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Aluminum rods in aluminum block? #2996681
12/19/21 11:26 AM
12/19/21 11:26 AM
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Frostbitefalls MN (Rocky&Bullw...
gregsdart Offline OP
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Found GRP rods for sale, 7.100 long, .990 pin, 2.200 rod size.618 grams . Seller claims they have never been run.
My question is about change in piston to deck height at running temps, and total required piston to head distance in a BMP Aluminum block. What should they be set up at 70 degrees, so they are safe at running temp?


8.77 153 mph best, 3055 lbs 528 indy 440-1 alky
Re: Aluminum rods in aluminum block? [Re: gregsdart] #2996705
12/19/21 12:18 PM
12/19/21 12:18 PM
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On the parachute mount
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n20mstr Offline
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I have run .035 below the deck . But I’m always using nitrous

Last edited by n20mstr; 12/19/21 12:19 PM.

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Re: Aluminum rods in aluminum block? [Re: gregsdart] #2996712
12/19/21 12:51 PM
12/19/21 12:51 PM
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Moparteacher Offline
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You should contact GRP for this question. I'm sure others know, but I would go straight to the source to confirm.

Also, I would suspect that piston weight, rpm, shift point and application i.e. drag, street/strip, marine, etc. would all be factors in anticipated growth similiar in manner to setting-up piston to cylinder wall clearance and ring gaps.

Re: Aluminum rods in aluminum block? [Re: gregsdart] #2996731
12/19/21 01:47 PM
12/19/21 01:47 PM
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Las Vegas
Al_Alguire Offline
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This is the second time this week this question has come up in my world. The simple answer is there is no magic number and blanket answer period! I have run aluminum block/rod engines for a long time. Unfortunately there is no one size fits all answer. Way to many variables, bore size, max RPM, piston design, piston weight, piston to wall, rod material, which block, blah blah blah. If you HAVE to get an answer a generic one is .050" for p to head on an NA engine. Especially for the first go around on a combo. We push things MUCH harder than that but if you want a safe set it and forget it that's where I would start. Again its a "best guess" scenario especially for a first go around on a combo. But you know what they say if you are not leaving witness makrs you aint trying hard enough.


"I am not ashamed to confess I am ignorant of what I do not know."

"It's never wrong to do the right thing"
Re: Aluminum rods in aluminum block? [Re: Al_Alguire] #2996760
12/19/21 03:39 PM
12/19/21 03:39 PM
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Frostbitefalls MN (Rocky&Bullw...
gregsdart Offline OP
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With steel rods in aluminum block I figured on .005 to .010 block growth, so if I run steel rods I had better have . 040 cold to be safe. Al rods and Al block figure would go possibly the other way, so add .005 to .045? That would be . 050, which Al mentioned.
On those aluminum rods, if run should they show a little bit of scuffing in the big end? Looking for ways to tell if they have only been used for mock up as seller says.

Last edited by gregsdart; 12/19/21 03:45 PM.

8.77 153 mph best, 3055 lbs 528 indy 440-1 alky
Re: Aluminum rods in aluminum block? [Re: gregsdart] #2996979
12/20/21 10:26 AM
12/20/21 10:26 AM
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Hot Rod Ridge
FastmOp Offline
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I was told just make the rod .010 shorter

Re: Aluminum rods in aluminum block? [Re: gregsdart] #2997114
12/20/21 07:05 PM
12/20/21 07:05 PM
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jwb123 Offline
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For about ten years I ran a world products aluminum block and Mickey Thompson aluminum rods with a set of Ross flat top pistons. The old rule of thumb is .040 for steel rods and .060 for aluminum rods. When I first built the engine I had my son who is a mechanical engineer do the calculations, he was very precise, I had to get the alloy of the aluminum being used etc. HIs calculation on that engine was .058 at 300 degrees, so I would say the rule of thumb is pretty close.

Re: Aluminum rods in aluminum block? [Re: jwb123] #2997209
12/21/21 12:16 AM
12/21/21 12:16 AM
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Frostbitefalls MN (Rocky&Bullw...
gregsdart Offline OP
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Originally Posted by jwb123
For about ten years I ran a world products aluminum block and Mickey Thompson aluminum rods with a set of Ross flat top pistons. The old rule of thumb is .040 for steel rods and .060 for aluminum rods. When I first built the engine I had my son who is a mechanical engineer do the calculations, he was very precise, I had to get the alloy of the aluminum being used etc. HIs calculation on that engine was .058 at 300 degrees, so I would say the rule of thumb is pretty close.

I called GRP today and they said aluminum block is and aluminum rod together, will grow together so run the same as steel rods/iron block. Hmmm, I think if I go aluminum rods I will start a touch safe and sneak up on it!


8.77 153 mph best, 3055 lbs 528 indy 440-1 alky
Re: Aluminum rods in aluminum block? [Re: gregsdart] #2997579
12/22/21 01:19 PM
12/22/21 01:19 PM
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Las Vegas
Al_Alguire Offline
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Well all I'm gonna say is that whomever told you that at GRP is dead wrong....They make all there rods out of plate not cast aluminum. The expansion rate is much different between a cast block that is much thinner with water running through than a plate of aluminum. We run alot less in the Vette with an alumium block and rods than what is being quoted here, even the number for steel rods being tossed around. We need every bot of compression we can get and have snuck up on that number. But we run a very light very rigid piston with a stupid low bobweight. The bottom line is all combos are different and Ill stand behind the number I used as a good starting point for a combo.


