# ‘65 389 overbore pistons



## Roqetman (Aug 31, 2015)

I recently took my GTO to a shop to have the rear main seal replaced which required pulling the engine. My mechanic discovered a piece of piston skirt in the oil pan and the #4 piston was cracked on one side up to the rings. The engine ran great but now I need new pistons. Amazingly the cylinder walls are fine. To prevent spark knock I’ll like to lower the compression to approx 9.6:1 so I don’t have to add 2.5 gallons of 110 to a tank full of 93 octane w/ethanol. The engine has already been bored .030 over and has ‘69 RAIII heads. My mechanic checked around has not been able to find lower compression pistons for the 389s w/.030 overbore in stock. He says if I want lower compression for today’s gas I should have it bored another .030 and use 400 pistons, which he has found in stock. So my questions are two fold. Does anyone have a good set of 389 pistons for .030 overbore? And, will boring it out to 400 cu. in. cause any overheat or other problems? It’s an AT with AC car. Thanks!


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## armyadarkness (Dec 7, 2020)

I'll let an expert give more definitive answers on the internals, but for the record, I have a 67 with 670 heads, and I run all day on 93 octane. Also, with a smaller and under designed cooling system, I run cooler, in a state which is hot and humid.

If you set the timing right and you maintain the cooling system, those things are not issues.


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## PontiacJim (Dec 29, 2012)

armyadarkness said:


> I'll let an expert give more definitive answers on the internals, but for the record, I have a 67 with 670 heads, and I run all day on 93 octane. Also, with a smaller and under designed cooling system, I run cooler, in a state which is hot and humid.
> 
> If you set the timing right and you maintain the cooling system, those things are not issues.



The 670 heads are closed chamber heads and it seems they can tolerate more compression and timing and run OK on the pump gas. It may be due in part to the larger quench surface of the closed chamber head and packing the air/fuel mixture in tight for a better flame travel when the plug fires.

And yes, cooling has a big factor.


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## PontiacJim (Dec 29, 2012)

Roqetman said:


> I recently took my GTO to a shop to have the rear main seal replaced which required pulling the engine. My mechanic discovered a piece of piston skirt in the oil pan and the #4 piston was cracked on one side up to the rings. The engine ran great but now I need new pistons. Amazingly the cylinder walls are fine. To prevent spark knock I’ll like to lower the compression to approx 9.6:1 so I don’t have to add 2.5 gallons of 110 to a tank full of 93 octane w/ethanol. The engine has already been bored .030 over and has ‘69 RAIII heads. My mechanic checked around has not been able to find lower compression pistons for the 389s w/.030 overbore in stock. He says if I want lower compression for today’s gas I should have it bored another .030 and use 400 pistons, which he has found in stock. So my questions are two fold. Does anyone have a good set of 389 pistons for .030 overbore? And, will boring it out to 400 cu. in. cause any overheat or other problems? It’s an AT with AC car. Thanks!


You have just seen what "detonation" does. Luckily you did not have a major failure and lose the engine.

Contact Butler Performance. They can have the Ross forged pistons made anyway you want. If you can keep the bore at .030", I would do so as that leaves you more cylinder wall should you need to overbore in the future.



https://butlerperformance.com/i-31643659-butler-ross-custom-pistons-early-or-late-389-3-75-str.html?ref=category:1234874



With the RA III heads, open chambers, I would go with a compression ratio of 9.2-9.3 to be safer, 9.5 might cause a problem and still need a small dose of octane booster. You cam spec's will also make a difference. A wider Lobe Separation Angle will help. Do Not use a Comp Cams 110 LSA as it will build more cylinder pressure and that means detonation.


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## BearGFR (Aug 25, 2008)

I'll echo what PJ said. 

I'll say up front that they're pricey, and right now with all the stuff going on with covid you'll probably have to wait a long time (8 weeks or more) to get them, but through Butler you can have a set of Ross pistons custom made to any bore size, dish volume, compression height, etc. 

