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Thread: Diesel Pistons

  1. #1
    Lead Engineer RWOLFEJR's Avatar
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    Diesel Pistons

    I'd like to get some thoughts on the pros & cons of using aluminum vs. cast iron for the pistons in a diesel engine. Just looking for any input from anyone who might know some underlying reasons for some of the choices made in materials today.

    Let's say you're making "X" hp with aluminum pistons. How much drop do you think a person would get switching to cast iron pistons... at a mere 1,500 rpm... roughly? My wrist pins are / should be the weakest link in the rotating assembly so any worry of any additional load would be there I'd suppose.

    Anything I've ever experienced says cast iron can handle more heat than the aluminum. Am I wrong in this application? Does burning diesel juice have any added issues on the cast iron that it doesn't on the aluminum? Like maybe a corrosion issue different than a gasser? I mean blocks and heads have been made from the cast iron forever until last what... 10-15 years? Aluminum would shed the heat quicker too though... The iron would build more up I suppose. I'm running the off road high sulpher stuff...?

    I was in a hurry getting my tractor together and put off adding a pyrometer to the manifold. The silly little extras you know... the gingerbread. It'll be allright I figured. (said with head hanging low...) Locked the motor up the other day... I'm 99% sure I fried the pistons. Doh!! Worst part of it was it was a joy ride. Went up a L O N G.... S T E E P hill down the road from my house weighing 17,000 lbs. (dropped my front weights off it) decided it'd be fun to blast up the hill in fifth gear full throttle and smoke the valley up. Oh it smoked allright... kept on smoking down the other side too.

    Little too much fuel for too long I suppose... for those dainty aluminum pistons. Water temp barely began to move before it started lugging under no load.

    "Dang me... Dang me... They oughtta take a rope and hang me... High from the highest treeeeeee... Woman wouldja' weep for me..."

    O.K. I'm over it now... It was a whole week ago and I did have fun with it for an entire... like three weeks.

    So moving on... I'm considering a few things prior to the official autopsy on this thing. Thinking "What about those iron pistons?" Also looking at an easier way to get to my rack adjustment. It's a pain to get at under a freeze plug typ thing except there's a shoulder it comes home on. Thinking I'll tap it with pipe threads and plug it when the side case is off. Looking at electric fan and pump or additional rad mounted underneath for added volume and shed. Although I really think the bulk of the issue (besides my shear stupidity) was an excess fuel and afterburning heat build issue. Oh... and need to figure a place in the dash to poke a hole for my new pyrometer.

    Everybody have a great labor day weekend...!!

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    RW, I would first be pulling the motor down and find what locked it up. My only experience with diesel are engines much bigger than the tractor, "small" Blackstones through to Mirleese 16s around (24" diameter pistons). They all used cast iron but they also had interesting swirl pockets on the top of the pistons which I suspect would collapse or deform in aluminum . For a Tractor Pull (TP) I would be opting for cast iron, better metal-to-metal sliding characteristics, less expansion.

    In that TP world I would assume you don't spend a lot of time getting the engine up to heat, with all the sliding parts working in harmony. It's probably a quick warm up and then full throttle for two minutes. Sounds a bit brutal on anything but the strongest of materials.

    A call to the engine manufacturer would probably be more beneficial and enlightening.

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    Lead Engineer RWOLFEJR's Avatar
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    Something occurred to me yesterday that... like you said... I'll need to run by the fellow who whipped these parts up for me. My stock bore was 6-5/8" and new bore 7-7/8"... Could be that the aluminum pistons he machined are approx. equal weight to the original cast ones and that is a way to eliminate need to adjust crank balance? At 1,500 rpm it's probably not all too critical... but same/ same can't hurt. My crank was in great shape so didn't need to pull it and he said he didn't need it. Started this project about four years ago and my memory isn't that good but I recall we did have conversation on balance and maybe that's where this line of thought came from. From the fading gray matter...

