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post #1 of 2 (permalink) Old 01-06-08, 10:19 PM Thread Starter
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Boring out 390 motor

I am currently restoring a 68 Mustand Fastback (Eleanor) and I have a 390 's' code motor. It has a FE Y Block with the numbers 352 stamped on it. I recently had it rebuilt and the block was bored out an extra 60 thou. After being started only the times the block has cracked. I have received differing information from various sources and I really need to clear up exactly whats going on here. I've been told that if you bore the 390 out 60 thou it is gaurenteed to crack, but then I have also been told that while the 427 was originally brought out with a different size block, when these were blown up the same block from the 390 was used. If this is so then surely boring it out 60 thou shouldn't cause it to crack. Personally I beleive that water inside the motor was the cause (no it wasn't me who put the water in it!) but I need to be absolutely certain before I go doing the same thing again.

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post #2 of 2 (permalink) Old 02-01-08, 06:20 PM
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Re: Boring out 390 motor

Boring a 390 block .060 it's pretty much at the end of it's natural limits. At standard bore it's 4.05 and at .060 4.11. That said if the block was sound at the time of the rebuild it should have been fine. At that overbore they can run a little hot. To do it right the rebuilder should have sonic mapped the block. Properly done this would have shown any thin spots especially in the major thrust area's of the cyls. This would have shown any problems area's that would be a concern. It's also possible the block was already cracked and should have been magnifluxed before any work was done. This would have clearly shown a crack in the cyl wall. Sounds like the shop who did the rebuild cut a few corners. Any competent shop that knows the FE engines would have magged and sonic mapped the block form the get go. I've had many 390's and some badly coreshifted only making a .030 overbore possible at max. Others have been fine at .060 but at the limit for overbore. And did run slightly hotter due to the thin cyl walls. Coreshift creates thin spots in the cyl wall that can limit the amount the block can safely be overbored. There are lots of variables. If the block is original or date code correct the cracked cyl can be sleeved. But rule of thumb you want at least .100 thickness on the other cyls for it to be worth while. Sleeving the block costs $100 a hole here. You can find a new 390 block for that in most cases that's got more life left in it. The 427 block although looking the same on the outside was a different animal internally. The 390 is a FE not a FE Y block. The Y block was a earlier series of engine and not really related to the FE. Although a few parts will interchange they are very different from each other. Just to clear that.....
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