I'd be really hesitant to bore your block any further than it already is if it doesn't actually need it. Once you get that 389 to + 0.060, what will you do when it eventually wears and needs more for you to keep running it? You'll be seriously short of options then.

If the block you have now is the one the car was born with, if it were me I'd do anything/everything in my power to preserve it for as long as possible. 

When I built my engine "the first time" more than 10 years ago now, I ran a set of real Ram Air IV #722 cast iron heads on a 461 at 9.5:1 using a moderately healthy solid roller cam from Comp with a 110 LSA, 236/242 duration at 0.050, and 0.600 lift at the valves and it was fine on straight 93 octane pump gas. I had to really do some 'ugly' things to it to get the compression down that far on a 461 with those 72 cc iron heads, including big ROUND piston dishes and fat head gaskets (which meant the quench properties in that engine were horrible), but it didn't rattle. Unfortunately, one of those heads eventually developed a crack in #6 exhaust port. The guy I bought them from had been running them on a 10.90 Firebird Super Street race car and his backyard porting job got them a little too thin there. I had them pressure tested prior to using them and they checked good, but I learned that running on the street with the associated heat cycling is a different proposition than running on a race car. I replaced them with a set of professionally ported 72 cc Edelbrock RPM's. The E-heads actually out flow and make more power than the RA IV's, but I really enjoyed seeing other Pontiac freaks get bug-eyed when they spotted those 722 casting numbers. I've since upgraded the engine even more and am in the middle of yet another build right now, and I'm still jumping through hoops to keep my original-to-the-car block despite having significantly 'surgically enhanced' it internally.

Bear


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## LATECH (Jul 18, 2018)

I have a 65 389 and it is bored to 400 (4.1200 )
RUNS KILLER.
I used set of flat top stock 400 forged piston (trw) NOS
I also used a set of heads from a late model to get a 82 CC chamber . Heads were milled pretty heavy to get there
Be aware the 400 piston valve reliefs are for a head with a different valve angle and the pushrods are different as well (longer)
custom pistons for the 389 might be the fastest easiest bet for you

If you do decide to bore is to 400 sonic check the block well before hand.
those 65 -66 blocks were pretty meaty so you should be OK


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## Roqetman (Aug 31, 2015)

Pontiac Jim, thanks for your input. I agree with you and really do not want to bore it another .030. It is the original 389 block (verified by the Protect-o-plate EUN, just in case there are any skeptics out there). The cylinder walls are fine, got lucky there! The RAIII heads are 67cc. The cam, installed by previous owner, is a mid rpm range cam for 2000-4600 rpm power. It, and the lifters are in good shape, and are already broke in. I will contact Bulter about the custom pistons. As BearGFR recommended, I will also check the LSA on the cam spec’s. I really do not want to over bore this block if possible. Thanks!


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## Roqetman (Aug 31, 2015)

Lateach, my ‘65 has heads off a ‘69 400 RAIII, so the valve reliefs for 400 pistons should be ok, correct? That is if I decide to go with the 400 over bore. Thanks!


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## BearGFR (Aug 25, 2008)

Roqetman said:


> Pontiac Jim, thanks for your input. I agree with you and really do not want to bore it another .030. It is the original 389 block (verified by the Protect-o-plate EUN, just in case there are any skeptics out there). The cylinder walls are fine, got lucky there! The RAIII heads are 67cc. The cam, installed by previous owner, is a mid rpm range cam for 2000-4600 rpm power. It, and the lifters are in good shape, and are already broke in. I will contact Bulter about the custom pistons. As BearGFR recommended, I will also check the LSA on the cam spec’s. I really do not want to over bore this block if possible. Thanks!


Have your heads been cc'ed and confirmed as being 67 cc's, or are you using the factory specs? I ask for several reasons. Pontiac heads are know to vary from the factory specs, and also your heads have been shaved at some point. It's not all that hard to build a tool to cc your own heads. You don't have to buy a read-made set to measure them. I can share how I made mine if you're interested. 

