Has anyone figured out the DOHC pistons

[sd_iconoclast]

I am starting a performance rebuild of my '92 TDC. I still cannot figure out which pistons to buy. The last time I looked into this, it turned out that most companies offered an early piston and a late piston. The consensus was that the early piston was the 9.25:1 early automatic piston. Nobody seemed to know what the results would be of putting the late piston in an early motor.
Which off the shelf piston can I buy if I want 9.75:1 (or better) compression.
Has anyone bought custom pistons? How much did they cost?
Bill Temple

[SappySE107]

The heads in 96 were what raised the compression. I havent CCed the newer pistons, but I doubt they will be raising the compression much if anything. Custom pistons will cost about 550-600 I believe. The place im trying to have make a set is the slowest company ever.

[sd_iconoclast]

Custom pistons will cost about 550-600 I believe. The place im trying to have make a set is the slowest company ever.

Ben,
What company is that? What will your custom pistons be like (compression, weight, forged? etc)? Would the cost or time go down if I ordered a set too?
Do the later heads have smaller combustion chambers? I hear about alot of people getting their pistons from Keith Black. What compression ration do these give with '92 heads?
Any info you have would be helpful.
Bill

[SappySE107]

I think its BRC...the phone number is 1-800-522-7478 and ask for Peter. He could give you any information, just let him know Ben gave you his number regarding 3.4 DOHC pistons. I sent him a rod, piston, piece of the head with the cumbustion chamber, and 4 valves so he can make custom sets. I dont know what compression is possible while keeping it non interference, but thats up to you. They are forged, weight unknown.

The 96 heads have 51.5cc chambers, the 91-95 heads are 54cc. I know nothing of keith black pistons, or compression ratio using them, you would have to talk to them.

http://www.brcperformance.com/

[Anonymous]

When Geddes Auto rebuilt my motor-I had forged aluminum pistons with a .30 over bore put in and the guy said they made for 9.75:1 compression along with the head decking. I do not know what brand of pistons though.

[3.4Z24]

i'm having wisco quote a set for me right now. i'll post their quote when i get it.

[Anonymous]

And my pistons do not interfere, the shop fucked the cam timing up badly, and when we turned it over they didn't go ca boom.

[SappySE107]

Interference would have the piston at TDC and the cam at full lift. The may not happen with bad cam timing. Even interference engines can get away from bending valves if none of the valves are at TDC, which our setup will stop the cams so none are at TDC. Its just when they are spinning from inertia after the belt breaks that they could be full lift at TDC on a piston or 2, or near enough full lift that they would hit (depends on how much interference there is.

[Anonymous]

true, so there is a chance that they would have hit or would have missed. They missed. But I wish they would have hit, then the shop would have had to pay for 24 new valves, 4 new cams, 6 new pistons, etc. That would be awesome!!!

[pocket-rocket]

I would see the lifters blowing before the cams getting hurt. Lifters aren't cheap either, lol.

[sd_iconoclast]

DOHC Lifters

That brings up the other issue I am trying to figure out. Where can I get the best proice on DOHC lifters? Are rebuilt lifters OK? I think I will start a new thread on the subject.

[sd_iconoclast]

I just Talked to Sealed Power Tech support (Phone number 800-325-8886).
They list two fifferent pistons for '91-97 3.4 DOHCs. PN H684CP and H813CP.
They said the following about their two different pistons available for the DOHC 3.4:
1] They cannot explain why H684CP is classified as "late" and H813CP "Early".
2] Both pistons have .040 domes
3] The domes are identical on both pistons.
4] The domes essentially cover the entire top of the pistons so both pistons look like flat tops.
5] Given the above, H684CP results in higher compression due to its higher compression distance.
6] Both pistons are hypereutectic. For some reason their listing on FederalMogul.com does not make this clear.

[BowtieKid79]

The dome on those pistons does NOT cover the whole top of the pistons... I know cause I have them. I cant say if they are indeed the same from one part number to the next, but I know the dome does not cover the whole top (not even close). I will try to find a pic of them when I get home. Another difference (that I know for sure) between the two part numbers is the rings. One # uses the same size 1st and 2nd ring and the other uses two diff size rings. I believe the 2nd design uses two thinner rings for less drag while the first design uses one ring that is thicker than the other.

[sd_iconoclast]

BowtieKid,
You are certainly right about the rings. I did not mention them because the Sealed Power catalog details that pretty well. I am VERY interested to see you picture od the piston tops though. I asked the guy at Sealed Power for a diagram of the pistons but he resused to give it to me. He said it was "against company policy". God forbid their customers should know what they are getting before paying.
I also thought it was interesting because the Sealed Power tech guy claimed the pistons were OEM reproductions. The pistons in my engines ('91, 92, and '95) all look like flat tops with eyebrows.
I would really appreciate those pics if you can manage it.

[BowtieKid79]

I will post a pic later tonight. The early style piston that they make is pretty close to stock. the piston is flat on the top like stock, but the skirt was redesigned (for the better). I honestly dont think they guy you talked to had any idea what he was talking about. The page with the piston specs that used to be linked from the 60 page (under DOHC pistons) is no longer up. I found the new version of that page from another site and they dont even list the old style anymore. Maybe they stopped making them? Anyway, the comp. height on the domed pistons is slightly lower than the height of the non-domed (they also have bigger valve reliefs because of the dome) so i think the CR is going to be very close no matter which style you use. The heads are what changed the CR for the most part. I'll get you some pics when i get home in a few hours.