Hot Rod Forum banner

Sbc piston choice?

9K views 26 replies 16 participants last post by  jokerZ71 
#1 ·
I have a mildy built 350 sbc that should be around 350 hp with stock pistons. There is a problem with the motor so I have to pull it out and am going to get it bored .030 over and not sure what pistons to get. I dont want some crazy dome pistons or forged or whatever. I was kind of just looking to get a step up over stock pistons, Any ideas?

The heads are promaxx vortec heads with 64 cc combustion chamber. Also does anyone know the specs on stock 350 tbi pistons? Thanks
 
#5 · (Edited)
Agree with johnsongrass. There are 3 types of piston construction commonly used in internal combustion engines. Cheapest is the cast aluminum piston. Fits tightly to the bore. Quiet. Not very strong if the motor is not tuned and goes into detonation. Matter of fact, no piston will survive detonation. Second in the lineup is the cast hypereutectic (high-per-you-tech-tick). Still a cast piston, but the manufacturer has used an overage of silicon in the mix that's used to cast the piston. This piston can be used in most of the mild to moderate builds that we hot rodders do. Some manufacturers of these hyper pistons specify unusual piston ring end gaps, that's why johnsongrass mentioned that. Hypers will usually work ok in a blower motor also, so long as detonation is not encountered. Strongest and most costly of the mix is the forged aluminum piston. Doesn't get any stronger, but may be overkill for a street motor, depending on the amount of abuse it will have to withstand.

Most of us would agree that the Keith Black line of hypereutectic pistons would work well in a modest rebuild. Some of them, the ones for a lower static compression ratio, have a D-cup arrangement on the crown that allows lower compression ratios and also feature a step-type crown that, when mated up against the underside of the cylinder head, creates a really nice squish.

Be careful about the compression height of the pistons you choose. 350 motors are designed around pistons with a 1.560" compression height. You'll want to stay close to that and perhaps even a little taller. Avoid pistons with a shorter compression height, they will ruin the squish and make the motor more detonation-prone unless you cut the block decks to tailor the squish. Still not a good idea. Avoid shorter heights.

Here's an inexpensive hyper piston that I might use in a budget rebuild....
http://www.summitracing.com/parts/sum-17350-30/overview/make/chevrolet

.
 
#7 ·
It isnt to much of a budget would just prefer to keep it under or around 200 for the pistons. I was looking at the pistons you posted, would those be considered d-cups or just 4 relief valve pistons? Is the lower the compression height the more compression it has?
 
#8 ·
Those are 4-valve relief, 4- valve notch or 4-"eyebrow" pistons. D-cup would be a shallow "D" shaped dish.

The lower the compression height, the shorter the piston is from the centerline of the wrist pin to the top of the piston, making both compression lower and squish worse. You want to avoid that.
 
#9 ·
X2 for me.

Don't just get "rebuilder" pistons. They are designed for budget rebuilds where there isn't any real blueprinting or measuring. They have a shorter compression height to keep the pistons away from valves and heads for when you're just doing a quick rebuild and don't care about the performance.

Since its apart, take the time and measure deck height, rod length, etc, and get a compression height that is right for your application.

In general, cast is hard but brittle. It also tends to grow not very much when its hot so you can run tighter piston-bore clearance. Forged is stronger, but malleable. It grows more so you need more tolerance between the piston and bore so its "right" when its up to temp. Forged is often used for forced induction, or where really high cylinder pressures occur because it can take some abuse while absorbing higher impacts.

Hypereutectics combine some of the better qualities of those two and are perfect for performance street use. I used Speed Pro Hypers on my last 454 build and I was very happy. Not a ton of money, anti-friction coated skirts, good piston.
 
