[matrix]

Just got the car. Previous owner put 6x heads on a 406 and used domed pistons to up the compression. I would like to switch the heads to a smaller chamber. Can I have the domes milled off the pistons and rebalance the assembly or should I just get new pistons. Its a very low mileage motor.

[firebirdcrazy1959]

You can if there is enough material on top.I think i saw some where that .120 is the min thickness you can get away with.I may be wrong.

[Old Man Taylor]

You will have to call the manufacturer to see how far you can safely cut the tops. I did that a long time ago to a set of TRW 12:1 pistons. As I recall I was only able to reduce the CR by 0.5 points, but that was enough to work for me at the time. In your case I would buy new pistons.

[Skip Fix]

If you look on the bottom side many domes protrude up and can;t be milled flat, but as Jim said maybe enough to help you out. If they are the old TRW dome they are pretty heavyalso.

[screamingchief]

I would like to switch the heads to a smaller chamber. Can I have the domes milled off the pistons?

Any particular reason why you desire to do that?

Would be interested in what you would expect to gain/change from that???

Even if those 6x's were dead-stock 92 CC 6x4's the compression with a L2279F-30 TRW/Speed-Pro forging (likely what you have,as they're the only commonly available "dome" pistons for 400's) is going to come in at appx. 9.75:1,unless the thing has been zero decked or such as those pistons are usually appx .020" in the hole on undecked blocks.

And at the 6x4's "nominal" 89 cc spec they would be around 10.0:1.

And if they're -8's,then your around 9.0:1 with 100 CC's,and appx. 9.25:1 at the "nominal" 98 CC spec.

Sooo,I'd have to ask again,just how much smaller a chamber are you looking to use,and what are you expecting to gain from doing as such?

The domes on those pistons are only like +10 CC's,not that big at all,just a tiny ridge really.

If you want a pic of the underside of one of these pistons,just ask,I have a set here and can post some if you want,but for now I'll post the generic "factory" photo of the tops so you can see the dome is'nt much to cry about,in spite of what 99% of folks here will regurgitate to you ad-nauseum.

And usually the naysayers will predict doom from using domes with no "real world" actual experience to back such dogma up,they just repeat all the standard pontiac anti-dome "cliches" to support their case.

When you do get a chance to talk to a few of the folks that have actually used these pistons,most reports say no problems with them,and the few that did'nt have good experiences using them,usually did'nt follow common sense building practices (like "safe" compression ratios and such).

I'll grant that the domes may require a few small "concessions",like I would'nt advise pushing the CR envelope with a set on the street with "pump gas" (though I do the same with flat-tops...),and they may like a bit of added timing as they slow the flame front in the chamber slightly,but again,that stuff should fall into the "common sense" catagory of the hot rod builders book,and be easily dealt with by anybody who knows what they're doing.

I know of more than a few 400's built with those pistons and 92-98 CC heads,they do just fine "as is",so I would'nt change it much if it were mine.

As for the piston weight concerns mentioned,same deal as all TRW/Speed-Pro pistons,the pins are way too heavy,much more-so than the pistons themselves,so a lighter pin yeilds an easy 50 gram loss per piston,other than that they're not that bad a piston in my book,especially when trying to hit that difficult 9.0:1-9.5:1 zone on a 400 without head milling or other additional machining mods or such.

Only other issue with those are most end up with the low tension oil rings on them as that is all Speed-Pro offers for them,but total seal carries them with a standard tension oil ring in their classic race moly "file fit" series,and would be my ring set of choice for those pistons.

HTH,Bret.

[matrix]

Screamingchief,
You hit the nail on the head. I want to be in the 9.5 to 1 compression range but have heard a lot of bad stuff about the domed pistons. I did suspect that much of the negative stuff was overblown but figured since Im pulling the motor anyway now would be the time to investigate and make any desired changes. Maybe I will just buy a set of the lighter wrist pins (if they are not already there) and rebalance. Too many possible ways to go. Thanks to all for the info.

[Skip Fix]

I would sure contour the dome taking all sharp edges around it off. Some domes even make a "fire slot" around the plug area to improve flame travel over them. Butler had some nice mini domes I saw at one PSN that looked like that.

[Bruce K]

For the cost of what you are going to do, you can buy alot of race/premium mix gas. High compression is sweet.

[goquick]

For too many years Pontiac owners were told not to use domes. Personnally, I would rather start with a dome piston and shape it to thw way I want that also promotes a good flame front. Takes some time, but never leave a sharp edge and get it right. The Total Seal Classic rings are very good on those old dometops on the street too.

[speedshopmike]

matrix, before you do anything, see what slugs you actually have.
for example, the mountains in my 455 are 27cc - pretty huge.
thowring my two cents worth, it's rather quite common knowledge domes have several negative impacts on combustion.
given a choice, i would never run them; a flatop or inverted dome is going to have more efficient, controlled combustion and ultimately a more efficient engine.
as firebird59 above mentioned, i also recall .120" is the absolute thinnest you want a piston deck to get.
look up inside the piston; it'll be fairly obvious if they're hollow or solid.

[screamingchief]

Comparing those 455 dome pistons and the 400 dome pistons is a night and day comparison,apples and oranges as is often said,the 455 domes were made to overcome the low compression 455's with their HUGE 455 "smog" head chambers to yeild an appx. 10.0:1 ratio with a 114 CC chambers,and thus the dome reflects that with their size and design.

The 400 domes on the other hand,were primarily designed to allow higher compression ratios (11.5-12.0:1) when used with the 72cc chambers,so thus are much less significant.

I have agreed in the past,and will agree here again,that I myself have little reason to use the 455 domes,and would recommend the same to others,especially considering a decent head casting choice can yeild virtually any compression ratio desired for the 455's.

But that's not always the case with 400's...

Those 455 domes were indeed often equipped with the flame groove Skip mentioned earlier as the domes are quite significant on those.

Again,the 400 domes are'nt that "big" a deal,,,JMO...

[screamingchief]

For reference: Pictures below.

[Skip Fix]

Look kind of like old 302/327 popups we ran all the time! but then gas was good then, and CHEAP.

I would think contouring that one would work decent. Look where the spark plug sits in the chamber also.