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whatsamerc??

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I have a 77 motorhome 440 that has dished .060 over pistons in it. The only numbers i can find on them are 1886 inside of the skirt. I cant find any info on them and was someone might know what they are.
 
I know the late motors were low compression with flat tops but this seems like it would be ridiculously low.

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Throw those cast pistons in the trash. They were junk before they were installed. And they didn't get better over the years
 
Oldmanmopar said:
Throw those cast pistons in the trash. They were junk before they were installed. And they didn't get better over the years
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Its already .060 over though. The bores are in really good shape but i hate to put new pistons in old holes
 
Do you have a deck height or a compression height measurement?
Agree with toolmanmike. Likely way under 8 to 1, unless an over two inch compression height.
 
33IMP said:
Do you have a deck height or a compression height measurement?
Agree with toolmanmike. Likely way under 8 to 1, unless an over two inch compression height.
Click to expand...
I'll get a measurement this friday. I dont mind fairly low compression because its going in a driver and id like to run cheap gas but 7.5 to 1 is pitiful.
 
Do you have a dial caliper? You can measure comp.height of the piston and figure out what the expected deck height should be.Compression height is the distance from the center of the pin to the flat area at the top of the piston.
Measure the pin diameter. Measure from the bottom of the pin to the flat of th top of the piston, then subtract half of the pin diameter.
Over two inches, you might have some compression. Zero deck, if I remember right, is somewhere around 2.06" . Too lazy to calculate it exactly right now.
Around 1.9" to 1.95, you might be looking at 7 to 1.
 
whatsamerc?? said:
Its already .060 over though. The bores are in really good shape but i hate to put new pistons in old holes
Click to expand...
Then hone it and run new pistons. You don't have to bore for new pistons.
 
33IMP said:
Do you have a dial caliper? You can measure comp.height of the piston and figure out what the expected deck height should be.Compression height is the distance from the center of the pin to the flat area at the top of the piston.
Measure the pin diameter. Measure from the bottom of the pin to the flat of th top of the piston, then subtract half of the pin diameter.
Over two inches, you might have some compression. Zero deck, if I remember right, is somewhere around 2.06" . Too lazy to calculate it exactly right now.
Around 1.9" to 1.95, you might be looking at 7
Click to expand...

33IMP said:
Do you have a dial caliper? You can measure comp.height of the piston and figure out what the expected deck height should be.Compression height is the distance from the center of the pin to the flat area at the top of the piston.
Measure the pin diameter. Measure from the bottom of the pin to the flat of th top of the piston, then subtract half of the pin diameter.
Over two inches, you might have some compression. Zero deck, if I remember right, is somewhere around 2.06" . Too lazy to calculate it exactly right now.
Around 1.9" to 1.95, you might be looking at 7 to 1.
Click to expand...
Yeah, ill get it measured friday. Thanks
 
33IMP said:
Do you have a dial caliper? You can measure comp.height of the piston and figure out what the expected deck height should be.Compression height is the distance from the center of the pin to the flat area at the top of the piston.
Measure the pin diameter. Measure from the bottom of the pin to the flat of th top of the piston, then subtract half of the pin diameter.
Over two inches, you might have some compression. Zero deck, if I remember right, is somewhere around 2.06" . Too lazy to calculate it exactly right now.
Around 1.9" to 1.95, you might be looking at 7 to 1.
Click to expand...
Comp height is 1.98. The piston is .050 down from the deck. The dish is .30 deep and the dish starts .500 in from the edge.
 
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Compression on that is likely down in the 7s.
 
At least it's not .150 in the hole, which I've seen.
Without a cc of the dish, an accurate cr number will be a guess. But I agree with RRR. Possibly high sevens.
You can cc the dish just like you would a cyl head combustion chamber.
 
Measure the diameter of the dish, at the top of the dish…….and the bottom of the dish.

This is only if you don’t have a way to actually cc the dish.

If it’s really .420” deep(nearly 1/2”), there’s quite a lot of volume there……… way more than a flat top down .150”.
 
Comp height is 1.98. The piston is .050 down from the deck. The dish is .420 deep and the dish starts .500 in from the edge.
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Does the block appear to be decked?
The nominal stock deck height for an RB block is 10.720”.
With an uncut block, pistons with a C/H of 1.980 should be close to .100” down the hole at TDC.

If the top of the dish is 1” smaller than the bore, and the bottom of the dish is .750” smaller than the top……. And you average those two bore sizes it’s a hole 3.905” …….. x .420” deep.
Which would be a dish that’s about 82cc worth.
Whereas a .060” over flat top piston that’s down the hole .150” is “only” 37cc’s.
 
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PRH said:
Does the block appear to be decked?
The nominal stock deck height for an RB block is 10.720”.
With an uncut block, pistons with a C/H of 1.980 should be close to .100” down the hole at TDC.

If the top of the dish is 1” smaller than the bore, and the bottom of the dish is .750” smaller than the top……. And you average those two bore sizes it’s a hole 3.905” …….. x .420” deep.
Which would be a dish that’s about 82cc worth.
Whereas a .060” over flat top piston that’s down the hole .150” is “only” 37cc’s.
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Wow! So basically what you are saying is ..... high sixes to one?
My 33 Imperial flathead has that much.
 
I’m mostly just trying to verify the dish measurements are correct at this point.

Silvolite lists a 413 truck piston that’s shown as “7.5”cr.
It’s 2.023”ch, and has a dish that’s 3.25” x .220”(29cc).
The 7.5 works out about right with a 78cc head on that one.

I can’t really imagine me reusing those big dished 440 pistons in anything I was hoping to make any kind of power with.
 
If your thinking about spending money on the original motorhome heads, don't, limited potential
 
PRH said:
Does the block appear to be decked?
The nominal stock deck height for an RB block is 10.720”.
With an uncut block, pistons with a C/H of 1.980 should be close to .100” down the hole at TDC.

If the top of the dish is 1” smaller than the bore, and the bottom of the dish is .750” smaller than the top……. And you average those two bore sizes it’s a hole 3.905” …….. x .420” deep.
Which would be a dish that’s about 82cc worth.
Whereas a .060” over flat top piston that’s down the hole .150” is “only” 37cc’s.
Click to expand...
It is a remanufactured motor so it could be decked. I re measured the dish and it is .300 deep 3.380 at the top and 3.88 at the bottom. The formula i found put it at 43cc. I used summits compression calculator and came up with 7.15 to 1 with a steel shim head gasket.
 
whatsamerc?? said:
It is a remanufactured motor so it could be decked. I re measured the dish and it is .300 deep 3.380 at the top and 3.88 at the bottom. The formula i found put it at 43cc. I used summits compression calculator and came up with 7.15 to 1 with a steel shim head gasket.
Click to expand...
See post #22. lol
 
whatsamerc?? said:
Ive been thinking seriously about a turbo. I wonder how much boost a cast crank can stand.
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I don't know, but I can tell you in horse power terms. The Chrysler nodular iron cranks are good for 500HP in stock trim. They can be lightened and balanced to the Nth degree and run a GOOD quality balancer and take more than that. What you're gonna have to be careful with are the pistons. Do NOT run it lean. I would run it a little on the fat side and you'll probably never have an issue under 10PSI. I'd run the 6-71 though. lol
 
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