"I am not ashamed to confess I am ignorant of what I do not know."

"It's never wrong to do the right thing"
Re: Aluminum rods in aluminum block? [Re: gregsdart] #2997651
12/22/21 06:19 PM
12/22/21 06:19 PM
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Bend,OR USA
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Cab_Burge Offline
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Don't forget about the head gasket thickness and rocking the piston in the bores to get the minimum piston to head clearances up
I'm going to use .055 P to V and P to head clearances on my KB aluminum wedge block pump gas build (572C.I.) with B1 heads up twocents
I'm shooting for right at 11.0 to 1 compression ratio and hoping to make near 1.5 HP per C.I. with one four barrel 1050 CFM Dominator gas carb using Oregon 92 octane non ethanol pump swill luck


Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: Aluminum rods in aluminum block? [Re: Cab_Burge] #2997829
12/23/21 11:09 AM
12/23/21 11:09 AM
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Posts: 1,854
Byron, NY
W.I.N. racing Offline
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Lets start here. The thermal expansion rate of aluminum (all types) ranges from .0000117" to .0000133" per inch, per degree F temp change
So a distance of 7.08" (aluminum rod for example) changes temp from 70F to 200F it will grow in length .0108" (7.0908") with an expansion rate of .0000117
and it will grow in length .0122" (7.0922") with an expansion rate of .0000133.
Next, a distance of 10.725" (std RB/Hemi deck height) changes temp from 70F to 200F t will grow in length .0163" (10.7413") with an expansion rate of .0000117
and it will grow in length .0122" (107435") with an expansion rate of .0000133. for a difference total of .0022.
That is a crossed the spectrum of aluminum types and grades.
With out including Piston height growth and a 0.0" piston to head clearance you would have a .0008" clearance or a -.0008 interference.
Anyone here sweating .0008+/- compression height variation?
Yes I understand this is a very basic example but the numbers still apply. Even if the block and rods heat to to different temp Deltas the growth numbers are still small relative to the basic clearances of a typical drag motor...Now if you go F1 racing, this stuff becomes important.
There has been no consideration taken for the expansion rate of steel (Push rods, Fasteners, lifters etc...) (about half that of aluminum) but that is a different question and where the introduction of an aluminum block starts to show.
Traditionally the main concern with aluminum rods is dynamic stretch that is effected by piston assembly weight...again not considered here.


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Re: Aluminum rods in aluminum block? [Re: W.I.N. racing] #2998183
12/24/21 12:17 PM
12/24/21 12:17 PM
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Posts: 256
NY
challenger451ci Offline
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Originally Posted by W.I.N. racing
Lets start here. The thermal expansion rate of aluminum (all types) ranges from .0000117" to .0000133" per inch, per degree F temp change
So a distance of 7.08" (aluminum rod for example) changes temp from 70F to 200F it will grow in length .0108" (7.0908") with an expansion rate of .0000117
and it will grow in length .0122" (7.0922") with an expansion rate of .0000133.
Next, a distance of 10.725" (std RB/Hemi deck height) changes temp from 70F to 200F t will grow in length .0163" (10.7413") with an expansion rate of .0000117
and it will grow in length .0122" (107435") with an expansion rate of .0000133. for a difference total of .0022.
That is a crossed the spectrum of aluminum types and grades.
With out including Piston height growth and a 0.0" piston to head clearance you would have a .0008" clearance or a -.0008 interference.
Anyone here sweating .0008+/- compression height variation?
Yes I understand this is a very basic example but the numbers still apply. Even if the block and rods heat to to different temp Deltas the growth numbers are still small relative to the basic clearances of a typical drag motor...Now if you go F1 racing, this stuff becomes important.
There has been no consideration taken for the expansion rate of steel (Push rods, Fasteners, lifters etc...) (about half that of aluminum) but that is a different question and where the introduction of an aluminum block starts to show.
Traditionally the main concern with aluminum rods is dynamic stretch that is effected by piston assembly weight...again not considered here.


Indy told me my Indy Maxx Low deck block would grow about 0.010" at 170F(call it ~100F temp increase). That comes out around 0.012" - 0.013" on paper based on expansion rates you found for aluminum above. It also jives with the lash change I get between cold and hot.

Re: Aluminum rods in aluminum block? [Re: gregsdart] #2998214
12/24/21 02:02 PM
12/24/21 02:02 PM
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B1duster Offline
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Originally Posted by gregsdart
With steel rods in aluminum block I figured on .005 to .010 block growth, so if I run steel rods I had better have . 040 cold to be safe. Al rods and Al block figure would go possibly the other way, so add .005 to .045? That would be . 050, which Al mentioned.
On those aluminum rods, if run should they show a little bit of scuffing in the big end? Looking for ways to tell if they have only been used for mock up as seller says.


Years ago I bought a mega block short block. Mopar 4.15 crank, BME rods and flat top pistons .030 in the hole.
It was my first aluminum rod motor and I didn’t know any better at the time, so I had the decks cut .030 and refreshed it.
Bought some used mildly ported -1s, .040 gasket, a used roller cam, dot to dot, slapped it together without checking
piston to valve,
Ran 8.90s with a shot of nitrous. After taking it apart a few yrs later there were no contact marks anywhere.
Btw I saw those GRP rods and knew they were going to turn out to be used







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