I am building a TPI 350 engine to replace the old 305. I use the little brother of the ZZ4 partial engine # 12556121 and have some questions. The pistons that are installed are hypereutectic but with a dish of -13cc this will not make a big compression and to bring the compression I am planning to change the pistons for SPEED PRO coated H631CP or the KB Claimer 3418H. The point here do I need to re-balance the rotating assembly or I can take the risk? Using selected pistons with the same weight? I don’t have the total information of the block just I found that is the basis of the 350 Ram Jet but this one is only two bolts, so I am confused. Does some one have the total part list of the 12556121 partial block? I want to have 400HP at the flywheel and will use the 180cc AFR heads (milled to 58cc) and the Isky CAM 257/265. Will this combo get that power? Or I am fool? I have check the set with Dyno2000 and with a compression ratio of 10.66:1 and I use the seat-to-seat cam spec’s is that OK? Regards, Cobra289

Whenever you change any rotating assembly parts, it should be rebalanced. even a few grams difference in weight will cause an imbalance especially above 5000 rpm. If you want it to live for a long time, get it balanced.

12556121 is a factory part number and means except to a dealer.

Wow this would be difficult here in The Netherlands, they ask more than $1000 is there a way to do it yourself?


I bought from e-bay, very good guy.
Does this mean that only dealers can have the information?
A very important item that I am looking if for the flywheel, do I need an internal balanced or external?
I have the 153 t external balanced from the 305.
If you can help me with information please let me know.

Regards,
Cobra289

Sure if you have a $10,000+ balancing machine.

Factory part numbers don't mean much except to dealers. If you told someone you have 882 casting heads, they'll know what you're talking about but a dealer will want the part number. To them, casting numbers or any other code numbers mean nothing.

OK!
So no change for new pistons!

I that case I should run the original pistons -13cc dish that will give with the milled AFR 58cc a compression of 10:1.

Will those pistons accept the power of 400HP the quench will be 0.041" plus the dish recess.

As I told you this partial block are used on a Ran Jet 350 but with 4 bolts, do you know if those pistons are OK.
The top of the piston is original coated (GM)

Thanks
Cobra289

If you build this engine with the stock pistons and 58cc head your cr will be 9.4:1. if yu use GM's .028" thick performance gasket you'll be at 9.7:1.

The pistons are strong enough for 5500 6000rpm and 400hp.

Changing to a flat top will require rebalancing. Unless you match the new pistons weight to the old piston.

A Isky cam with 257/263 duration @.050" is pretty big.
Exactly which cam grind # did you choose.

Y not order the engine built the way you want it to be ( wth performacne flat top pistons etc) from one of the performance oriented engine builders that ship world wide.
There are many to choose form in both The US and Canada
That can supply a motor built to your specs by a a pro.

F-BIRD’88

Thanks for your replay, here is the situation, I have all ready the engine and also the Isky CAM that is the part # 211-257/265 and grind # RR-257/265 the figures are:
Hydraulic roller

Intake Timing Exhaust Timing
Duration 257° Duration 265°
Open 16.5° BTDC Open 64.5° BTDC
Close 60.5° ATDC Close 20.5° ATDC
Cam lift 0.310 Valve lift 0.465 Cam lift 0.323 Valve lift 0.485
Lobe center 112° Overlap 37° CAM advance 0°

I don’t think that this is a big cam those 257/265 are advertised figures, it is a nice cam with lots of torque and will work from 1200-5000 rpm
The duration @ 0.050 is: duration INT 209 and EXH 217. That is better figures. I will use 1.6 rockers.
Using Dyno2000 should I put the cam spec’s for seat-to-seat or @ 0.050 lift, because it give an other results, so I am confused.

The pistons are the same as the as the 330HP 350 Crate engine # 12361371. but the top is coated I think that the coating is Teflon.

The engine is new never used and I like it because it has the same block as the ZZ4 and it is 4 bolt.
The pistons and the crank are different.

It is called L31 4-Bolt Main GM Performance Parts. (Same name as the Vortec heads??????)

I found that the original GM piston can be around 800 gr. And see that my optional pistons are 535 gr. It would impossible to do a kind of cold balance matching the pistons would be impossible.

I was thinking is using the head gasket shim from FEL-PRO FEL1094 that has a thickness of 0.015 to bust some extra CR to get the 10:1

This the link of the Scoggin-Dickey

http://www.sdpc2000.com/catalog/514/...e-Assembly.htm

Thanks for your help.
Regards,
Cobra289

Here is a picture of the block before I painted in Midnight Blue

I didn;t know that you already had the motor. thought you were just planning to order a motor.

Are you sure that the piston weight specs include the piston pin in both cases. The 800gram weigth sounds like w/pin weight and the 535 gram spec sounds like w/o-pin spec.

That cam should work well for ya.
the thin shim gasket should work well. I've used it on many motors including a 12.65:1 compression motor. Never had a gasket problem with them.

I recommend you use a good anti corrosion additive in the cooling system (galvanitic action between the aluminum heads and steel gasket) and retorque the heads after a few heat cycles. 9 (running time)

Yes I know the pins are not included but both pin weights are equivalent +/- 145 gr. so that will not be a problem compared to the piston weight.

I am glad to hear that because this was also a point of concern for me. I am not afraid of the quench.

Certainly that is a point all ready take in account and I will clean the radiator before the engine is installed to get rid of all the old stuff there.
What do you mean with “9 (running time)” I understand the heat cycles but sorry not this one.
I will follow your advice of re-torque the head bolts because you have those good results, some web sites tells that this is not necessary, but I trust you more than the fast talk! Thanks.

Do you think that this pistons will held the 400 HP?

Gratings from the Netherlands,
Cobra289
http://www.donostia.demon.nl

Sorry just a typo. The pistons are strong enough for the power( cylinder pressure) and rpm. Cast pistons are not as strong as forged thou. The main advantage of a forged piston is its ability to withstand ***some detonation***.

The key here is "some". You want to avoid detonation and preignition in any case. ( too high compression ratio , over advanced ignition, lean AFR's, too ow octane gasoline
tuning mistakes)