[n2ogpsj462]

You guys ok with .004? I knows its a bit loose but SRP website says 4-4.2 bore 2-25thou and 4.2-4.4 bore 3-35thou. Adding .001-.003 for boost or nitrous. I'm at 4.185 using common sense I think 4 is fine but my machine shop wants new pistons and 40 over bore.
Positive and negatives. New piston will cure my questionable compression but I didn't plan on 900$+ extra for pistons and boring. Taking my roller cam money away!

[Tom Vaught]

Have them give you dimensions on the bore in the following locations:

1/2" below the deck

a) 12:00 down to 6:00 then 9:00 across to 3:00

1-1/2" below the deck

a) 12:00 down to 6:00 then 9:00 across to 3:00

2-1/2" below the deck

a) 12:00 down to 6:00 then 9:00 across to 3:00

3-1/2" below the deck

a) 12:00 down to 6:00 then 9:00 across to 3:0

Once you have those numbers you can tell how much taper/wear you have in the block.
(in basically 1" increments of ring travel downward).

Then post up the numbers. We have some very good block machinists here who can say if the block actually needs a rebore or not
and new pistons for your application.

Tom V.

Should take them no more than 20 minutes to check 2 bores in those locations with a good dial bore gage and write down the numbers.
If they are charging $100 per hour, then $50 and you will have your answer and can decide what you want to order from a good piston manufacturer if necessary. You can get pistons made with mm ring packs for practically any bore these days.

[n2ogpsj462]

Thanks Tom but I think I'm just gonna bite the bullet and get dished pistons. Deep down I kinda knew I'd be headed this way. My wife don't know money is quickly turning to wife do know money! I'm still going to smooth back the valve shrouds to get rid of sharp edges so they should end up around 100-101cc. Not sure how accurate the Wallace calculator is but with a 4.190 bore, 15 cc dish 100cc chambers and .005 deck it has me right at 8.6

[Tom Vaught]

Nothing wrong with a good dish piston and 8.6 to 1 is a nice boosted compression ratio for gas or with e-85 as high as 38 psi (ask Charlie66 about that deal).

Tom V.

[taff2]

I ran .0045" on my 421 turbo combo and with the 3/4 filled block that was a bit tight- had some scuffing on the skirts.

[n2ogpsj462]

im going up there tomorrow to talk with him anyway to go over a plan. he told me over the phone my bores were 4.185 but didn't mention taper. Ill get pistons anyway but if I can get without boring I will. everywhere I look I see guys going 4-5.5 with there boost builds. This is a VERY reputable but anal shop but I don't want him putting these in at 2-2.5. The icon pistons are 2618 vs 4032 that the SRPs are made of. cant remember which needs a tighter specs but I think its the srps because of less expansion. Either way these SRPs are basically new. 10yrs old but barely 5000 miles and literally look new so Ill sell them to offset

[chiphead]

Are these 2618 alloy? Those (at least from Ross) want .005 to .006" clearance in a nitrous/boost application. 4032 will want .0025 to .004 typically.

Are torque plates being used during the measurements? If the taper and OOR is within .001", I'd run it without hesitation. .002" or over taper or OOR gets dicy for lifespan of ring seal.

[n2ogpsj462]

Originally Posted by chiphead

4032 will want .0025 to .004 typically.

that's why when he called and said its at about 4thou I said "cool then im good". im going to talk to him and get the taper tomorrow. Im buying dish pistons but id rather not bore the block unless necessary. if Icon wants these 2618 pistons that loose then he can hone for clearance

[n2ogpsj462]

Taper was 1.5-2. While discussing my goals and needs I decided upgrading to pro gram caps and line boring. So it looks like my freshen up turned into rebuild plus upgrades. Are most guys just doing the center 3?

[Tom Vaught]

install the Pro Gram caps on the center 3 unless you have a belt driven Centrifugal Supercharger set-up.
Then a good idea to install a Pro Gram cap on the front main too. NA Just the 3 center typically.

Tom V.

[mgarblik]

You are getting ready to spend allot of money on your engine. As such, if you are going to be spraying N20 in any quantity, I would spend the extra $100.00 and put a front cap on. That cap is pretty wimpy and tend to crack at the dowel holes. The shock load from N20 will aggravate that situation. No need to worry about #5. It is a big, beefy cap and has 9/16" fasteners. Piston wise, and there is some disagreement in the forums, I would use 2618 T6 alloy in any race application. The high silicon 4000 series alloys are fine for street cars. For further information about why 2618 is the preferred material for race pistons, look over the tech briefs in the www.bmeltd.com web site. Good luck with your build.

[Tom Vaught]

Thanks Mike for the NOS info and the front main cap. Do not do NOS so no real knowledge on that and making the engine live.
You had tons of experience with the Grocery Getter and NOS.

Tom V.

[n2ogpsj462]

Thanks, the Icons use the 2618 and its also pure luck they stock a shelf 15cc dish which is exactly what I needed to get to my compression goal. As far as the caps, ill likely do the front as well even if it is overkill. Like you said, another 100$ is a no brainer