Dished pistons for 1966 389 - Pontiac GTO Forum
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post #1 of 16 (permalink) Old 01-28-2019, 05:15 PM Thread Starter
 
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Dished pistons for 1966 389

Hi Guys, looking for some input on dished pistons. I am starting a restoration on my 1966 GTO Hartop. It has original 389 Tripower which I believe has 10.75 :1 compression ratio. I have read many posts on how to lower the compression so that I can run the rebuilt motor on 91 or 93 octane. Unfortunately I am unable to find someone who sells BRC dished pistons which I am told will do the job of lowering compression. Can someone point me in the right direction or is there a better solution. The posts i was reading were older from around 2013. It seems that Pontiac dude in Florida is not operating anymore?

thanks
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post #2 of 16 (permalink) Old 01-28-2019, 06:28 PM
 
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Originally Posted by opieone28 View Post
Hi Guys, looking for some input on dished pistons. I am starting a restoration on my 1966 GTO Hartop. It has original 389 Tripower which I believe has 10.75 :1 compression ratio. I have read many posts on how to lower the compression so that I can run the rebuilt motor on 91 or 93 octane. Unfortunately I am unable to find someone who sells BRC dished pistons which I am told will do the job of lowering compression. Can someone point me in the right direction or is there a better solution. The posts i was reading were older from around 2013. It seems that Pontiac dude in Florida is not operating anymore?

thanks
You will probably have to get them custom made. Butler Pontiac uses Ross pistons. bigD should chime in as well as he can usually come up with a couple sources that offer custom dished pistons.

You want o be around 9.0-9.3 on the compression, but closer to 9.0 is a safer bet with iron heads.
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post #3 of 16 (permalink) Old 01-28-2019, 07:34 PM
 
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Right off, I don't know of any 389 dish shelf pistons. But I've never built a 389, so I'm not up on what's out there. But, I assume Auto Tec can make a set, using shelf blanks in a size that will work for you.

From a quick search, looks like the 389 stock bore size is 4.062. So, .030 over is a 4.092 bore, & .060 over is a 4.122 bore.

Your machine shop will have to determine the smallest over bore your block will clean up. Let's assume that it will clean up at around .030 over. That means that a 4.10 bore piston should work. Don't know if there are any other popular engine combos that have a common 4.10 bore over size piston.

I'll do some searchin & see what I can find.

OK, there looks to be lots of 4.080 & 4.090 pistons out there. Looks like 360/390 Fords, some Mopars, and they even have lots of 4.080 pistons for sbc engines. So, Auto Tec should have some shelf blanks in both sizes.

https://www.summitracing.com/search/...iew=Horizontal

https://www.summitracing.com/search/...iew=Horizontal

Shannons Engineering lists some Auto Tec 4.080 bore pistons for LS engines.

https://shanonsengineering.com/colle...at-top-pistons

Of course, if you can(and want to) bore to 4.120, there are some Pontiac 400 std bore pistons that will work. Lots of guys have had a dish machined into the SP forged pistons. It is said that the tops are plenty thick enuff to machine a nice D-shaped dish. That should make up for the difference in valve relief location. After looking at the SP piston tops. You might get enuff relief by just machining out that center section, between the valve reliefs. You know they can be cut at least as deep as the valve reliefs are.

https://www.ebay.com/itm/Speed-Pro-T...vh9s:rk:4:pf:0

Then there are these Icon pistons which have reliefs for 389 & 400 engines. The reliefs have a 10.80cc volume. That's similar to a small dish. Could remove some more material by just grinding down the sharp edges of the reliefs.

https://www.summitracing.com/parts/u...saAiNHEALw_wcB

https://butlerperformance.com/i-2445...ef=brand:99961

Last edited by bigD; 01-29-2019 at 08:56 PM.
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post #4 of 16 (permalink) Old 01-29-2019, 01:09 AM
 
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A decent set of of forged TRW can be dished d by any good machine shop. I had my 400
done with with 72 cc 12 heads dished 14 cc for cheap money. I think he charged me less than 10 per piston and got C/R down to 9.5/4 . Its a kick ass motor 360hp with 460 lbs of torque and runs on 89 octane gas. . Doug
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post #5 of 16 (permalink) Old 01-29-2019, 10:23 AM
 
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A decent set of of forged TRW can be dished d by any good machine shop. I had my 400
done with with 72 cc 12 heads dished 14 cc for cheap money. I think he charged me less than 10 per piston and got C/R down to 9.5/4 . Its a kick ass motor 360hp with 460 lbs of torque and runs on 89 octane gas. . Doug
Yeah, that's probably the cheapest way to go, IF he is willing to bore his block big enuff for the 4.12 std bore 400 pistons.

