Diamond Pistons Group Buy

I'm going to need a lot of help. Here are the questions I need answers to. Or tell me which ones I don't need to bother with.

With .030 pistons and 6.125 rods.

Finished Bore Size? Not necessary is it, as I'll determine that with the pistons in hand? Or do I put something there?
Stroke: 3.4" right?
Rod Length: 6.125" right?
Compression Distance: How would I calculate this?
Block Deck Height: Book says 9.2"
Piston to Deck: What would this number be?
Piston Type: Are we flat top with reliefs, or Dish?
Ring Groove Info: What is our specs for top, 2nd and oil?
Valve Pocket Info: Intake Depth, exhaust depth, valve angle, valve spacing, valve diameters?
Pin Lock Type: What pin lock type do you suggest?
Chamber Volume: Book shows 61.2cm^3 is that accurate?
Head Gasket #'s Do we need these? Do you know what the MLS or felpro numbers are?
Angle Cut Degree or Amount of Cut?
Rod Info: What are the small end and thickness above the pin for common 351 rods?
skirt shape: What shape are we going to want for ours?

I guess that's about it.

money is sent.
 
Last edited:
Mike8675309 said:
I'm going to need a lot of help. Here are the questions I need answers to. Or tell me which ones I don't need to bother with.

With .030 pistons and 6.125 rods.

Finished Bore Size? Not necessary is it, as I'll determine that with the pistons in hand? Or do I put something there?

This is necessary. They need to know the target overbore size - .020, .030, .040 etc. Stock bore is 3.81 for reference.

Stroke: 3.4" right?

Not exactly. 3.39" ;)

Rod Length: 6.125" right?

If that is what you want, then yes.

Compression Distance: How would I calculate this?
Deck height - rod length - 1/2 stroke - desired piston to deck clearance = compression height.

Block Deck Height: Book says 9.2"

Get a new book! lol Seriously you can't round off numbers like that. Deck height is 9.232 (theoretic, actual will vary). You will have to measure yours and then have the block decked to a specific value. I will go with 9.220 on mine. This leaves plenty of room to square the deck (they are never square) and put on an RA30 finish. Intake manifold will need to be trimmed an equal amount to match.

Piston to Deck: What would this number be?

Minimum piston to head clearance is .035". Most gaskets are .040" thick but they can vary. I will be running 0.000" deck clearance.

Piston Type: Are we flat top with reliefs, or Dish?

Dish. (flat tops would give 10:1 compression and more like 10.5:1 if you run 0 deck clearance.

Ring Groove Info: What is our specs for top, 2nd and oil?

OE is 1.5/1.5/4mm. Select your rings first, that will have impact on what you end up running.

Valve Pocket Info: Intake Depth, exhaust depth, valve angle, valve spacing, valve diameters?

I believe that Diamond has the basic layout of the piston orientation on file. You need to know what diameter valves you are using or at least will want to specify something larger than the largest you will be using.

Pin Lock Type: What pin lock type do you suggest?

It's up to you. There are different designs and each has benefits. Do a little checking.

Chamber Volume: Book shows 61.2cm^3 is that accurate?

Unless your heads have been milled or chambers modified....For a performance engine the chambers should be checked.

Head Gasket #'s Do we need these? Do you know what the MLS or felpro numbers are?

If you are running forged pistons I sure as all heck HOPE you are running MLS. I don't know the exact crushed thickness. I've heard .040".

Angle Cut Degree or Amount of Cut?

This is the angle of the cut. Heads can be cut at an angle to improve flow. I don't imagine this will be a factor for any of us.

Rod Info: What are the small end and thickness above the pin for common 351 rods?
Depends. 6.125 is not a stock 351W rod so you need to either check with the manufacturer or measure them.

skirt shape: What shape are we going to want for ours?

Unless anyone has any better ideas, I will leave this up to Diamond.

I guess that's about it.
Whew!
 
