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Old 12-14-2010, 05:41 AM
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Bearing scuff marks 496 BBC build

Assembling a 496 BBC,Scat crank that mics right on the money. Plastigauges at .002 on the main journals. I would really like a tad more clearance.

What concerns me is this scuffing you will notice in the photos from just a few revolutions. Tell me what you think? Lot of $ in this build
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Old 12-14-2010, 07:52 AM
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Did you use the Plastigage only on one place on the journal?

Give us a clue as to what parts you are using and who did the machine work.

Vince
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Old 12-14-2010, 08:30 AM
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Have the block bearing bores been align bored or honed? Have you checked the crankshaft journals for taper? What kind of clearances are you getting?

tom
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Old 12-14-2010, 11:52 AM
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[IMG]I did have the block align bored at a very reputable machine shop that has been in business doing primarily circle track engines for a long time. Most of the local area machine shops use him for their align bore work.

It is a '76 959 casting block. The crank is a 4.25 Scat cast steel. Clevite bearings. Only plasigauged under main caps in one spot.

I realize plasigauge is not the best method. I was really wanting .0025-.0027 as this will be street/strip car.
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Old 12-14-2010, 12:04 PM
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Your picures show either crank taper, housing taper, or both. Also possible out-of-round in the housing bore. You can check the crank with a micrometer.

tom
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Old 12-14-2010, 12:26 PM
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Quote:
Originally Posted by machine shop tom
Your picures show either crank taper, housing taper, or both. Also possible out-of-round in the housing bore. You can check the crank with a micrometer.

tom
That aint good, I will investigate a little further.

Thanks
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Old 12-14-2010, 03:24 PM
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Quote:
Originally Posted by GMC boogie
Assembling a 496 BBC,Scat crank that mics right on the money. Plastigauges at .002 on the main journals. I would really like a tad more clearance.

What concerns me is this scuffing you will notice in the photos from just a few revolutions. Tell me what you think? Lot of $ in this build
These are all uppers. Two questions: which are these by station? And what do the lowers of these look like?

Bogie
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Old 12-14-2010, 04:01 PM
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Good point! I had just today thought about checking the lowers. I am going to turn the engine upright on the stand and spin it over a few times and check the lowers.

Thanks
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Old 12-14-2010, 05:16 PM
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Quote:
Originally Posted by GMC boogie
[IMG]I did have the block align bored at a very reputable machine shop that has been in business doing primarily circle track engines for a long time. Most of the local area machine shops use him for their align bore work.

It is a '76 959 casting block. The crank is a 4.25 Scat cast steel. Clevite bearings. Only plasigauged under main caps in one spot.

I realize plasigauge is not the best method. I was really wanting .0025-.0027 as this will be street/strip car.
The more professional way would be to line hone not line bore when machining for a performance build. I line bore when putting new caps on and always finish with a line hone. I have always said Line honing always brings out the imperfections of line boring. I have seen this to many times!!!

Like Tom said there are signs of taper some where of even a crank that is not straight.
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