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Discussion Starter #1
Well, just got off the phone with the mechanic. Looks like my rear main valve has started to leak even more.....probably time to pull it and replace.

He mentioned last time I was in about a rebuild on the motor. It is the original and hasn't had a major overhaul in a good 15-20 years. So he's getting me a rough price for a rebuilt and the main seal replacement.

My question is, since I'm likely doing a rebuild is there anything I should have done that is extra? I'm not looking for a huge increase just something that if I spend a few extra bucks would be worth it.

Also, IYO what do you think a normal rebuild on an original 389 would cost? Besides leaks there is nothing really wrong with the motor. Would it even be somthing I should do?

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A normal rebuild on a 389 will cost you about 3 to 4k if you stay stock or close to it. Less if you assemble it. Why does it need a rebuild? burning oil? Knocking? Here's my story, and I'll try to be short: I rebuilt the 389 in my '65 in 1981....a year before I got the car. (already had the engine). It's still in there today. 32 years later. And it runs great. Yeah, it only has about 50k on the rebuild, but it's still leak free, which I can't figure out. (Must be those good old asbestos based gaskets!). I rebuilt the 400 in my '67 in 1988, at 173,000 miles. It actually only needed a timing chain and a valve job, but I didn't know as much then as I do now. I pulled it last year to replace the leaking rear main and the other leaking gaskets....a complete reseal job. Cost less than $200. It had run about 77,000 miles since I overhauled it. I inspected the bearings and checked the clearances. Still like new. I put it back together and put it back in the car. Been fine for the last 4,000 miles since then. My point is, it doesn't really matter how much TIME goes by when it comes to rebuilding (or changing oil for that matter). It's the actual condition of the engine. How it was stored. How it was treated. If it runs well but leaks oil and is grimy, pull it and re-seal it. Don't rebuild it. You can only bore a block so many times, and less is better.
 

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I completely agree. Don't tear the engine apart without good reason.
But, if you pull it, don't forgot all the little dress ups. Paint the engine bay and whatever else you can touch. Not gaudy, just clean it up.
 

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This is good information. My '68 GTO convertible did not have an engine, so I picked up a '70 400 that presumably runs. I was wondering how far I should "rebuild" it. It is good to know that a rebuild is not necessarily required. I am thinking replacing all gaskets, plugs, wires, points, filters, etc. Maybe even new timing chain and replace/rebuild Carb. Then finally, a possible valve job. Does this sound about right?
 

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This is good information. My '68 GTO convertible did not have an engine, so I picked up a '70 400 that presumably runs. I was wondering how far I should "rebuild" it. It is good to know that a rebuild is not necessarily required. I am thinking replacing all gaskets, plugs, wires, points, filters, etc. Maybe even new timing chain and replace/rebuild Carb. Then finally, a possible valve job. Does this sound about right?
Basically the right approach in my book, with one addition. I'd pull the heads and pan and take a look at a couple of bearings and the cylinder walls. That will tell you for sure whether or not you need to re-ring and replace bearings. If not, you've cleaned and painted everything and you'll have fresh heads.
 

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Basically the right approach in my book, with one addition. I'd pull the heads and pan and take a look at a couple of bearings and the cylinder walls. That will tell you for sure whether or not you need to re-ring and replace bearings. If not, you've cleaned and painted everything and you'll have fresh heads.
Well I assumed I was going to take the heads off, if for nothing else, I figured new head gaskets would be in order, plus the heads would need to come off for the valve job, so I figured that would be a good time to take a peak into the engine to see what I really have. On pan, I think you mean oil pan? I planned on that too for three reasons, 1) to replace gasket, 2) to see if there is sludge or metal shavings in pan, and 3) my oil pan is dented so I am thinking to replace it.
 

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Discussion Starter #7
A normal rebuild on a 389 will cost you about 3 to 4k if you stay stock or close to it. Less if you assemble it. Why does it need a rebuild? burning oil? Knocking? My point is, it doesn't really matter how much TIME goes by when it comes to rebuilding (or changing oil for that matter). It's the actual condition of the engine. How it was stored. How it was treated. If it runs well but leaks oil and is grimy, pull it and re-seal it. Don't rebuild it. You can only bore a block so many times, and less is better.
Not burning oil, except where it is leaking from, no knocking. Runs fine besides the fuel issue, which I believe was the timing being off and a loose distributer. And how hot she gets, even with the 2 row aluminum rad and electric fan(220-225 in AZ Summer)

Honestly the rear main is just leaking a little more. I was seeing maybe a 1/4 quart loss every other month....maybe less than 1/4. Its been garage kept unless I drive it obviously. Was garage kept for the last 15 or so year prior to me buying it too. Guess I'll just see about pulling to replace the rear main. I have a buddy that has an engine lift, would cost a lot less to do myself if its not too hard.
 