Last edited:
I don't know. My engine guy says that wire locks are best. They charge $2/pin extra for them. I guess I'll go with wire locks. The OE 95 SC rods had wire locks.
 
Dave,

I'd like to have you do the heads so I look to you for thoughts on the valves. I just haven't talked with you about what we'll do as I want the auto rotor on this motor. The only reason why I'm jumping on pistons now is because now is the time. I won't be ready to build this block and do heads until December. (Note my book was the spec sheets off the tech articles pages here, those factory pdf files. Alldata seems to show better numbers like you showed)

Have you considered taking the piston above the deck? Re the article with the comments by Vernon, he seemed to feel .0015 above deck was more than safe and beneficial.

I'm going with Total Seal rings. So 1.5mm/1.5mm/4mm is correct I believe. What would the radial widths be or is that not important?

Regarding the chamber volume. Do they need that information if we are specifying a standard piston top structure and can finalize chamber volume when the deck hights are adjusted and all? I just don't have a method to measure that currently.

Thanks for your help.

o.k. another thing. Alldata shows 2.4266-2.4274 crank bearing diameter. How does that work when figuring out big end size on the rod? Eagle stock 351 rods have a listing of 2.310, but their longer sizes all show 2.100. That seems a tad small to me. Big end width also worries me with the longer rods at .940 up from .8135 for the stock length. I'm wondering more and more if I'm asking for trouble with going longer...

Another item... Pin lock type. Round wire says see pin chamfer. Is that just a note on that pins must be chamfered for use with these?
 
Last edited:
I have given this some long thought.

I am not able to keep a trans in my car.
Now I find out that I have been splitting the transmission cases.
The last two.

I have been tring to get my car tuned for over two years now.

So as I see it ,it is senseless to be building a higher horse powered
SC engine.

So I have no interrest in purchasing better pistons for another motor
build up.I have no way to tune.I have maybe my problems cured with
my trans issues.I am told.I don't beleive they are.Especially if I am
going to apply more HP to a AOD.

With the trans and tuning issues.There is no sence in going any farther
with this type power for this car.

If your wanting to build a 500 rwhp SC .I have every part needed
except pistons,rods,cam,I have from the blower to the bottom of
the block all new parts.The way things are going for me.An not
being able to get the car running right.I will be putting up for sale
soon.

RANDY
 
Sorry, not a pharmisist.... Keep in mind my 500rwhp motor will be attached to a 5 speed trans and will be tuned by XR7 Dave. It's also going to take a good 4 years to get there. But ya gotta start inside the motor, so you gotta get pistons.

from the perspective of someone building for under 400rwhp. I'd still be interested in this deal as this gives the opportunity for some much stronger rods and a nominal price increase above using stock replacement Sealed Power pistons.

Besides people using questionable or no tunes with their high powered bolt ons, rods are the next big weakness in our bottom end.

I'm still conteplating piston material. I'd prefer the less expansion to allow tighter tollerances.
 
Well, if Randy doesn't want a set then we are down to 4 committed to buy which is not enough. Jason is the 5th person here and he has not said that he is definite.

Wire locks require machining which costs $2/pin more.

Mike, you will have to find a 2.311 rod or you can't use it. You cannot use a 2.100 rod. Scat has 2.311 rods as do some others.

People have used 6.2 rods and in fact you could go longer in theory but sometimes moderation is best. Due to the small bore of the motor many engine builders recommend not getting too carried away with rod length. I am using 6.125 in mine.

I don't know the small end width nor the height above the pin. However I do know that stock rods have a lot of material above the pin whereas aftermarket rods are pretty darn thin. Stock rods have a big lump of metal up there that they shave off for balancing.

Guys I'm leaving and won't be back now until Tuesday at the soonest. Try to answer your own questions by calling connecting rod manufacturers or checking websites.