    My pistons have (or maybe had...) a shallow dish with a cone shape in the middle to get the fuel swirl going on. Contrary to what I've always heard, this fellow tells me that on a dyno the most power is coming from this set-up when cold. Said the cooler air temp does more for it than the heated up walls of the bore. I always thought the heating of the bores etc. helped to get a more thorough burn.

    I'll be tearing into it here shortly and see what we have. Want to know before we leave for this hunt coming up so I know what I'll be getting into instead of speculating for another two or three weeks.

    Side note... spoke with a fellow pulling yeasterday at our local fair who was running turbos and all. He told me he runs his exhaust gas temperature up to 1,800 with aluminum pistons...!!! I don't know how he's getting away with it? Maybe he was feeding me a line? Can't picture how he could get more than a run at that? Stuff I've read suggests that anything over 1,400 is "gambling against a stacked deck."

    Well... we'll see in the next couple few days... Once I get it apart I'll give the fellow who made these parts a call and get my scolding. He warned me more than once to put EGT pyrometer in and watch I didn't cook it.

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    Quote Originally Posted by RWOLFEJR View Post
    He warned me more than once to put EGT pyrometer in and watch I didn't cook it.
    There are so few things in life that are perfect, but Hindsight, is certainly up there with the few that are. LOL

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    Lead Engineer RWOLFEJR's Avatar
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    Got to thinking again...
    The crank assembly in these old 2 cylinders is balanced with the crank weights, flywheel, and the counter balance located on the other end that drives the clutch. Theroretically this is all balanced from the factory and takes rods and pistons into account. Sooo... I'm thinking that since the throws of the crank are 180 degrees apart it won't make much difference in balance so long as both pistons weigh the same. Acceleration isn't an issue so I'm going to look hard at the possibility of using iron pistons next go at this.

    Looking like we're supposed to have rain the next few days so it's looking like my open air garage will be closed for the season. We're blasting off for Wyoming next Monday so it's looking like I won't be doing the exploratory tear down until we get back... if then. The lights start to go out in my garage earlier and earlier nowadays and by the time we get back I'm gonna be busy catching up at work. Looking like a spring project. I'd just assume keep it all buttoned up instead of parts scattered in my basement and ... in the house garage all winter long anyway. This way nothing has the chance of exposure to the elements while it's apart.

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    Quote Originally Posted by RWOLFEJR View Post
    it's looking like I won't be doing the exploratory tear down until we get back... if then.
    In that case, it might be an idea to take a peek into the radiator and check for water loss and in the sump for water in the oil. It is not beyond possibility that the lock up caused some damage to water seals, maybe head gaskets etc. I would not want to be tearing down to find 6 months of rust.

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    Project Engineer CCR5600Design's Avatar
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    A few things I have learned about pistons in my life:

    1) Forged aluminum pistons are the way to go in high performance applications. Due to the extremely high cylinder pressures encountered under the increased volumtric efficiency and higher compression ratios of racing engines, The aluminum offers much better stability and durability. Cast pistons are prone to cracking under the forces of detonation (this is, afterall, how a diesel engine operates).

    2) When ordering pistons from a custom piston manufacturer, make sure piston rings are readily available for the bore size you wish to use. Any piston builder worth his salt can machine a piston for you. The hard part is sealing it in the bore without having the proper sized rings.

    3) Forged pistons grow at a different rate than do cast pistons when subjected to heat. Allow for proper piston to cylinder clearance with the forged pistons.


    Also, you friend's thoughts that the engine makes more power when it is cool are viable. The cooler and denser the air that enters the combustion chamber, the more oxygen will be present. With the additional oxygen, more fuel can be burned, thus resulting in more power. The intake charge can absorb heat from the intake manifold, the cylinder head and the cylinder walls. For years when we drag raced, we would run an electric heater on the oil pan to keep the oil warm to minimize warm up time and put ice on the intake manifold to achieve the coolest intake charge possible. Regardless of the fuel used, the more air you can get into the cylinders, the more power you can make.


