H405CP versus stock magnum 5.9 pistons

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HankRearden

HankRearden

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So since I ruined the pistons in my 5.9 build I thought maybe I could get s small compression bump without spending much money.
I stumbled onto the h405 cp. It lists a compression height of 1.637 in. Everything I found on the dished magnum piston shows 1.612. The magnum pistons were .045 down in the cylinder. Am I right. Will this be an acceptable replacement with a low price?
 
You need to be careful and dot your i's and cross your t's, because the Magnum blocks have a shorter deck height than the LA. I believe those 405s are actually for the LA, so make SURE you do the math before you buy.
 
RustyRatRod said:
You need to be careful and dot your i's and cross your t's, because the Magnum blocks have a shorter deck height than the LA. I believe those 405s are actually for the LA, so make SURE you do the math before you buy.
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Yep that's why I asked. I have learned not to trust catalogs and their specs. Fifteen years in the parts business was good for something.
If I recall from my days in tech school, compression height should be a measurement of the center of the pin to the highest part of the piston with deck height not having any bearing. If these is accurate though Ill still be about .020 below the deck.
But you and others are a hell of a lot more knowledgeable on this stuff than I am. I whit wrenching almost twenty years ago and have only started dabbling again the last three years.
 
Was wondering the same considering the difference in balance between a magnum and an LA 360.
 
The measurement is from the center of the wrist pin to the measured top of the piston. So that doesn't necessarily mean the highest part. If it's a domed piston, the dome is not included. Just the top of the piston opposite the dome.
 
Trust the math. If you accurately measured 045 in the hole with the stock pistons and you added 025 to the compression height of the new pistons you should measure 020 in the hole with the new pistons and you should see a compression bump. The key here is accurate measuring. The balance is a non issue because any time you change the bob weight you should balance the rotating assembly.
 
TT5.9mag said:
Trust the math. If you accurately measured 045 in the hole with the stock pistons and you added 025 to the compression height of the new pistons you should measure 020 in the hole with the new pistons and you should see a compression bump. The key here is accurate measuring. The balance is a non issue because any time you change the bob weight you should balance the rotating assembly.
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Yeah. I think you are right. As long as the catalog is accurate the math works.

I don't mean to be argumentative,but I'm not balancing what was supposed to be a low buck hop up for a pickup.Mallory metal isn't cheap.

We didn't even pay to balance the dirt track stuff we built when I was kid. We just kept the weights close and rolled with it. Guys thought I was wasting time by weighing each piston and rod combo and lightening them all to match the lightest one.

I would posit that of the millions of engines bored and rebuild with replacement pistons very few are actually balanced. I've seen big block Chrysler with 60 over cast pistons that weren't balanced and there had to be a lot of weight difference there.
 
I have a magnum short block, was running, heads cracked.

I thought the compression height was 1.605.(mag)

My pistons were .050 in the hole. (magnum)

The LA 360 pistons i have, 1 set dished 1 set flat top both weight about the same 584 grams with out rings or pins. LA pins 153 grams. LA rods about 758 grams. Pistons were down .095 on a 7/1988 360.

Looking through FABO, magnum pistons were 469 grams, 135 on the pins. Couldnt find anything on the rods.

The KB 190's i have are 595 grams with rings. The pins that came with it are 129 grams.
 
HankRearden said:
Yeah. I think you are right. As long as the catalog is accurate the math works.

I don't mean to be argumentative,but I'm not balancing what was supposed to be a low buck hop up for a pickup.Mallory metal isn't cheap.

We didn't even pay to balance the dirt track stuff we built when I was kid. We just kept the weights close and rolled with it. Guys thought I was wasting time by weighing each piston and rod combo and lightening them all to match the lightest one.

I would posit that of the millions of engines bored and rebuild with replacement pistons very few are actually balanced. I've seen big block Chrysler with 60 over cast pistons that weren't balanced and there had to be a lot of weight difference there.
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With 20 over replacement pistons, there was a significant weight difference on the 5.9 magnum I did. Just saying.
 
Can buy new h655 for 168 bucks on summit...if that's what you're looking for.
Whatever you end up doing..buy the right piston.
 
