The Stovebolt Site Forums The Shop Area Engine Shop Blowing Smoke

Got my 235 block down at the machine shop, and the machinist mic'd it out and said he'd have to bore it .060 oversize. The truck had run ok prior to pulling the engine and had compression of 105-110 in 4 of the cylinders, 1 cylinder was 90-95 and one was 60. The one at 60 had a nice crack in the head and a sticky exhaust valve. The engine wasn't blowing smoke or not anything noticable, but I'm wondering if the machinist is ?
By the way the block had never been bored and currently has standard size piston and rings, does it make sense that if could run ok, but need to be bored from standard to .060 ? :rolleyes:

Thanks
Kevin

I would get a second opinion from another machinist. Have the cylinders mic`d and see if you can get by with a .030 over-bore.

It could need .060" and still not smoke to bad. If the rings were good and all the seals tight it might not smoke at all. Also if the valves were sealed tight, the compression could leak past the rings and this could keep the oil out. Take you manual back with you and remeasure the holes at the top just under where the ring ridge is.

Another way to look at it is, the bigger the hole the more cubic inches which means more power!!

Joe

Usually when a motor is rebuilt for the first time, its cylinders are only worn .010-.018". Then the bore to .030" will straighten the walls back up because .030-.018 gives you a .012" cut into the maximum wear in the cylinder. Divide .012" by 2, and you're cutting .006" into each wall.

So if your motor is worn too close to .030", they may have to bore to .040". But usually you don't go from standard bore to .060" unless:

1) You're going for maximum cubic inches
2) You already bought a set of .060" slugs
3) There is a gouge in a cylinder
4) .040" pistons are not available (for particular app)

I don't know why the machinist would lie to you, he gets the same money whatever size he bores it. Unless he took a bad measurement, or thinks .040" aren't available.

On a related note, have the block decked .010-.015", because the oversize pistons sit a little lower in the holes.

good luck

chip

But the wear is predominantly on the thrust side - your .018 of wear may .012-.015 on the thrust side and won't clean up at .030. Learned that from my auto parts days from guys complaining about blocks that had to go .060.

Too, a lot made the problem worse with a ridge reamer - .010 wear until you get to where they had "overbored" it with a ridge reamer.

Another question would be does it have deep scoring, i.e. from something getting jammed under a ring. That would necessitate a big bore.