'69 512 block overbore

[G A.]

I suppose this is chapter two of an earlier post. Originally started as an oil consumption issue. Since I've now moved on to the block, I've started this one.
The story.
Upon removal of the heads and delivery to the machine shop, they discovered that 5 of the stem seals were damaged. Some kind of off-brand, cheap seal was used. They also determined all of the guides had spiral liners installed and one guide was not geometrically lined up in the valve train causing damage to the rocker stud, rocker arm and spring retainer. Half the valves had unacceptable stem wear. Since then, they have installed all new bronze guides, valves and seals. They also determined that the spring rates were entirely wrong for the cam. They believe that the springs were intended for a hydraulic roller based on dimensions/rates. The proper springs have been installed and shimmed. Along with these issues the intake gaskets were allowing oil into the intake runners.
The machinist kindly came out to the house and measured the cylinder bores which are .100 over. He also checked, with a compressed air vacuum "gun" the ring seal quality. The gun has a sealing surface when set on the block deck over the cylinder bore, and pulls air up through the bore while a gauge on the gun registers this as "pressure". A perfect seal in the bores, as he stated, would be 21 inches of vacuum, but since there would be leakage due to the ring gap 16-18 would be the norm. The cylinders all tested out at 13-14 inches. His conclusion was that the ring gaps were probably over the tolerances. He also felt that based on a couple of thousand miles per year this would not adversely affect things. The bores also were out of round by a thousandth to a thousandth and a half.
Now for my question.
Based on the above I have very little confidence that the cylinders/pistons/rings were done correctly. I don't have confidence that the oil rings will seal either. Since I've got the heads off my urge is to pull the block and have the bores, pistons and rings fixed. I will certainly sleep better knowing things are right. I have been reading in earlier posts that mid-year blocks can go .125 over. The cast date on this 512 block is D28 9. Can this block go .125 over? If so, would you overbore, or would it be better to have all the bores sleeved?
This is the original LR block with an intact stamp pad. The entire drivetrain remains original as well.
You guys have been a great help to me and really my only source of guidance. Without y'all I feel I'm operating in a vacuum. Thanks!!

[Doug F.]

Re: '69 512 block overbore

check the top of the pistons, if they are replacements the will be stamped, .010 is 10 thousandths over, .030, i hope the guy read the micrometer wrong or you wrote it down wrong, .100 over is a ton and i'm sure .125 won't happen, there is nothing wrong with sleeves if they are installed correctly, that is you must leave a lip so the sleeve bottoms out, if you just power hone the whole cylinder and press in a sleeve they usually slip, move and leak,

[Ken A.]

Re: '69 512 block overbore

If money is no object. then pull the engine and sonic test the bores for core shift. If ok, then an 1/8 inch TOTAL overbore is OK. I have not heard of a .100 overbore. 8 sleeves is NOT a good idea. Make sure he knows to final hone the block with the mains installed & torqued, as well as a head torque plate. Also, do not cut the compression.
If core shift is present, find another block.

[Clem Z.]

Re: '69 512 block overbore

i have no idea what a 512 block is BUT if it is a 427 4 bolt main block it will take .125 over. you can't check the "out of round" without installing a gasket and "deck plate" on the block. most after market piston companies can make you piston in any over size like .105 over so you don't have to bore to the max.

[G A.]

Re: '69 512 block overbore

i have no idea what a 512 block is BUT if it is a 427 4 bolt main block it will take .125 over. you can't check the "out of round" without installing a gasket and "deck plate" on the block. most after market piston companies can make you piston in any over size like .105 over so you don't have to bore to the max.

Clem,
Sorry, I thought it was obvious. I'll try not to be so clipped in my descriptions. It's a 3963512 part number block as opposed to a 3955270 part number. I felt the part number information would be critical to the process. Yes, it's a 4 bolt main, the original L-71, LR code.

Dan

[Clem Z.]

Re: '69 512 block overbore

Clem,
Sorry, I thought it was obvious. I'll try not to be so clipped in my descriptions. It's a 3963512 part number block as opposed to a 3955270 part number. I felt the part number information would be critical to the process. Yes, it's a 4 bolt main, the original L-71, LR code.

Dan

i never learned those GM codes except maybe L-88 and always describle the engine by cubic inches and HP.

[G A.]

Re: '69 512 block overbore

Clem-

What would be the consequences of sleeving all cylinders as opposed to going another .005 to .010 over what I have now? As I understand, the quality of the sleeves is such that, if I went that route the motor would outlast me. I would really prefer a solution that I don't have to go back at some point and do over.

Dan

[Clem Z.]