    Ron

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    I would not be overlooking the in-cylinder temps for aluminum pistons. High heat, high compression, high work load? Hmmmm.

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    Project Engineer CCR5600Design's Avatar
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    Quote Originally Posted by PinkertonD View Post
    I would not be overlooking the in-cylinder temps for aluminum pistons. High heat, high compression, high work load? Hmmmm.
    Forged aluminum pistons hold up for nitrous, turbo, blown, methanol, gasoline and nitromethane applications. I can't imagine any more rigorous test of a piston than supercharged on nitromethane.


    Ron

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    Lead Engineer RWOLFEJR's Avatar
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    Thanks for the input guys.
    Hey Ron... Was wondering if you've ever utilized a EGT pyrometer on a nitromethane fueled engine? I'm just curious what sort of temperatures develope when using it? Stuff I've been reading suggests I might have had my pump racks set to high and apparently with diesels if you overdo it with the fuel the burn will continue to burn after the power stroke and that's what developes damaging high heat. Guess the burn is still taking place on the exhaust stroke and sort of doubles the time for exposure to the heat causing the heat to build up more than it would otherwise?

    Dave... drained my tires... capped my intake and exhaust.... took my breather off the oil fill and capped it... cracked my oil drain plugs and no water from any of them. An old buddy used to tell me to do that after they sit a bit and before lighting them up for the season. With all that iron you'll get a little condensation at the plugs after a long winter. My focus has now shifted to the elk hunt coming up... Leaving in just 5 days. Gonna go shag some dog food and treats so my mum is set for her dog-sitting adventure.

    I'll see what the weather's like when I get back and maybe dig into it. Either way the problem will still be there whenever I decide to dig into it...

  11. #11
    Project Engineer CCR5600Design's Avatar
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    Quote Originally Posted by RWOLFEJR View Post
    Thanks for the input guys.
    Hey Ron... Was wondering if you've ever utilized a EGT pyrometer on a nitromethane fueled engine? I'm just curious what sort of temperatures develope when using it?
    I have not personally utilized an EGT pyrometer on a nitro engine. In a supercharged nitro application, there is a large amount of unburned fuel that ignites in the exhaust header. This is the cause of huge flames coming from the headers as the engine is under a load. I have talked with several nitro guys and they said they experience EGT's as high 1800° F. Turbocharged gasoline EGT's will get up there, too. I have seen stainless steel exhaust headers and turbo housings glow bright red from the heat during dynomometer pulls. I always cringe, thinking that at that temp and pressure the stainless might fail, but I have yet to witness it.

    Stuff that was originally designed for utilitarian or agricultural use was not intended to be put into a performance application, but I know plenty of gearheads who sit there, drinking Diet Bud and comparing notes with each other. Inevitably, the poor ol' tractor/SUV/station wagon or, in my case, 1972 New Yorker, get pressed into performance use based on the "whadda ya think would happen if we tried this?" philosophy. I know for a fact that a 5200# 4-door Chrysler can knock at least 2 seconds off its quarter mile elapsed time with nothing more than a healthy shot of nitrous oxide.



    Ron

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    Well I put in for a couple vacation days tomorrow and Friday to tear down my 820 and find out what the damages are.

    Tearing it down certainly won't be as bad as when I had the entire tractor tore down and sitting on cribbing and all, but still turns into a BIG pile of parts when they're tore down. Been running through my head all the things that will need to come off to get to the heart of the problem and as simple as they are there are still a boatload of bolts to be turned.

    This time around I'll try to make re-assembly a little easier on myself by keeping bolts and things more organized. Last go at it I assumed I'd remember which went where and it added a lot of time to the re-assembly.

    My "garage" is supposed to be sunny and high 50's to low 60's the next three days so I can't ask for nicer wrenching weather. Warm enough to be comfortable but not so hot that I'll be sweating.

    I'm getting anxious to see what I'm gonna be dealing with in repairs. Hoping it's not tore up too bad...

