The cylinder walls on my 389 have some pitting and need to be bored out. The problem is that they are already at .060 over. Can they be safely bored greater to clean this up? What do you do about pistons for something this size?

Would I be better off looking for another block, and if so, where do I find a good core?

The 389 is from a 1966 GTO.

Thanks
Ken

The cylinder walls on my 389 have some pitting and need to be bored out. The problem is that they are already at .060 over. Can they be safely bored greater to clean this up? What do you do about pistons for something this size?

Would I be better off looking for another block, and if so, where do I find a good core?

The 389 is from a 1966 GTO.

Thanks
Ken

When I built my 389, I had it bored to 0.060 over, but Jim Butler said that it used to be common to bore them to 0.090 over A 0.090 over 389 gives you almost 406 cubic inches. And, you can run later 400, big-valve heads that breathe better. Those heads, with dished pistons, can get the compression down to 9.5 to 1 so you can run pump gas. Give Butler a call and talk it over with him.

Punch it .60 and go with the standard bore 400 pistons! Easy job! [img]/infopop/emoticons/icon_biggrin.gif[/img]

Usually I'm found working on my Pontiac. If you want to find me Look for the white picket fence and all the old Pontiacs parked outside. And the half dissembled engine sprawled all over the driveway!
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If your .060 over 389 needs to be bored again you can go to.090 over and use .030 over pistons for a 400. Caution between 66 and 67 Pontiac changed valve angles so if you do this and use your 389 heads you may have a valve clearance problems depending on the cam you use. The plus is you can use newer style heads to get your compresion down so you can run pump gas. I've been through this and its a pain in the ass unless you do it right the first time, hope this helps.

About what year heads or numbers do you run to get the 9 or 9.5 comp ratio after getting to the .030 over 400.

thank you
stano

Any of the '67 -'70 400 72cc (actually between 72 and 78) big-valve heads will get you the 9 to 9.5:1 ratio with dished pistons. Some have nearly 80ccs of volume and can put you at 9.5 without a dish. Look for casting #670 ('67), #16 ('68), #48 and #62 ('69), and finally #12 and #13 ('70). The '69 #62 will usually be about 75cc or more, and the '70 #13 will be at least 78cc (best choice).

Another route would be to use the '75 5C or '76 -'77 6X 350 heads. Both should have a '4' stamped on the 'accessory' boss next to the valvecover rail to ID the smaller 92 to 94cc chamber volume as compared to the 98 to 101cc 400 heads stamped '8'. You will have to mill these heads pretty much to the max of about .060" to get into the low 80cc range (intake side of heads need to be milled too for intake fit) to see 9:1 on a 400 engine with flat-top pistons.

Use a compression calculator available for your use on a number of sites (or do the math yourself) to see exactly what volume head you need. Compression ratio also depends on how far the piston sits down from the block deck (zero-deck is best for efficiency and detonation resistance) , gasket thickness (usually .039"), as well as piston valve relief/dish volume so you must know what you have in the engine to know the proper chamber volume needed.

Let's go racing!
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Hobby,
What causes the valve clearance issue when boring to .090 over? Or is it the use of the 400-.030 pistons?

I want to keep the 389 heads - They are ported and built.

Ken

1967 Pontiac went to open chamber heads and changed there valve angle. This means the valve comes down at a slightly different angle. The reliefs in the tops of your pistons are cut to the same angle. If you use a high lift cam the valve may hit piston if you use 389 heads with 400 pistons or vis virsa. If you want to use your 389 heads you probably will have to buy custom pistons like from Ross. Nobody makes 389 forged piston anymore.