Also, unless we get 4 people (in addition to myself) this won't go through. Lets get the rest of these questions answered best we can and I will call Diamond when I get back to confirm any remaining questions.

People paid:

Mike.
 
lilredstang said:
Known Gasket Volume?
Small End Width Of 351 Rod And Thickness Above Pin?
Support Rail?

I think if you just have the thickness and diameter you'll be ok.

We need data on the rod ends. Diamond may have this depending on the rod, but in our case I doubt it's going to be a problem because we are not using super low compression heights.

I don't know about the support rail at all. ?
 
I'll be calling both Diamond and Eagle/Scat and maybe others. on Monday to review my questions and try to get a reasoning behind why one might choose this version of material or that version.

I'm sure these guys are very intelligent and I think the main issue for myself is my really wanting to have a street capable machine that has more than enough power on weekends.

We've gotta drum up a couple more folks. This is a great opportunity for the SC owners.
 
The standard lenght 351 rod (5.965)with a custom piston application has been the standard for year swith building upour motors. If you go to use a 6.125 rog I believ eyou will have to offset grind your crank.

As far as material for the pistons..If you go forged your motor will have to be built looser to take into consideration the expansion of the piston.

A Hypertectic does not expand nearly as much as a forged. You get less engine noise(No piston slap at warmup) and can run tighter clearences.

However forged will hold up better to high HP applications.

That being said on a v8 of mine *supercharged" I'd bend a rods before I melted a piston.
 
Why do you feel you'd need to offset grind anything if you're getting pistons made to fit? That doesn't make sense to me. If you're willing to run the pin under the oil ring you should be able to go even longer than 6.2 long rods with no crank change.

The issue with the piston material is the question of 2618 or 4032 forgings. Diamond doesn't make cast pistons. 2618 requires even more room for expansion than the 4032 and Diamond is recommending 2618 for our application.
 
Mike8675309 said:
Why do you feel you'd need to offset grind anything if you're getting pistons made to fit? That doesn't make sense to me. If you're willing to run the pin under the oil ring you should be able to go even longer than 6.2 long rods with no crank change.

The issue with the piston material is the question of 2618 or 4032 forgings. Diamond doesn't make cast pistons. 2618 requires even more room for expansion than the 4032 and Diamond is recommending 2618 for our application.


YEP HE IS RIGHT. THEY SAY 2618 FOR NOS OR BIG BOOST. I WILL TRUST THAT THEY KNOW WHAT THE HELL THEY ARE DOING, SINCE I WAS CLUELESS ON 25% OF THE QUESTIONS ON THEIR ORDER FORM AND THEY HAVE BEEN THE BIZ FOR A FEW YEARS.

I SAY THAT SINCE THE TEACHER(DAVE) WILL BE GONE. THAT WE DO A BIT OF "CHEATING". ;) POST A QUICK SUMMERY OF WHAT YOU WANT TO DO(5.956 RODS, SAFE TO 6000 RPM, BORE .030 OVER, CAPABLE OF 500HP) ECT. THEN LIST YOUR "ANSWERS" THAT YOU WROTE ON THE ORDER FORM. MAYBE SOMEONE WILL CATCH A BLUNDER AND SAVE SOMEBODY ELSE A SERIOUS PAIN IN THE A$$. IT MAY ALSO GIVE US AN IDEA OF WHAT QUESTIONS WE NEED TO ANSWER. JUST A THOUGHT. :D
 
Well I'm bummed that we may not have 5 people though my money is already sent. Here is my list of items based on a converstation with Mike at Diamond this morning. Still waiting on hearing back from Scat regarding the rod info.