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HankRearden said:
So since I ruined the pistons in my 5.9 build I thought maybe I could get s small compression bump without spending much money.
I stumbled onto the h405 cp. It lists a compression height of 1.637 in. Everything I found on the dished magnum piston shows 1.612. The magnum pistons were .045 down in the cylinder. Am I right. Will this be an acceptable replacement with a low price?
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Look into the KB107 pistons. Thats what are in my 2000 360 magnum engine
 
I am curious because I have 2 360 magnum blocks (one already 30 over) and my son has a set of the 405 pistons in 30 over that have very low miles on them, he built a 76 360 then rebuilt it again as a 408, one day I'd like to combine his pistons and one of my blocks, if that would be possible. I know that the 360 mag while still external balance, was a lower balance factor than the old LA version. If we were to use everything else Magnum, but with the 405 pistons would we use a balancer and flywheel weight for a magnum or for an LA? And for the record this will be a "one day down the road" build, as we have too much other stuff going on besides, right now to worry about it right now. Just convenient timing for someone to bring up this topic is all. Putting it out there for the benefit of the group "only" at this moment.
 
The crank is what determines the balance factor. Magnum vs LA. But I don’t understand why anyone would build an engine with aftermarket parts and not at least have the balance of rotating assembly checked by a machine shop. Yes you can be out a few grams and it won’t be an issue but why spend the time and money to build an engine just to have it vibrate itself to death.
 
I have both Magnum and LA cranks (stock) sitting here Good shape, and ready to go.
I can't speak for 360 engines, but for the 318, I know for sure, that the same crank from an LA crossed over to the Magnum years. I remember spending lots of time in a crankshaft book, when I built my 1st 318 Magnum. I might have to look into that, as it relates to a 360.
I have a good, machine shop polished '76 LA 360 crank (really my son does, but I can have access to) and I have 2 good, Magnum 360 cranks sitting here. One '00, one '01. Like I'd said, right now this is strictly a "bench race" exercise for me, for the moment// though I dunno what my son has in mind for his '72 land yacht Fury wagon. He was talking about taking one of my 318 Magnum blocks and making it into a 390, (318 with 4" crank) once he gets to that point. His other 72 wagon had a 440 in it (he has one of those from a motorhome, sitting in his garage that could go in) he wants an OD with 3.something gears/ and knows that a small block based engine, would be easier to run with that OD (probably would be a 518) This wagon is an original 360/727 car.
I have a Volare to finish, and thought at one time thatI knew exactly what engine I wanted to stuff inside, (318 Magnum with a 5 speed) but now "project creep" has sunken in. and I have more questions and parts hanging around, than answers. (which I thought at the time, I knew exactly what I wanted to do with that car/ not so sure I know anything these days haha)
 
I am currently doing a 360 magnum block, with LA aluminum heads for a customer, so on your piston question, I used the h116cp pistons in a un-decked 360 magnum block, with the magnum crank and rods. The pistons sit in the hole a average of .006 we are using a .045 head gasket, 63.5 cc heads, and the pistons of 5cc of valve relief. It works out to about 10.3 to 1 static, perfect for the aluminum heads and hyd roller setup. The crankshaft DID require rebalance, it was out about 40 some grams if I remember correctly.

20220309_163725.jpg
 
replicaracer43 said:
I am currently doing a 360 magnum block, with LA aluminum heads for a customer, so on your piston question, I used the h116cp pistons in a un-decked 360 magnum block, with the magnum crank and rods. The pistons sit in the hole a average of .006 we are using a .045 head gasket, 63.5 cc heads, and the pistons of 5cc of valve relief. It works out to about 10.3 to 1 static, perfect for the aluminum heads and hyd roller setup. The crankshaft DID require rebalance, it was out about 40 some grams if I remember correctly.

View attachment 1715888521
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I dont remember if my block was decked or not. But with the kb107 pistions, felpro 1008 0.038 gasket and the new trickflow 190cc heads with a 0.060 cc chamber i am right at 11.1 compression. That is if my math is right. Sounds like you are building a very nice engine
 
Service replacement pistons (ie. Badger) are the same weight as stock pistons. If your new pistons are lighter, you dont need $75 tungsten, you need to drill the counterbalances. Get smart and buy lighter pistons.
 
