The Stovebolt Site Forums The Shop Area Engine Shop A Boring Question: To bore or not to bore?

I am disassembling a 46 GMC 228. My hope was to clean it, flush it, reassemble and paint. I measured the depth of the ridge at the top of the bores. The deepest wear point is .010.

I have not run this motor.

What can I expect in performance loss with this? And knowing that I like to have things in good running order (although this motor will be run in the summer months only) would a rebore and oversize pistons be justified?

Just gathering input to help me get off the fence and make a decision.

Thank you in Advance. Tom

I should add that my intention is to restore/rebuild this 1946 GMC to the best standard I can do myself. Stock. And I will be installing a NOS speedo gauge into the cluster. A fresh built motor would correspond to the mileage.

Performance is not really a factor that comes up much with used engines. If you are going to haul firewood all summer, a rebuild might be needed. If you are just cruising around town a few months out of the year, then follow through with the cleaning, new gaskets and seals. Blow-by from wear and piston slap is real possibility with worn cylinders, but then these engines pretty tough and can take a lot of abuse so who knows, it may not cause any problem.

Rebuilding is expensive, $3000 range plus or minus depending on your location and parts availability. Are you willing to part with that much? Will it run that much better, I doubt it.

Measure all the bores at a few depth and head over to your local machine shop. Talk about the engine and look around to see if you feel comfortable with them. Ask about old engines and see what they know. A lot of shops only deal with late model stuff.

Joe

If you have it all apart, might as well have it bored and new pistons if it's .10 over. My last engine rebuild the machinist told me if its .05 over best to have a rebor and new pistons.
I guess it all depends on what you what to do with it and how long you want to keep it.
I'm working on a 55 right now.. It came with a 350 block from the previous owner and I took the block to the machinist and he said the block was good no need to have it bored. But I decided on putting new piston and rings in it since it was all apart.
Restored old trucks, aren't like the old ones, mine probably won't put on 1000 miles a year, but now is not the time to try and save a few bucks. New pistons and rings, less than two hundred bucks.
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just my two cents..

A friend reminded me that one ridge depth measurement at .010 means that the cylinder is .020 worn to about 1.5 inches below the top of the cylinder ... And to straighten out the bore(s) may requie a .030 bore and oversize pistons...depending on actual measurements from my machinist.

...Means a rebore...

You'll be a lot better off if you go ahead and bore the engine and put new pistons and rings in. The cylinders will be tapered as well as out of round. Causing new rings to have to constantly flex to conform. Even then they may not seal very well against an out of round and tapered bore.

If they do seal, they won't last long and will fail to seal.

As was mentioned, another very good possibility is the piston slapping in the bore. Annoying at the least, but a skirt could break off the piston from slapping the cylinder.

yes, you are right. That seems to be the consistent opinion from this discussion and about ring failure and slap - talking to a couple local guru's and a PM posting I received.

Always learning at the expense of my shallow pocket book! After-all this hobby isn't about saving money anyhow. Thanks for all input.

Tom