Tweet
I noticed a post where Joe Lucia was saying that one should never bore out "routinely" to .030". I think I understand why "routinely" is never recommended if there is not a need. But on the subject of over-boring, what are the negatives of going .030 - .060" if you absolutely have to...? THANKS,J
-
-
Re: Boring Out a '62
You miss the oportunity to go .020, .030, .040, and .060.
But at the miles most drive these cars anymore, boring to .060 will probably last more than the owner's lifetime.
Still, I've known a few who had overheating problems with .060 engines that went away as soon as they put a standard or .030 block in. I don't think this is the norm, just a case of the oddball thin block. -
Re: Boring Out a '62
John and Mike-----
There are things that can happen to cylinder bores besides normal, mileage-induced wear. For example, a broken piston ring can score a bore and then all bores must be overbored to match the overbore necessary to "clean up" the scored cylinder. There's no sense in removing more material than necessary from a cylinder bore and reducing your future options.
If the block has to be bored out to, say, +.040" or +.060", then you do what you have to. That's a totally different situation than just routinely overboring to +0.030" because that's what the machine shop is set up to do or because that's the piston oversize they have in stock.
There are risks which come with greater overbores, though. Core shift can result ending up with too thin of cylinder walls. Beyond +.030" I, personally, like the sleeving option for all cylinders. It's expensive and it only makes sense for valuable blocks. Otherwise, just get a new block. But, for valuable blocks, I'd sleeve all 8. After all, if it's a valuable block, you want to do the best you can by it.In Appreciation of John HinckleyComment
-
Re: Boring Out a '62
THANKS GUYS......makes sense to me......appreciate the info. JComment
-
Re: Boring Out a '62
The reason most blocks are "rountinely bored" to .030 or even .060 is because the machine shops have a flat rate for boring 8 cylinders. If they clean up 6 bores at .040 for example and # 7 takes .060, now what do they do. Go back and bore the first 6 for free. I don't think so!
You can head this off by telling them ahead of time what specific overbore you want. Of course, most posters here would not have any idea what that might be because they don't have the experience or measuring devices to determine such. That's why you sometimes get an "automatic" .060 overbore.Comment
-
Re: Boring Out a '62
JOHN....MY '64 BLOCK WAS FACTORY MACHINED OFF CENTER, AND I MEAN "BIG TIME". WHEN IT NEEDED A .030" OVERBORE MY MACHINEST GOT INTO THE WATER JACKETS IN 3 HOLES. HE SAID "OH LET'S JUST GET ANOTHER BLOCK" AND I SAID "NO YOU DON'T UNDERSTAND". ANYWAY HE SLEEVED THOSE HOLES AND EVERYTHING HAS BEEN FINE EVER SINCE (27 YRS). JOE MENTIONED A RING SCORE ON ONE WALL, BUT IF THE REST OF THE BLOCK IS UP TO SPECS I WOULD SLEEVE THAT HOLE AND LEAVE THE REST OF THE BLOCK ALONE. JUST MY THOTSComment
-
Re: Boring Out a '62
Frank,
The problem that you describe is/was very common in engine blocks 40 years ago. Was called core shift. As casting and machining processes improved over the years, cylinder location and wall thickness were much more consistant which allowed the newer thin wall castings.
Cylinder walls can be sonic tested to determine their thickness before boring.
It's also possible to bore one or two cylinders larger than the others, if necessary. This was common years ago and I've seen brand new Pontiac engines with one or more odd size cyl's right out of the engine plant.Comment
-
Re: Boring Out a '62
As I understand it the oversize piston set a little lower in the bore so that the factory compression ratio is unchanged. This alows one cyl to be over-bored and the engine will run smooth. That's what I was taught in trade school. Back in the day when I had drag cars we bored the block .030 and then brought the deck back down to the stock height to boost the compression.Comment
-
Re: Boring Out a '62
Robert,
I suppose it's possible that some piston mfg's did drop the top surface, or possibly, raise the piston pin location slightly, but I would have to guess that this didn't happen with low/mid compression flat top pistons. The small amount of CR gain when increasing the bore dia of, for example, a 327 by .030 raises the CR by less than .2, from roughly 10.5 to 10.68. To end up back at the original 10.5 CR, roughly .005" would have to be removed from the top of a flat top piston.
It was common practice, however, to reduce the top of the dome of higher CR pistons to compensate for CR increase for larger bore dia as this was already a machined surface.
MichaelComment
-
Re: Boring Out a '62
At trade school I was told it was a SAE thing. Just a standard used in the mfg industry.Comment
-
you were LUCKY!
Imagine if the machinist had just MISSED hitting the water jacket and the wall thickness was only about .010! Probably would have BLOWN when you ran it for the first time. Much better to find out before assembly!
MarkComment
Comment