#16 ·
Here's a hypereutectic from Summit that I would use without hesitation in a street or street/strip application....
http://www.summitracing.com/parts/sum-17350-30/overview/make/chevrolet
And here are the rings....
http://www.summitracing.com/parts/sum-133-139-30/overview/
These are cast iron rings, use the proper hone for them. They will seat quickly and last for roughly 60,000 miles. If you want rings that will last longer, use plasma-moly rings.
http://www.summitracing.com/parts/sum-134-cm553230/overview/

Whatever rings you use, you will have to use the proper grit coarsness for that ring material. Cast iron rings will tolerate a rougher finish, while plasma moly will require a smoother hone job.

Here's a tutorial....
http://fme-cat.com/docs/1110.pdf

.
 
#17 ·
Mikes1500, Here is the part number for the piston and ring set you want. http://www.summitracing.com/parts/uem-9902hcktm030/overview/make/chevrolet. The compression ratio using a .039 thick head gasket with these pistons will be 9.773. If you are going to have your block bored take it to the shop first to make sure .030 will clean up the piston walls. The machinist will measure the block and tell you what size pistons you need. I have heard people on this site say you need to take the pistons with the block to the shop. I don't agree because what if you buy the .030 pistons from Summit Racing and then the machinist says .030 won't clean up the block that you need .040 pistons. Then you have to send your pistons back to get the right size. Maybe you can price the pistons and rings from the shop you use. If the price is close get him to order the pistons. The reason for him to have the pistons is the machinist bolts a torque plate to the block to simulate the heads and then he bores and hones the block to get the cylinders as perfectly round as possible. You should give us all the info about your build as you can it makes it easier to recommend what parts you will need.
 
#20 ·
Are they any good? $288 at Summit
D.S.S. is a really good piston, equal in quality to Probe or Keith Black FHR.

I would rather see guys use one of these three brands of tighter fitting forgings, and $35-50 standard moly rings than use a hypereutectic piston with fancy $100 plasma moly rings.

The forged piston with a regular moly ring is more durable, makes no sense to me to put more extreme duty rings on a hyper piston.

If you want to step up beyond that, put plasma moly rings with the forged pistons.

Sounds like for the OP's needs, the cheaper $100 Summit hyper's or $200 KB hyper's with a $40 regular moly ring set would be more than sufficient. :cool:
 
#21 · (Edited)
Mikes1500, this may help to understand the difference between the D-dish (also called reverse dome, d cup, or d deck dish), a regular circular dish piston, and a flat top piston with valve reliefs. The D dish piston is better for hi performance than a circular dish because you still have a quench pad that the circular dish loses.

 

Attachments

#22 ·
. If using a brand of pistons other than KB or custom made, measure and make sure the compression height (top of piston to center of pin hole) is actually what is spec'd... we've seen some that aren't... that are lower... if pistons sit .025" below top of stock block deck, I'd suggest about a .015" thick head gasket for good engine quench effect...

http://www.crankshaftcoalition.com/wiki/Quench

What Is The Ideal Quench Height? - Hot Rod Magazine

. You can also get into milling the block and different gasket thicknesses and piston compression heights, but milling may add $200... and weaken the block deck... and add other complications...
 
#24 · (Edited)
I' not a big fan of Hypereutectic pistons they are better than cast and less expensive than forged but I don't think this is a good place to go cheap.
I would (and do) buy forged "D" dish (reverse dome) pistons and tool steel rings with a moly top ring.

Be mindful of the deck clearance and head gasket combo you run you should be shooting for a total of .035 piston to head clearance to get the quench / squish area correct. A lot if felpro gaskets have a compressed thickness around .038 and if your pistons are .020 below the deck the squish won't be correct which could make your motor more detonation prone.
 
#25 ·
I'm not familiar with the ProMax heads.If they are aluminum,the $100 set of Summit hyper pistons will work very well for your use.For an iron head,the KB-193 D cup would be your best bet.Either measure,or,have the bores measured befor buying pistons as stated above.If the heads are aluminum,I don't think you're gonna want to use a .015 shim gasket.Have the block decked so that you use a gasket with adequate thickness for aluminum head to iron block.(I recommend a minimum of .028)This will give you a good seal,proper quench,& compression.Do the math.It will make or break your build & performance of it.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top