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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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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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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/part-type/pistons/bore-in/4-080-in?gnview=Horizontal

https://www.summitracing.com/search/part-type/pistons/bore-in/4-090-in?gnview=Horizontal

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

https://shanonsengineering.com/collections/pistons-rings-rods-parts/products/copy-of-auto-tec-small-block-ls1-ls2-ls6-l92-flat-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-TRW-Pontiac-400-Forged-Coated-Skirt-Flat-Top-Pistons-MOLY-Rings-std/171736599188?hash=item27fc4c8a94:g:-BYAAOSwG-1Wvh9s: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/uem-ic9946-std?seid=srese1&cm_mmc=pla-google-_-shopping-_-srese1-_-icon-performance-pistons&gclid=Cj0KCQiA7briBRD7ARIsABhX8aDh1xcWbx-QqKijzvsvJLjUpzfzFXUJJNA9KXJo_6nCqOrtDIYJ_jsaAiNHEALw_wcB

https://butlerperformance.com/i-24453315-icon-fhr-forged-flat-top-pistons-3-75-str-400-std-bore-389-058-over-bore.html?ref=brand:99961
 

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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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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-L2262F-Forged-Pistons-8-PACK-Flat-Top-for-Pontiac-400-6-6-STD/263691997897?hash=item3d654492c9:g:E9kAAOSwKPNTxsfK:rk:2:pf:0
 

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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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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-24453535-ross-racing-custom-piston-set-any-bore-any-stroke-flat-dish-or-dome-set.html?ref=category: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-CATALOG/54-55/#zoom=z

https://www.summitracing.com/search?gnview=Horizontal&SortBy=BestKeywordMatch&SortOrder=Ascending&keyword=dss 389 pontiac pistons pistons

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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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-TRW-Oldsmobile-Olds-455-Forged-Coated-4-Barrel-Piston-MOLY-Rings-STD/171057398913?hash=item27d3d0c481:g:0LgAAOSwr41XH4BP:rk:5:pf:0

https://www.ebay.com/itm/Speed-Pro-TRW-Oldsmobile-Olds-455-Forged-Coated-Skirt-4-Barrel-Pistons-Set-8-STD/171057390597?hash=item27d3d0a405:g:UOYAAOxyA9dSY5wx: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.
 

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Discussion Starter #10
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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Just had the 389 to my '65 GTO built this past year. My block had to be bored .060 which leads to using standard bore 400 pistons.

My machinist used Keith Black Hypereuctectic dished pistons. They have a 16-17cc dish and the valve pockets will work with the 389 heads. Take a look at Spotts Performance web site. They offer them for $339 a set unless the price just increased. The part number is KB346.

My engine has 9.5:1 compression and it made 340 H.P. @4,800 rpm/443 torque @3,000 rpm on the dyno. I use the stock points distributor, stock intake manifold with the original 500 cfm Carter AFB.


Best of luck to you.
 

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Just sent you a link on PY. They are talking about same issues. Best luck Doug
Thanks, but I started that thread, trying to find somebody who has used the DSS 389 dish pistons. Nobody posted that they have used them. So, I haven't found anybody yet who can give us 1st hand info.
 

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"...My machinist used Keith Black Hypereuctectic dished pistons. They have a 16-17cc dish and the valve pockets will work with the 389 heads. Take a look at Spotts Performance web site. They offer them for $339 a set unless the price just increased. The part number is KB346..."

They're $384 + shipping at CNC. So, that Spotts price may be old. But, if it's current, its the best price I know of.

https://www.cnc-motorsports.com/kb346-std-keith-black-pistons-dish-4120-bore.html


I wouldn't use the KB Hypers. Lots of guys have had piston failure, with those. The problem seems to be with the top ring. Unless you go with a huge end gap, the ends will butt together after the pistons warm up. Also, they are cast--not forged.

If you go with a 4.120 or bigger piston, there are stronger forged piston options.

I mentioned the SP L2262F pistons, with some material removed from the center, to reduce CR. I think this might be my choice, since I've had good luck with all the SP/TRW forged pistons I've raced.

https://www.ebay.com/itm/Speed-Pro-TRW-Pontiac-400-Forged-Coated-Skirt-Flat-Top-Pistons-MOLY-Rings-std/171736599188?hash=item27fc4c8a94:g:-BYAAOSwG-1Wvh9s:rk:1:pf:0

I also mentioned the KB Icon forged pistons, which have 8 valve reliefs. These will probably work, with some material removed from the valve relief area.

https://www.summitracing.com/parts/uem-ic9946-std/overview/make/pontiac

Again, if current, the Spotts price is cheaper, @ $399 + shipping.

http://www.spottsperformance.com/Pistons

Then there are the SP L2323F 4.125 bore Olds 455 pistons. Here's a set for $250. Will need to take a little off the top, to achieve zero deck height. They already have a .142 dish. So, machining the tops should be easy.

https://www.ebay.com/itm/Speed-Pro-L2323F-std-455olds-pistons/372581322266?hash=item56bf93ce1a:g:LdcAAOSwgotcTk0Z:rk:1:pf:0

I suppose you could lower the pistons in the holes, a slight amount, by offset grinding the crank, for a slightly shorter stroke. Don't know if that costs more than a regular crank grinding or not. The less the crank is worn, the more offset can be ground in. Might need to cut to a .030 undersize to get any significant stroke reduction. But, if you ever need another crank, I assume that most any Pontiac 350, 389, or 400 crank can be used. So, a good core crank should be real cheap. Lots of guys have tried to give Pontiac 350's away & couldn't. And, there are lots of extra 400 cranks, because of all the 400 block stroker builds, & because of so many cracked or otherwise unusable blocks.
 

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If you don't mind boring to 4.125, AND are OK with cast sealed power pistons, these might work without cutting any off the tops, & give you near zero deck, since they have a 1.725 pin height. They have a .120 deep dish.

https://www.ebay.com/itm/Oldsmobile-Olds-455-4-Barrel-Cast-Pistons-Cast-Romgs-Set-8-1968-76-9-75-1-STD/172122791986?hash=item2813516032:g:gUYAAMXQinlRk4Js:rk:1:pf:1&frcectupt=true&autorefresh=true

A guy on PY posted that he thinks the forged L2323F pistons, with their 1.735 pin height will work, without cutting any off the top. He thinks that you might need to grind just a little on some of the edges of the dish, for a valve reliefs. I assume that might depend on what cam you use.

"...The olds sealed power 455 piston is a close zero deck drop in for a 3.75” stroke pontiac with a 18cc dish, 1.735 ch. It won’t need any machining on the top. Might have to grind some off the edges of the dish though to clear the valves on a big cam..."

I've never heard of using that much pin height in a 3.75 stroke Pontiac. But then, I'm not an engine builder. Anyhow, the tops may not need to be cut any, or at least very little, unless a lot has been machined off the decks. So, you may be able to take advantage of the entire dish, or most of it.
 
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