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#1
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421 question: Cost to machine recess to lower compression?
(looking at you, Bart Orleans!)
I have a line on a set of (8) 4.125 / 4.000 stroke Ross forged pistons that came out of a very low-time '65 421 build. My block could easily take the .032 overbore, and would like to keep the stock 4" stroke. Issue: The pistons are flat tops, which means only a pair of notches for valve clearance, and a static CR way too high for -092 /-093 heads if zero decked. If you have done this, what is a typical cost for the setup and cut from a reputable shop? Or does that compromise the piston strength/durability? Thanks! |
#2
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Orleans is a different Bart, not me.
Might not be enough material in the piston top to cut a dish deep enough to get where you want to be. My advice would be to have Ross make a dished piston set to your specs like I did. Expensive but worth it, you end up with what you want with no guessing. Cost would likely be in the $200 range to cut a dish, you really might not save much buying lightly used pistons and having additional machine work done to them. However again there likely isn’t enough material in the piston top to cut a dish deep enough to effect a drop in compression. Ross flat top pistons are lightweight with not much material there as compared to a heavy TRW or Speed Pro piston, a Ross dished piston is made differently to accommodate the dish.
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1964 Tempest Coupe LS3/4L70E/3.42 1964 Le Mans Convertible 421 HO/TH350/2.56 2002 WS6 Convertible LS1/4L60E/3.23 |
#3
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Many thanks.
I guessed that would be the case. Suspect that Ross may have different blanks for different purposes. Not building a 'points' car, but do want to keep it as original but rational for today's gas. Thanks! |
#4
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You have a few other options to consider. If you are sure you want to stay with the 093 heads, you may want to investigate having the chambers modified to an open chamber design similar to the 670 shape or go all the way to a heart shape if the casting will allow. SD Performance may be worth a call to see what they can do. Of course you can always add some race gas to take care of the possible detonation and a re-curve of the distributor will help too. I ran a 421 HO with BME flat top pistons, 2 valve notches and a .045 compressed thick head gasket. Static compression was 11.2:1. Light throttle, light cruise it was OK with 32 degrees total timing and limited vacuum advance. Rules would not allow any chamber modification for pure stock drag racing. But I drove it on the street allot. If I was going to get after it on the street I put 2-3 gallons of Turbo Blue 104 octane gasoline in 15 gallons of unleaded premium. Never had an issue with detonation. If allowed, I would have dropped the compression to 9.8-10.00 to 1 by opening the chambers.
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#5
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Thanks for the ideas on the -092/3 heads. Much appreciated.
B-Man: I've re-read the thread on your 421 build at least a dozen times, learn something new every pass, much better and more complete than most magazine stories. Hope the mill is putting the convert into orbit. Question on the early head, because so few people even mess with them other than a straight rebuild: Is there any gain "unshrouding" the intake by laying back the chamber wall a bit? Thinking a gentle cut could pick up some breathing room, so to speak. Also, for a 'zero deck' build, what is the minimum safe clearance between the piston dome and the deck surface. Know that it all grows when warmed, wondering how much will still leave room for thermal expansion, carbon buildup, etc. I will need therapy to cut the decks on my block; they still have the factory over/under markings on the deck from the trip through Plant 9 lo those many years ago. Thanks for the knowlege and patience! |
#6
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Quote:
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“It takes considerable knowledge just to realize the extent of your own ignorance.” Dr. Thomas Sowell |
#7
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I agree with first decking the block square (of course your machinist will be removing the least amount of material to achieve this) and then ordering your pistons with the proper pin height to achieve as close to zero deck as possible.
In my case I was looking for the pistons to be at .005”’above the deck since I was trying to get a .040” quench distance using a .045” crushed thickness Butler head gasket. As you probably already know my pistons ended up.007” to .008”’above the deck giving me a .037” to .038” quench distance. Generally most engine builders look for .040” for steel rod engines and .050” for engines equipped with aluminum rods. As tight as .035” is perfectly fine and I’m pretty sure some might even get away with a .030” quench distance. Carbon buildup will be minimized with a nice tight quench and a good tune. I really can’t say what laying back the chamber wall might achieve, on my 421 I pretty much let my friend who built the engine have free reign when he opened up the chambers. I think he was mainly looking to add a few ccs to help lower the compression and perhaps soften up the chambers a bit to help prevent detonation.
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1964 Tempest Coupe LS3/4L70E/3.42 1964 Le Mans Convertible 421 HO/TH350/2.56 2002 WS6 Convertible LS1/4L60E/3.23 |
#8
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The old TRW's that we would dish had a massive amount of meat in the piston top and we could remove a quarter inch and there was still a very safe amount of aluminum left. Modern piston design just doesn't have any excess thickness at all. Flip the Ross piston over and note that the bottom contour follows the eyebrows and is consistently thin. The underside of the old TRW's was as flat as the top.
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Mick Batson 1967 original owner Tyro Blue/black top 4-speed HO GTO with all the original parts stored safely away -- 1965 2+2 survivor AC auto -- 1965 Catalina Safari Wagon in progress. |
#9
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Just for reference, I just bought a set of Ross dished through Butler for my 389 with #76 heads. 12cc dish, pins, locks and rings ran $955 with shipping. It took about 6 weeks to get them.
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"I know just enough to keep me here, but not enough to get me out" |
#10
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DDS makes a 389, 14 dished piston that you can have the pin height changed on them. Their pistons are of real quality and much nicer then Ross. They will also make you anything you may need. I use they pistons on all our builds
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#11
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Might wanna consider Auto Tec. pistons. They're made from the 4032 material, rather than the 2618 stuff, that race pistons use. It is stated that the 4032 material expands less, therefore requires less piston to cyl wall clearance. Guessing that the cost of pistons with pins will be less than $600, depending on what dealer you buy thru.
http://racetecpistons.com/pages/autotec.php When I called 'em, they told me I could buy cheaper from some dealers, than directly from them. When I ask, they recommended Shannons Engineering, as one of their high volume dealers. At the time, he priced me some custom 350 pistons for just under $500, shipped. https://shanonsengineering.com/produ...at-top-pistons I've read here that Paul Knippen & Paul Carter can also order 'em for you. https://www.facebook.com/PaulKnippensMuscleMotors/ Last edited by ponyakr; 08-21-2019 at 11:03 AM. |
#12
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DDS pistons are also 4032 alloy.
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