[blueghoast]

Can you bore a 389 safely to a 400or even a 406. Is there enough
Meat in the bore's.

Thanks GT.

[Tom Vaught]

I have a 64 GTO block that was bored to a 400 size (.060) and drove the car for several years once I put a good (new) radiator (high fin count per inch) in the vehicle.

I have posted in the past about this radiator built up by Superior Radiator in Mt Clemens Michigan. They did a LOT of Circle track radiators for the local racers.

Learned a lot about radiators from the owner too. Engine is mounted on a stand and has been there for several years once the $$$$$ of 64 GTOs with original engines went up. But I will never sell the car.

Tom V.

[Bill Eveland]

I bored out a 66 389 out .055 to 4.120 and didn't have any problems. Unless you sonic test it , its a chance you take.

[Radman]

Sonic check the block. That will tell you the maximum overbore that you can use. If you do not have enough room to bore, and still want to use that block. There is always the option of sleeving the block.

[steve25]

My first ever rebuild I did like Tom did and took my 65 389 out to the Bore of a 400 without any ill side effects and put tons/ years of daily miles and 5800 rpm shifts on that motor !

Don't for get that with modern boring equipment if a cylinder checks out to be a little thin on one side you can offset Bore it.

I would do this anyway to shuffle the Bore over to the Intake valve side of the head to assist in unshrouding it with out resorting to much of a valve chamfer in the Bore,especially if open chamber 14 degree heads may be installed down the road!

[Cliff R]

A .060" overbore is only taking .030" out all the way around. Most blocks will handle that without issue unless you run into one with casting or core shift issues........

[Gary H]

Quite a few years ago we bored a 62' 389 engine out to 4.150. That's around .180 overbore. The engine had no issues and it ran great. But, as others have said, have the block sonic tested for thickness and then you'll know before wasting a bunch of many on machine work. Sonic testing is normally around $100 or less in my area of the country

[Sirrotica]

FWIW, back in around 1971 I bought a 65 GTO that the previous owner had bored .060, installed TRW 12 to 1 pistons. He had all the receipts from the shop that I got with the purchase.

Nine days later I owned a 65 GTO convertible with a mass of fractured cast iron and mangled aluminum under the hood. Upon examination the cylinder wall had collapsed, and the piston had entered the water jacket, breaking the connecting rod crank, etc.

This was roughly 1971 before sonic checking was around, so the block was never checked. I was pretty disheartened having paid a fair amount of money for the car, purchasing it mostly for the built engine, with receipts accompanying it.

I did some digging around and found out after 1964 Pontiac had a program called, "thin wall casting" that had taken some of the meat out of the post 1964 castings.

From my past experience I would be looking at 64 and earlier castings if I were thinking about going .060 over.

Just one of those life experiences that when it cost you a bunch of money to learn something, you seldom forget it. I paid the dues, and have the memory.

[steve25]

A lot of build issues and or fuel octane levels could have easily been the root cause of the distruction also in the 389, and with a failure of that level you will never know if the it was a thin cylinder wall that was the starting point.

[Tom Vaught]

Originally Posted by Sirrotica

I did some digging around and found out after 1964 Pontiac had a program called, "thin wall casting" that had taken some of the meat out of the post 1964 castings.

From my past experience I would be looking at 64 and earlier castings if I were thinking about going .060 over.

Just one of those life experiences that when it cost you a bunch of money to learn something, you seldom forget it. I paid the dues, and have the memory.

I worked for three Speed Shops (including Parker Automotive in Kalamazoo Michigan) (who campaigned a Twin Turbo AND Supercharged dragster)).

The head machinist (a pontiac guy with a Pontiac Funny Car chassis race car)
told me exactly the same thing when I asked about going to a later 65-66 389 block vs using my 64 GTO block. He said when you decide to do it stay with the 1964 389 block as the 65 & 66 blocks are (in his words) "thin wall" blocks. Great memories from the past. Always wondered what happened to his 1968 GTO funny car.

Tom V.

[77 TRASHCAN]

Originally Posted by Tom Vaught

I worked for three Speed Shops (including Parker Automotive in Kalamazoo Michigan) (who campaigned a Twin Turbo AND Supercharged dragster)).

The head machinist (a pontiac guy with a Pontiac Funny Car chassis race car)
told me exactly the same thing when I asked about going to a later 65-66 389 block vs using my 64 GTO block. He said when you decide to do it stay with the 1964 389 block as the 65 & 66 blocks are (in his words) "thin wall" blocks. Great memories from the past. Always wondered what happened to his 1968 GTO funny car.

Tom V.

Tom,
Was this the guy that was close to Wangers and worked for Jim at his dealership in Milwaukee. He kept running Pontiac engines, long after they fell out of favor vs. Hemi's...???

[steve25]

At any rate if I was going more then .060" over on any factory block then even for street usage I would do a half fill on the block since there's no issue with them running any hotter after that.

[racegto65]

My street/bracket Firebird has a 389 that is .090 over no issues.

[Tom Vaught]

Originally Posted by 77 TRASHCAN

Tom,
Was this the guy that was close to Wangers and worked for Jim at his dealership in Milwaukee. He kept running Pontiac engines, long after they fell out of favor vs. Hemi's...???

I was never really sure where the 1968 GTO funny car came from but it was a very nice car and the machinist really knew his Pontiac engines.
It might very well have been the same person you mentioned.
For sure He was in the Rick Johnson "knowledge level" or Nunzi "knowledge level" for Pontiac engines and how to build a Pontiac engine properly.

Tom V.

[Dragncar]

I would never bore a 66 389 .060. Known for thin walls. PMD had a lot of warranty work and overheating because of it.
It is the reason the 67 400 blocks are known to have thicker walls. 1st year of a new engine, went on the heavy side to ensure no comebacks.

[Tim Corcoran]

A sonic check will answer your question no two blocks are the same.

[6-pack]

I have a 1960 389 with 1969 400 pistons in it with no issues, pistons were the same except the diameter.
6-pack

[Poncho60]

From what I understand the 59 & 60 blocks are some of the beefiest tha Pontiac produced.

[lust4speed]

Won't bore a 65 or 66 block without a sonic check. Had one .030" over 389 come in that had walls only .090" thick on two cylinders, and a second block was almost as bad. Great coffee table material at that point.

[Schurkey]

Originally Posted by Cliff R

A .060" overbore is only taking .030" out all the way around. Most blocks will handle that without issue unless you run into one with casting or core shift issues........

...or water-jacket corrosion issues, where the cylinder walls have eroded from the water jacket side.

Or cylinders with slag inclusions in the iron. I'm told that Ford was particularly bad for that.

It's always a risk. Nothing is guaranteed. Not even with a sonic test.