Author Topic: Do I need to replace my pistons?  (Read 1747 times)

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Offline TimL

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    • My '59 Cadillac Journey
  • Location: Melbourne
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Do I need to replace my pistons?
« on: April 18, 2015, 08:25:59 PM »
I have just pulled down the 390 engine out of my '59 for a rebuild.  The odometer says 56,650 miles, and based on the condition of the motor, I wonder if this may be correct as it is a tidy car. The engine was running on 7, however I am confident this was an ignition issue.

I used an inside micrometer to measure the ridge on the bore, and it is around 0.001". All of the bearings have worn evenly, as have the pistons.

The new factory piston to bore clearance is 0.0013" to 0.0017". The clearances in this engine range between 0.0035" & 0.007".

The new factory compression ring to groove clearance is 0.0022" to 0.0035". The lower rings measure 0.0003", however the top rings have 0.008" clearance.

They pistons look forged, and have CADILLAC and LO EX embossed in them (I assume they are the original ones).  I would rather use original quality pistons rather than take my chances with new stuff from who knows where.

My question is could I successfully re-use the pistons with a new set of rings and a ball hone?

Online Bruce Reynolds

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Re: Do I need to replace my pistons?
« Reply #1 on: April 18, 2015, 10:55:29 PM »
They are forged pistons, and if not too badly worn, as shown by the measurements, I would have no hesitation in simply fitting new rings, and bearings.

If the bearings are still good, and the rings still have tension, I wouldn't have any hesitation in re-using them.

I have just rebuilt a Buick Nail Head engine, and had to replace the pistons because of two broken ones, and everything measured up so well, that I got standard pistons, and re-used the original bearings.

I reamed the ridge, deglazed the bore, polished the crank, and put it back together.

Bruce. :devil:
Bruce Reynolds, Lindisfarne, TASMANIA
'72 Eldorado Convertible (LHD)
'70 Ranchero Squire (RHD)
'74 Chris Craft Gull Wing (SH)
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Offline MY 59

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Re: Do I need to replace my pistons?
« Reply #2 on: April 19, 2015, 08:34:17 AM »
agree with Bruce, new rings and a hone :)
'59 Sedan Deville
'67 ZA Ford Fairlane

Offline TimL

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Re: Do I need to replace my pistons?
« Reply #3 on: April 19, 2015, 09:53:10 PM »
Thanks Guys.

My only concern is the loose top rings.  I have sent an email to Hastings to see if they have 2mm rings which would fit the Cadillac pistons.  This would add 0.006" to the ring thickness, and close the gap back up to 0.002"!