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Discussion Starter #8
Ok, so bumping this for an update and opinions.

We pulled the motor out this weekend to replace the rear main seal. Pulled the oil pan and the crankshaft has some blemishes on the outside counterweight lips. Like it was grinding a little. Also the journals aren't as smooth as they should be. I can feel a small nick, can't really describe it but they need to be smoothed out.

Didn't look at the pistons or much else, but probably doing a rebuild or at least a partial cause of the crank. So what should I look into? I plan on it dipping to get the old paint off. Again not sure if it needs bored out.
 

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In this day and age, I would take the whole thing to a reputable Pontiac oriented machine shop and have them prep the block, crank, and new rods and pistons. The block needs to be boiled out and the oil galleys need to be rodded out. You can't do it yourself. All of the galley plugs need to be replaced. Chevy machine shops don't do this, and huge problems arise. You should have the block zero-decked. You will need aftermarket upgraded con rods, and dished pistons for todays poor gas. This alone can be about $800.00. There are many, many threads on this forum about this exact question, with a ton of information. The biggest thing to remember is to NOT let a non-Pontiac machinist near the engine. Most machinists don't know the quirks of a Pontiac, and inadvertently do a substandard job. With luck, Mr. BearGFR will chime in and provide further information. He is very much "into" this topic.
 

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Discussion Starter #10
In this day and age, I would take the whole thing to a reputable Pontiac oriented machine shop and have them prep the block, crank, and new rods and pistons. The block needs to be boiled out and the oil galleys need to be rodded out. You can't do it yourself. All of the galley plugs need to be replaced. Chevy machine shops don't do this, and huge problems arise. You should have the block zero-decked. You will need aftermarket upgraded con rods, and dished pistons for todays poor gas. This alone can be about $800.00. There are many, many threads on this forum about this exact question, with a ton of information. The biggest thing to remember is to NOT let a non-Pontiac machinist near the engine. Most machinists don't know the quirks of a Pontiac, and inadvertently do a substandard job. With luck, Mr. BearGFR will chime in and provide further information. He is very much "into" this topic.
So will the afermarket con rods and dished pistions lower the compression? I don't want to lose HP.
 

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The con rods are so superior to the originals they are a must have. Durability, not HP issue. The lowered compression will be at 9:1 with the right amount of dish. If you run the right cam, no power loss. Built a 9:1 389 a few years back with a comp cams xe268 cam in a 4 speed 65, and it is a screamer. Runs on 89 octane, too. Does your current engine ping or detonate on pump gas? They usually do, IME.
 

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If you take it to a machine shop, tell them you want the cam degreed and a balance rotating assembly, don't just assume they will do that.
 

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Discussion Starter #13
The con rods are so superior to the originals they are a must have. Durability, not HP issue. The lowered compression will be at 9:1 with the right amount of dish. If you run the right cam, no power loss. Built a 9:1 389 a few years back with a comp cams xe268 cam in a 4 speed 65, and it is a screamer. Runs on 89 octane, too. Does your current engine ping or detonate on pump gas? They usually do, IME.
I had planned on them just machining the current cam, if it can be done. So whatever the original one is. I don't have either on pump gas.

If you take it to a machine shop, tell them you want the cam degreed and a balance rotating assembly, don't just assume they will do that.
Will do, anything else? I'm not very mechanically inclined.
 

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Discussion Starter #14
Ok so another question, I know the engine was rebuilt 20+ years ago but unsure if it was bored. Friend of mine is asking if I can find out whats the most you can bore these 389's out too? Just incase it has been done to a certain point already.
 

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.060". And it should be sonic checked. '66 389's are known to have thinner cylinder walls and core shift issues that the earlier blocks don't seem to suffer from. Nothing wrong with a '66 block, just saying less overbore is better. If it's already .030", go .035" or .040" if you can. After a 389 is punched .060", that's the last life it gets to live, unless you sleeve the block ($$$$).
 

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Discussion Starter #16
So after being gone for 6 weeks in San Antonio and a baby due any day, I was able to stop by the machinest and get whats been done and a final price.

Apparently I was close to spinning a bearing and the cast pistions and rods were decent but needed replaced. Decided with the forged pistions and everything with them. He will have to bore out the cylinder walls slightly. The cam degreed and a balance rotating assembly was included, came out to just over 2700 for total rebuild.
 
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