When you're building an engine that may be sensitive to compression, precision matters a lot.

Bear


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## PontiacJim (Dec 29, 2012)

Roqetman said:


> Lateach, my ‘65 has heads off a ‘69 400 RAIII, so the valve reliefs for 400 pistons should be ok, correct? That is if I decide to go with the 400 over bore. Thanks!


Yes, you want the valve reliefs to match the 400CI heads. Sometimes not really needed with a deep dished piston because of the dish, and/or if the valve lift/valve overlap is not too radical and kept closer to factory specs. Some aftermarket pistons will have both valve reliefs for the 389 and 400. But if you go custom pistons through Butler, they will add the correct valve reliefs.

If someone put the 400 heads on the 389, the scallop cut into the block around the valves is different to allow better breathing of the larger 400 valves. I believe you can use a 400 head gasket which will indicate where this scallop should be made in the top of the cylinder bore. Obviously you can go without it if your engine has not been modified, but the valve can get close to the bore and is shrouded by the cylinder bore. The pushrods from the 400 should be used as one of the 389-to-400 builds I have says the pushrods are slightly longer than the 389.


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## lust4speed (Jul 5, 2019)

The great thing about the custom Ross pistons is if your bore looks good at .030" then your machinist can simply hone a few more thousandths and the pistons can be ordered for .035" over. As long as you are at it figure out what piston dish is needed to arrive at no more than 9.3:1 compression if you have 91 octane in your area. If you have a very mild cam then 9.1:1 is a little safer. Let the Butler tech know that you have later heads or they will see "389" and order pistons with the earlier eyebrow locations.


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## BearGFR (Aug 25, 2008)

And, since we're in "custom piston" territory you don't have to stick with standard sizes and the normal overbore sizes. I recently had to pull mine apart again (long, boring story) due to an issue in one cylinder that my previous (and now former) machinist told me "would be fine". It wasn't, so my block had to be punched out another 0.005 to clean it up. My finished, honed bore size is now 4.160. When I ordered pistons, I provided that finished bore size to Ross and they made me a set to match with the proper clearance "baked in", as well as with D-shaped dishes with the volume I specified to put my compression ratio exactly where I wanted it for the heads I'm running. I had the the cylinders prepped before I ordered the pistons. With custom, you can specify every critical dimension: finished bore size, dish (or dome) volume, compression height, ring groove width, valve angle, etc.

Bear


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## Roqetman (Aug 31, 2015)

Bear, thanks for input on the 400 head gasket and push rods. As an update, I decided to go with the overbore and use Keith Black forged pistons for a 400, I hope this doesn’t come back to bite me! The Ross pistons from Bulter are about 8 weeks out and since I have 400 heads I decided to go that route. My mechanic verified the heads are 67 cc. Final compression will be 9.5:1. My car has factory AC and we’ll be replacing the water pump. I’ve read that cast impellers flow better than the steel impellers, steel is what’s on it now and it’s relatively new But the AC belt seems to be causing some water pump chattering when engaged. What’s a recommended water pump for this set up? Thanks everyone for you input. Sorry it’s taken so long to get back to you.


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## Roqetman (Aug 31, 2015)

Another question regarding the water pump and plates. The 389 plates have one hole on one side whereas the 400 has one hole on each side. Which plates should I use for a 389 bored out to 400? Also, my harmonic balancer has no weights. Will I need to change it out too? If so, what is recommended?


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## BearGFR (Aug 25, 2008)

Use whatever matches your timing cover, that's the determining factor. Yes, the cast impellers work better than the stamped steel ones, and getting the clearance between the impeller and divider plate is of critical importance to cooling.

Balancer: I know on the flywheel/flex plate it matters to know whether your rotating assembly is internally balanced or externally balanced. The most important thing about the balancer is what kind of shape it's in, if the rubber has started to degrade, if the outer ring has slipped, is it the right one for the engine, etc. If there's any doubt, replace it.