Finished Bore Size: 3.84" (Stock 3.81 + .030 over)
Stroke: 3.39"
Rod Length: 6.2"
Compression Distance: 1.326"
How do i figure out the compression distance (this is the distance from the center of the wrist pin to the top of the piston crown)?
Take your Rod Length and add it to 1/2 the stroke of the SC motor (Stroke is 3.39, 1/2 is 1.695)
Then subtract Piston to Deck clearance you want. (Stock clearance is .0215 below the deck)
Then subtract the total from your finished block height. Stock block height is 9.232 but you need to consider the resizing that may occur due to decking.
I'm using what Dave mentioned, 9.22 as a calculation point. The deck can be brought down to that when machined. Head gasket thickness will also impact this.

For a 6.2 rod with the piston coming slightly above deck the calcuation is:
9.22 - (6.2 + 1.695 - .001) = 1.326 compression distance. (Stock is 1.602)
Block Deck Height: 9.22
Stock SC Block Deck Height is 9.232. Final deck height will be dermined by your machinst when decking the block. For purposes of pistons, assume .012 lost when squaring the deck giving a 9.22 deck height for piston ordering.
Piston to Deck Height: .001 Above the Deck
This is how far above or below the block deck you want the piston to travel.
Stock is about .0215 below the deck.
The closer to the actual deck, and in some cases over it, the better the "quench" of the gases entering the engine. This can help to deter engine knock and improve combustion efficency. Manufacturing tollerances from the factory help determine the stock height.

I'll be going slightly above the deck. .010. You could easily zero the deck... which means a deck hight of -0-.
Length of Cylinder: 5.375"
Piston Type: DISH
Ring Groove Info:
Top: 1.5mm
Second: 1.5mm
Oil: 4 mm
Standard sizes are 1.5mm, 1.5mm, 4mm which are slightly thinner than the 1/16, 1/16, 3/16.
The available Total Seal rings are the 1.5/1.5/4mm.
Being thinner they don't apply quite as much pressure on the cylinder walls.
Rings with Order: NO
Valve Pocket Information
Intake Valve Diam: 1.84"
Exhaust Valve Diam: 1.60"
Wrist Pin Info:
Diameter: .912" Length: 2.75"
Wall thickness: .155
Pin Lock Type: Double Spiral
It really depends on the engine builder. Round wire is stock and is more difficult to install. Double spiral is about as good and is easier to install.
Desired Compression Ratio: 9 to 1
With the longer rods, and slightly above deck location for the piston, quench should be increased and support a higher compression ratio than stock configuration.
Chamber Volume: Stock is 61.2cc (this will be changed by any porting done on your heads - this number helps Diamond shape your piston head for the given compression ratio.)
Comp. Gasket: .040 (standard)
Gasket bore Dia.: 3.9"
Angle Cut: NO
Rod Information
Material: Steel
Small End Width: ???? (not yet received)
Thickness above pin: ???? (not yet received)
Piston Material: 2618
Skirt shape: Profiled Side (This is basicaly a stock design. more of a preference than anything)

If anything looks wrong or you have a question why I chose this rather than that.. just ask.
 
One item I did talk to them about, and one Item I forgot that we may want to know bout.

#1 - colapsed skirts in forged pistons. It was brought to my attention that many custom pistons are designed with an out of round shape geared towards higher rpm use than what is going to be typical in our cars. This out of round shape is needed due to the expansion of the material as it heats and forces in the engine work on it. The oval shape is due to the less growth that occurs along the wrist pin line.

If the out of round shape is designed to be round again at 6000RPM and above, we could find our selves with possibly out of tollerance pistons while we cruise around at 4000-5000 rpm.

I addressed this with Diamond and he said it wasn't an issue. But I do think it's something we want to make sure they are clearly aware of.

#2 - I forgot to ask about detonation grooves being cut into the piston above the first ring. Current technology pistons have seen improvements in controlling detonation and improving engine efficiency by cutting grooves into the piston. Is this something Diamond does? I didn't see it in their extra machining operations and I don't kow if it would apply that much to my pistons which are going to go above deck slighty. But it's something to consider. (See thiese wiseco pistons here:http://extremev6racing.com/Forum/showthread.php?t=1330)
 
Back
Top