[AMX guy]

I have a very rare 1966 389 block that is .058 over the best we can measure.

Since I'm not sure .060 over would clean it up is there any room left to bore oversized and use 400 pistons ? or is .060 all there is to work with?

I don't want to buy it if I can't use it.

[Tom Vaught]

I put 400 std bore pistons in my 389 block many years ago as I wanted a forged piston
and the ability to run a "Big Valve" (2.11") Cylinder head.
So the pistons measured in at 4.126" bore size and they wanted .005" Piston to wall clearance. So basically the block bore was 4.130"-4.131". So my block was bored .068" from stock. Never had an issue with the 1964 block and the engine ran 11.55 at 117 mph in a heavy convertible body.

So using your .058" over bore and my .068" overbore, there might be enough room but a 64 389 block is not a 66 389 block. Hard to say on that one.

Today I would have put in a custom .010" bore piston with 400 valve reliefs. The bigger bore is basically nothing in power. But that was 40 years ago.

Tom V.

[AMX guy]

I'm pretty sure .010 over 400 pistons would clean up the bore, so you think there is enough cylinder wall to get away with it then ?

[Gary H]

I've bored many 63' - 66' 389 blocks .060 & .070 over but you never know. I'd sonic test it to make sure. Pretty cheap money spent in the overall scheme of things.

[JLMounce]

I would say have it sonic tested to see what's available to work with. That's a pretty decent gamble to make odds off of asking an internet forum. It's a little bit extra expense, but not nearly as much as paying for the machine work only to discover there's not enough meat left to safely work with.

Do you need this engine for a number matching restoration or are you wanting to use it because you have it? Unless you need the engine in the car from a restoration and value standpoint, I would way all options on the table, including sourcing a different engine block.

[Chris65LeMans]

I'll bite - why is a 1966 389 block "very rare?"

[Sirrotica]

Originally Posted by AMX guy

I'm pretty sure .010 over 400 pistons would clean up the bore, so you think there is enough cylinder wall to get away with it then ?

NO, starting in 1965 Pontiac took a lot of meat out of the 389 block. In 1971 I bought a 65 GTO punched .060. Within a week I was cruising and shoved in the clutch to rev the engine as it was loaded up from city driving. At about 3000 RPM I had a cylinder wall break and stuff a piston into the water jacket. The engine stalled and I popped the clutch to re start it, because the engine was effectively seized I broke almost every part inside the engine. Water and oil running out everywhere, balancer was cocked sticking out of the timing cover, worst Pontiac destruction I have ever seen personally, and I've destroyed a few Pontiac engine as well as worked on other peoples blown up stuff.

I was able to salvage the top end of the motor (heads intake and for some dumb reason the rods never touched the cam). Crank was broken in 5 separate pieces one attached to each main saddle, and there were 4 TRW pistons not broken. All the rods were either bent or broken. A piece of one cylinder wall had broken about midway between top and bottom, and the broken cylinder wall and a piston were jammed into the water jacket.

Upon doing some reading I found out that 64 still had reasonably thick cylinder walls. In 1965 a block redesign took too much cylinder wall thickness out (Pontiac termed it thin wall castings). 65-66 were considered to not have enough beef to go more than .030 over safely. Your already beyond that factor. If your still considering taking your block .070 over, I would strongly recommend sonic testing it. FWIW, I would look for another block alternative.

[AMX guy]

It's a WV code, date correct for the 66 tri power GTO I bought. it was for a California tri power only, which my car was built and bought in California. the engine is in another state and is owned by a friend of the 2nd owner I bought the car from.

He thinks it's from my car since he took it in trade 30 years ago and the 2nd owner is now is deceased so he's no help . I tried to confirm it and cannot, there is no way to confirm a 66 block to the car according to PHS.. I'd like to salvage it if possible since it almost has to be my original block. .


I'm asking because I have to buy it and ship it, if I had it in my hands I'd just take it to my machine shop.

So id boring it isn't possible what about sleeves? maybe that's my only option. now I'm starting to wonder if it's all worth it.

[JLMounce]

I think without the engine in hand, you kind of have to assume it won't take the overbore. You could get lucky, but since you don't own the engine anyhow, why take the chance?

I go back to whether or not the car actually needs that specific engine. If you're holding on to something that has serious value as a concourse numbers matching car with it's factory engine, then the expense of doing something like sleeves on the engine may be worth it. I honestly don't know of anyone that would ever recommend doing 8 sleeves in an engine, but if you're at that point where you've got 50K in value riding on the factory engine, doing so could be worth it.

The guys at RPM Engines in California recently did such a job on a RAII 400.

If you're not in that position, I would search for a different block.

[Sirrotica]

Sleeving 8 cylinders is expensive and takes a lot of strength from a casting because of removing support material from all the cylinders. Overlapping liners can be a problem from what I've heard from machinists.

I'm not going to stick my neck out with sleeve advice, probably need to consult with someone familiar with installing sleeves in a Pontiac block, not my forte. I'm sure one of the resident machinists will be along to comment on it.