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## lust4speed (Jul 5, 2019)

The 8-bolt water pump used through the 1967 year does have water passages on each side and will cool just as good as the 11-bolt water pump that has the more easily seen water channels. Three of the coolest running cars in our GTO club have the early style pumps. It does take more care to get things right since the early pumps have two plates and more fitment time is needed. As BearGFR said, the important thing is to get the impeller/plate clearance down. Tighter the clearance the less cavitation and more volume the pumps will produce.

Both the early and late balancer assemblies have weights. The later style has the big ring on the outside of the hub while the earlier balancer assembly with the 8-bolt setup has a weight that bolts in front of the pulleys. Best thing about the early bolt on weight assembly is the hub is solidly attached to the crank and the timing mark is mechanically linked. The later style weight ring with the timing mark rides on rubber and can slip on the hub giving false timing readings.


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## Roqetman (Aug 31, 2015)

Regarding water pumps and fitment of divider plates, I see that Ames has their version of the cast impeller pump similar to Flow Cooler, is the Flow Cooler worth the extra money? And… What are the clearance specs for the plates and impeller. I’ve yet to find a good YouTube video on adjusting that clearance so if anyone knows of one please let me know. Thanks!


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## lust4speed (Jul 5, 2019)

Simplest answer is any clearance is enough. The impeller pushes coolant towards plate as the coolant slips past the vanes so if anything the clearance will increase very slightly in use. I set the water pump in a vice with the impeller facing up and set both plates on the pump. The clearance can be seen through the center hole and with the 8-bolt pump you want just enough clearance so there is no rubbing as the shaft is turned. Some pumps will have the impeller a 1/4" away from the inner plate and other manufacturers will be almost close enough out of the box. Just tap the plate towards the impeller evenly until you are happy with the gap. Also those two plates should set in the timing cover housing and be flush with the gasket surface and the bent edges should form a fairly decent fit to the two side outlets. Not much finessing you can do here but usually you can improve on the fit. I've used dabs of JB Weld where the plates rest on the timing cover to help bring the plates up to the gasket level.

The 11-bolt plate is easier to fit since you can tap the plate down until it just touches the impeller since there is a gasket between plate and pump that adds to the clearance.


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## BearGFR (Aug 25, 2008)

The official clearance is "as tight as you can get it without it actually rubbing" 😁


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## LATECH (Jul 18, 2018)

Roqetman said:


> Lateach, my ‘65 has heads off a ‘69 400 RAIII, so the valve reliefs for 400 pistons should be ok, correct? That is if I decide to go with the 400 over bore. Thanks!


YEP


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## Baaad65 (Aug 29, 2019)

I don't mean to hijack anything but reading about the water pump adjustments here I have a question or concern, I have an 11 bolt flow cooler pump on my 461 that was all assembled by Butler and a couple of years ago I blew out the pump bearings running my belt to tight. So I ordered the same flow cooler pump and slapped it on not touching any of the plates, so is that ok or should I have done any adjustments to anything? Thanks for letting me butt in.


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## lust4speed (Jul 5, 2019)

BearGFR said:


> The official clearance is "as tight as you can get it without it actually rubbing" 😁


That's what I was trying to say, but you said it better.

Baaad65, if the plate was adjusted to the first pump then you should be fine since Flow Cooler should have pretty tight production tolerances on the same model water pump. Most manufacturers error on the loose side since too much clearance doesn't get noticed while not enough will generate noise complaints and a returned product. So if you didn't clearance the first pump, the second will probably be off by a like amount.


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## Baaad65 (Aug 29, 2019)

lust4speed said:


> That's what I was trying to say, but you said it better.
> 
> Baaad65, if the plate was adjusted to the first pump then you should be fine since Flow Cooler should have pretty tight production tolerances on the same model water pump. Most manufacturers error on the loose side since too much clearance doesn't get noticed while not enough will generate noise complaints and a returned product. So if you didn't clearance the first pump, the second will probably be off by a like amount.


Ok that's a relief, no noises 👍 here's why you don't overtighten your belt 😉


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