dan

[buster73]

Was reading mixed opinions about plastigage, but still decided to used it for crank and rod bearings. Not many alternatives for a DIY job anyway. My experience was quite positive. Easy to use, easy to read results and with pretty reproducible values. Just follow the instructions and the advice given by Pete. Helps to give you some peace of mind and should tell if something is really wrong.

[PeteKz]

Dan sent his case, crank, rods, and flywheel to Ollie's so I expect they set it up right. However, it's always a good idea to check it yourself, and that's what Plasitigage is excellent for.

[rudedude]

If you have measured the assembled case bearing saddles, crank diameters and bearing thickness and done the calculations do they concur with what the plastigage tells you? I haven't done the plastigage but have done the calculations and I wonder if it is another somewhat necessary step?

[porschedude996]

I was dissuaded to use plastigauge and use the measured dimensions. I had the crankshaft line-bore checked, thumbs up, and the crank checked, two thumbs up, measured both rod ends, three thumbs up. I checked the measurements of the machine shop and compared the data and was convinced to go with that. I did check one rod to prove the numbers.The crankshaft rotation felt great, rod side angular play was consistent with the one rod I checked.

[dannichols1474]

My intent with plastigage is to do a sanity check on the work performed by the machine shop. As Wayne states in his 911 engine rebuilding book, you may trust your machinist but what about the shop assistant or the drunken partner that actually takes the measurements....

I can measure the crank journals OD directly with a micrometer. The crankcase bearing IDs I have measured on other engine types with a bore gauge and micrometer, then subtracted crank OD from bearing ID to get clearance. The split case 911 engine I am not seeing how to measure bearing ID's of an assembled case - my inexperience I guess.

[targa72e]

You should be able to measure the case bore with a standard bore gauge. You need to assemble the case with a modified #8 bearing. You need the #8 bearing as it aligns the case halves but you need to modify it so that the hole is big enough to get the head of the bore gauge thru. I understand there is a VW bearing that has the same outer dimensions as a 911 #8 but with a bigger hole (I dont know what its used on). I just used a old #8 and turned the center larger in the lathe. One assembled you can measure thru the front and rear and get to all the bearings. I like to measure with micrometer and bore gauge and then double check with plastigauge.

john