Here's a set for $306 shipped.

https://www.ebay.com/itm/SPEED-PRO-L...xsfK:rk:2:pf:0

Last edited by bigD; 01-29-2019 at 10:42 AM.
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post #6 of 16 (permalink) Old 01-29-2019, 06:42 PM
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Helped two friends with two 389's, both .030" over '65 engines. Both guys ordered Ross pistons with a dish of about 13cc's. Price was about $550 and this was 8 or more years ago. Both engines scream on 89 octane fuel.
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post #7 of 16 (permalink) Old 01-29-2019, 07:34 PM
 
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Helped two friends with two 389's, both .030" over '65 engines. Both guys ordered Ross pistons with a dish of about 13cc's. Price was about $550 and this was 8 or more years ago. Both engines scream on 89 octane fuel.

I think the shipped price for Ross dish pistons, plus pins, plus shipping is over $800.

https://butlerperformance.com/i-2445...tegory:1234850

Auto Tec pistons will be closer to $600, or a bit less. Also, the Auto Tec pistons are made of 4032 material, which expands less. The Ross pitons are made of 2618 material, which is used in racing pistons, & expands more when heated.Many agree that the 4032 material is better for street engines. 4032 pistons require less piston to cyl wall clearance.

http://racetecpistons.com/pages/autotec.php

I think I've mentioned that if you want Auto Tec Customs there are some Pontiac engine builders who can order 'em for you. When I called Auto Tec, they told me to contact Shannons Engineering. Apparently he's a high volume dealer for Auto Tec. He quoted me a price of about $450, plus shipping, for some Pontiac 350 pistons. That was maybe 3 or 4 years ago, now. They have gone up a little since then. But, I'm guessing that $500-$550 plus shipping, should buy the pistons & pins.

Next cheapest piston choice appears to me to be the 4.120 Icon pistons I mentioned, then grind off the sharp edges of the valve reliefs, to reduce compression a bit.

Cheaper than that is the Speed Pro L2262F 4.120 bore pistons, then remove enuff material in the valve relief area to reduce the CR.

Did a little digging in the DSS piston cat. THEY OFFER A 389 PISTON WITH A 14cc DISH. The part number looks to be 6105X. Their 389 pistons come in 4.080, 4.100, & 4.120 bore sizes. Don't know what their direct price is. Summit sells their 2-relief pistons for $570 shipped, but don't stock any. So, I assume they could order any DSS pistons & have 'em shipped direct from DSS.

https://dssracing.cld.bz/DSS-ONLINE-.../54-55/#zoom=z

https://www.summitracing.com/search?...tons%20pistons

I called DSS. They do not stock any of those pistons. But, they can be ordered thru Summit & shipped direct, after they make up a set.
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Last edited by bigD; 01-30-2019 at 03:40 PM.
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post #8 of 16 (permalink) Old 01-30-2019, 01:51 PM
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That is some EXCELLENT information, bigD!!! Thanks!
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post #9 of 16 (permalink) Old 01-30-2019, 02:42 PM
 
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That is some EXCELLENT information, bigD!!! Thanks!
And here's more.

For those who can, and don't mind boring to 4.125, there are SP L2323F std forged 455 Olds dish pistons. The pin height is 1.735, so you'd need to have a little machined off the tops, to get down to zero deck height. But, they have a .142 deep dish, so that would be OK. These look to have the same pin size as Pontiac. So, unless there is something I'm not seeing, this should be a good piston for a 4.125 bore 389. Not sure how much cam these pistons would allow. ???

And the price is quite reasonable, similar to SP 400 Pontiac pistons.

https://www.ebay.com/itm/Speed-Pro-T...H4BP:rk:5:pf:0

https://www.ebay.com/itm/Speed-Pro-T...Y5wx:rk:2:pf:0

Here's a quote for a guy on the PY forum. "I and many others here have taken a 389 out to .090" over without issue when staying within a 3.750" stroke."

Obviously, each block must have the cyl wall thickness checked, to see how much they can safely be bored.

Last edited by bigD; 01-30-2019 at 03:17 PM.
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post #10 of 16 (permalink) Old 01-30-2019, 04:11 PM Thread Starter
 
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Thank you so much for the great information. I will be sure to pass this along to my rebuilder and we can make an informed decision on how to proceed.
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