I know of 3 automotive machine shops, that are semi local. All would rather sell me a Scat or Eagle crank than to do any crank work. None do crank balancing.
I've asked elsewhere, what it would take to convert a 360 crank to neutral balance. I'm not wanting to buy a crank if mine is perfectly serviceable unless I was doing a stroker. Further I don't know where I would have to take it, in order to have any sort of balance work done. but if I have to take my crank elsewhere to have balanced that means I'd have to have them do the whole machining job. The shop I use does a great job at a fair price, for anything else. They do send any crank work (beyond a basic polish) out, I do not know where that would be.
 
pishta said:
Service replacement pistons (ie. Badger) are the same weight as stock pistons. If your new pistons are lighter, you dont need $75 tungsten, you need to drill the counterbalances. Get smart and buy lighter pistons.
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in my case I would like to be able to use at least some of what I have here and available if possible. I never said anything about buying pistons. (someone else might though)
 
HankRearden said:
..
I stumbled onto the h405 cp. It lists a compression height of 1.637 in. Everything I found on the dished magnum piston shows 1.612. The magnum pistons were .045 down in the cylinder. Am I right. Will this be an acceptable replacement with a low price?
Click to expand...

I must have missed the part where low price and replacement didnt equate to a purchase? "Get smart' wasn't directed to you , it was refering to a planned purchase on the cheap if anyone did not want to have to buy slugs of $$ Tungsten to counterbalance a heavier piston.
I would not trust a machine shop around here to balance a crank as I have 3 places in a 10 mile radius that only balance cranks. Im spoiled living in a large metroplex with a heavy aerospace presence.
 
volaredon said:
I know of 3 automotive machine shops, that are semi local. All would rather sell me a Scat or Eagle crank than to do any crank work. None do crank balancing.
I've asked elsewhere, what it would take to convert a 360 crank to neutral balance. I'm not wanting to buy a crank if mine is perfectly serviceable unless I was doing a stroker. Further I don't know where I would have to take it, in order to have any sort of balance work done. but if I have to take my crank elsewhere to have balanced that means I'd have to have them do the whole machining job. The shop I use does a great job at a fair price, for anything else. They do send any crank work (beyond a basic polish) out, I do not know where that would be.
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In order to ballance your 360 crank internally, will likely require heavy metal. Nothing really wrong with leaving it external on a mild build, will definitely be cheaper. You still need a competent machine shop to do it however.
 
replicaracer43 said:
In order to ballance your 360 crank internally, will likely require heavy metal. Nothing really wrong with leaving it external on a mild build, will definitely be cheaper. You still need a competent machine shop to do it however.
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Ok but balance to what? LA or magnum specs?
 
volaredon said:
I know of 3 automotive machine shops, that are semi local. All would rather sell me a Scat or Eagle crank than to do any crank work. None do crank balancing.
I've asked elsewhere, what it would take to convert a 360 crank to neutral balance. I'm not wanting to buy a crank if mine is perfectly serviceable unless I was doing a stroker. Further I don't know where I would have to take it, in order to have any sort of balance work done. but if I have to take my crank elsewhere to have balanced that means I'd have to have them do the whole machining job. The shop I use does a great job at a fair price, for anything else. They do send any crank work (beyond a basic polish) out, I do not know where that would be.
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Yeah I have no one local to do crank work. The local shops all gather up a bunch and then take them to some place in ohio.
I would spring for the KB 107 if i didn't have to have the rotating assembly balanced.
I had called around looking for a shop to deck the block to zero deck the stock pistons and they were all over a month out and no one was real interested in taking that much off the deck. Whichever way I decide it will have to be a home job.
I may just grab one of my other 5.9's and throw it together with this cam and valve springs. Save this block for the eventual stroker.
 
volaredon said:
Ok but balance to what? LA or magnum specs?
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It's not really about "magnum" or "LA" specs, it just depends on what the bobweight ends up being. In my example, we used magnum crank, rods, and weighted flexplate, with a summit SFI external weighted ballancer, and I had to add a bit of weight to both, due to the heavier piston/pin combo
 
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