Re: '69 512 block overbore

Clem-

What would be the consequences of sleeving all cylinders as opposed to going another .005 to .010 over what I have now? As I understand, the quality of the sleeves is such that, if I went that route the motor would outlast me. I would really prefer a solution that I don't have to go back at some point and do over.

Dan

sleeves work great if installed properly. as far as .125 oversize i had a 454 bored .125 over in my pickup for years with no problems.

[Dick W.]

Re: '69 512 block overbore

I suppose this is chapter two of an earlier post. Originally started as an oil consumption issue. Since I've now moved on to the block, I've started this one.
The story.
Upon removal of the heads and delivery to the machine shop, they discovered that 5 of the stem seals were damaged. Some kind of off-brand, cheap seal was used. They also determined all of the guides had spiral liners installed and one guide was not geometrically lined up in the valve train causing damage to the rocker stud, rocker arm and spring retainer. Half the valves had unacceptable stem wear. Since then, they have installed all new bronze guides, valves and seals. They also determined that the spring rates were entirely wrong for the cam. They believe that the springs were intended for a hydraulic roller based on dimensions/rates. The proper springs have been installed and shimmed. Along with these issues the intake gaskets were allowing oil into the intake runners.
The machinist kindly came out to the house and measured the cylinder bores which are .100 over. He also checked, with a compressed air vacuum "gun" the ring seal quality. The gun has a sealing surface when set on the block deck over the cylinder bore, and pulls air up through the bore while a gauge on the gun registers this as "pressure". A perfect seal in the bores, as he stated, would be 21 inches of vacuum, but since there would be leakage due to the ring gap 16-18 would be the norm. The cylinders all tested out at 13-14 inches. His conclusion was that the ring gaps were probably over the tolerances. He also felt that based on a couple of thousand miles per year this would not adversely affect things. The bores also were out of round by a thousandth to a thousandth and a half.
Now for my question.
Based on the above I have very little confidence that the cylinders/pistons/rings were done correctly. I don't have confidence that the oil rings will seal either. Since I've got the heads off my urge is to pull the block and have the bores, pistons and rings fixed. I will certainly sleep better knowing things are right. I have been reading in earlier posts that mid-year blocks can go .125 over. The cast date on this 512 block is D28 9. Can this block go .125 over? If so, would you overbore, or would it be better to have all the bores sleeved?
This is the original LR block with an intact stamp pad. The entire drivetrain remains original as well.
You guys have been a great help to me and really my only source of guidance. Without y'all I feel I'm operating in a vacuum. Thanks!!

I have no problems with the .125 bore, nor do I have a problem with 8 sleeves. More than one race engine has been built with multiple sleeves. BUT......you may have a problem finding someone that can sleeve the block without cutting the deck, and ruining your stamp pad. It can be done, as I have had it done, but most machinists will not do it. The block will have to be decked under the cylinder head surface only. The only way I know of is to use a Bridgeport, or similar, vertical mill. Somewhat time consuming, but is worth the cost to save the stamp pad.

[Joe L.]

Re: '69 512 block overbore

Clem-

What would be the consequences of sleeving all cylinders as opposed to going another .005 to .010 over what I have now? As I understand, the quality of the sleeves is such that, if I went that route the motor would outlast me. I would really prefer a solution that I don't have to go back at some point and do over.

Dan

Dan------


I think I would go with a minimum overbore from where you're at and use custom made pistons for that bore size. Custom made pistons will be very expensive but I don't think it will be anywhere near as expensive as 8 sleeves, especially considering the special machine work necessary to preserve the stamp pad. Plus, you'll maintain a "monolithic" block.

Do this right and I don't think you'll ever need to go back into the engine unless you drive this car day-to-day.

[G A.]

Re: '69 512 block overbore

Joe-Clem-Ken-Dick

Thank you all for the advice and personal experience. I feel much better about the engine having been educated by you guys. I will go for the custom pistons.

Dan

[Joe L.]

Re: '69 512 block overbore

Joe-Clem-Ken-Dick

Thank you all for the advice and personal experience. I feel much better about the engine having been educated by you guys. I will go for the custom pistons.

Dan

Dan------


While the custom pistons will be expensive, you'd need to buy new pistons no matter what you do. So, the cost difference is really only the difference in cost between off-the-shelf pistons and the custom pistons. JE Pistons in Huntington Beach, CA is one source for custom pistons.

My feeling ALWAYS is that it's best to use the smallest possible overbore on any engine. Many times, machine shops will go with a 0.030" overbore on an engine's first rebuild. Not for me, though. I want a 0.020" overbore on an engine's first rebuild.