[Tom Vaught]

Like I said you are a flip of the coin on that deal and since then my original 64 Tri-Power engine (while not hurt) has never left the engine stand.

I have even thought about putting a custom sleeve in each hole (say a Chebby Bore Size sleeve or Mopar bore size sleeve) where the sleeves needs a very slight press in the current block and having custom pistons made for the 389 original heads. Original 64 GTOs and original to car 64 Tri-Power engines do not happen often these days.

Pontiac Bore is 4.0625. SB Chebby bore is 4.000" so the bore would be .0625" smaller vs stock. If the sleeve wall thickness is .060" then you add .125" to the 4.00" bore and you get 4.125" same as stock 400 piston bore. No extra metal removed from the existing block I have and close to the Bore he has now. But a lot of "picky-uni" work to make it happen.

Tom V.

[AMX guy]

I'm not feeling a confident vibe here. I wanted the original block but maybe it's not worth it.

This is a rust free real tiger gold tri power 4 spd car but I have no intentions of making it a concours car in any event. just a really nice drivable original restoration is my plan. a 400 tri power with the correct 093 heads may have to do.

Sounds like I need to have him take it to a shop and check it out at my expense. if it's good I get my block if it's not I 'm not out much.

[Tom Vaught]

Agree, sonic check would tell it all.

Tom V.

[hurryinhoosier62]

Originally Posted by Gary H

I've bored many 63' - 66' 389 blocks .060 & .070 over but you never know. I'd sonic test it to make sure. Pretty cheap money spent in the overall scheme of things.

X2! I know of two early '66 389 blocks that had incredibly thin cylinder walls(less than .090). One was in my uncle's' special order Tempest. The other was in a friend's '66 GTO drag car. The one in the Tempest failed while in service, the one in the Goat was detected while the block was being ultrasonic tested.

[AMX guy]

What would be considered a minimum wall thickness fir a street engine ?

[hurryinhoosier62]

Originally Posted by Sirrotica

Sleeving 8 cylinders is expensive and takes a lot of strength from a casting because of removing support material from all the cylinders. Overlapping liners can be a problem from what I've heard from machinists.

I'm not going to stick my neck out with sleeve advice, probably need to consult with someone familiar with installing sleeves in a Pontiac block, not my forte. I'm sure one of the resident machinists will be along to comment on it.

We had a company policy on sleeving gas blocks: no more than two sleeves per deck, no side by side sleeves period in non-siamesed "v" blocks. Sleeving simply takes too much material out of the base cylinder walls unless you use thin wall sleeves(1/16th wall .0625) which are NOT recommended for performance use.

[hurryinhoosier62]

Originally Posted by AMX guy

What would be considered a minimum wall thickness fir a street engine ?

.125 on the non-thrust sides. That is assuming you're using light pistons.

[6-pack]

I had my 60 Bonneville 389 bored to some stock 69 400 pistons I had and have had no problems with it. I have had it idling for and hour and a half in New Mexico mid day heat and still no concerns. I'd be ok with doing it again!
6-pack

[lust4speed]

I had two 1966 389 blocks brought to me that were too thin. One was at .030" and the sonic check found two cylinders that had wall areas under .070" total thickness. The block could have failed even at .030", but led a normal life. There wasn't a hope of going any more. Other '66 389 block was standard bore, but had three cylinders sitting at just under .095".

Just when I was ready to condemn all of that year's blocks, the third 1966 389 engine came in a few months later also sitting at .030" over and I had the shop sonic it. That one came in thicker than just about anything I've seen - think the thinnest wall was around .210". So all I can say now is I'd sonic test any '66 block and hope for good news. First two blocks showed signs of pretty severe core shift which is why they ended up thin on the same side wall in every cylinder. Third block not only had more meat in the walls, but the casting was very nice and even.

[geeteeohguy]

Originally Posted by lust4speed

I had two 1966 389 blocks brought to me that were too thin. One was at .030" and the sonic check found two cylinders that had wall areas under .070" total thickness. The block could have failed even at .030", but led a normal life. There wasn't a hope of going any more. Other '66 389 block was standard bore, but had three cylinders sitting at just under .095".

Just when I was ready to condemn all of that year's blocks, the third 1966 389 engine came in a few months later also sitting at .030" over and I had the shop sonic it. That one came in thicker than just about anything I've seen - think the thinnest wall was around .210". So all I can say now is I'd sonic test any '66 block and hope for good news. First two blocks showed signs of pretty severe core shift which is why they ended up thin on the same side wall in every cylinder. Third block not only had more meat in the walls, but the casting was very nice and even.

Great post! There IS a way to tell if a specific engine belongs to a specific car....the Protect-O-Plate. It has the Engine Unit Number stamped into a metal plate on the back cover. No arguing with THAT. I have the P.O.P for my tripower '65 GTO, which is one reason I never bothered finding a 'numbers matching' WS block for the car and settled for a WT engine instead. The only way I could make my car numbers correct is to throw away the P.O.P and then install a 'correct' coded block. No way is that happening! AMC guy, you should just run the 400 the car came with. It's no big deal. The WV at .060" is a no-go, IMO.....good luck. And please, post some pics!!