    Looked into capped pistons a while back that are used in diesel locomotive engines. They're aluminum with a steel cap on the top. Saw a lot of different means of internally cooling them too. Also as Ron mentioned I dug around and saw the mention of cast vs. forged and I'll need to find out what stock the guy who made these was using so I can get a better grip on where this thing is going to be at the elevated temperatures

    First thing is to see what's up in there. Hoping for enough clean areas to gather some good measurements of pistons, bores, ring gaps etc. This isn't to say I'll necessarily be changing any dimensions when rebuilding... It did what it was supposed to do... it won the pull it was intended for. I just over did it with my joy ride. Kind of like taking a dragster out for a trip to get groceries. But if little tweak here or there or a material change might look more appropriate then I'll have to give that consideration.

    I'll be back with autopsy results soon...!!

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    Administrator Kelly Bramble's Avatar
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    Interesting pictures please....

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    Bob, I set up a digital camera on a tripod when I disassemble complicated stuff. I lay out the groups of bolts and whatever in sequence. I have an older Sony that has the thread for a cable-release to take the pic. You could probably do similar with the eraser end of a pencil for a camera without the cable-release option so greasy hands don't help to rust-proof the camera.

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    Lead Engineer RWOLFEJR's Avatar
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    I did take a ton of pictures last go at it and yep... the camera is rust proofed. The pictures came in handy a couple times.

    Here are a few pictures from the assembly the last time. Or pre-lock-up...

    First picture was part way though the tearing it down phase for a good cleaning and inspection. I was lucky and it appeared the tractor wasn't used very heavily for how old it is. (1957) And hey... it's a fact... They don't make 'em like they used to...!!

    Second shows the rear of the modified block. This rear part slides back inside the main case and shows how cooling between the cylinders is now impossible. Hard to tell in the picture but the rods are offset a little to the inside of centerline to the pistons so the original crank could be used.

    Third shows the head side of the block. There's a little cooling goes on here for approx. top 3" or so of the cylinders. You can see the heavier section of the block casting in the last picture where some coolant can run. Can also see that the pistons are stepped down to allow use of the original head.

    Fourth shot is the head. The exhaust valves were the old intake valves. Head was ported out big-time... and now has 3-1/16" diameter intake valves.

    Last shot shows the block and head torqued down and ready for another pile of parts to be put back on it. Reminds me that I'd better check my Advil supply before I start this...

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    That all looks pretty cool Bob, but I gotta say, is that some kind of Ninja tractor in the first pic? It is almost invisible with it's surroundings. The US Army could use you in hiding tanks.

    Pic #4, "no stinking Hemi-anything around here."

    Good luck with the rebuild, it looks like fun. What's "Advil" Some kind of gasket sealant or something?

    p.s.
    More importantly, what's the blue motorcycle in the second pic? Drop down rear wheel stand makes it kinda old. Motorcycling minds need to know.
    Last edited by PinkertonD; 04-05-2012 at 10:35 AM. Reason: afterthought

  17. #17
    Administrator Kelly Bramble's Avatar
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    So, this is a John Deere? Model - 730?

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    Lead Engineer RWOLFEJR's Avatar
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    Well I completely wasted a good chiropractor visit the other day. Guessing I pretty much undid any good he did for me over the weekend.

    Although I did get a good peek at the bores etc. I'm two nuts away from getting the block, pistons, and rods out. (Both literally and figuratively...) The position it locked up in has made getting the two lower connecting rod nuts loose look like an impossibility. The angle they're positioned in isn't allowing room for any lever arm. The block won't budge off the case since the pistons are still connected to the rods which are still connected to the crank, which is connected to the case.

    Had one of those spells where you just stand there and stare at it. After a lot of that I decided to see what I could do with the leverage of the flywheel. Put a big ratchet strap on the flywheel in a choker fashion and tied the other end off to the front axle. With everything I had on that ratchet and some coaxing with a fine piece of locust it became apparent that this is not an option.

    One thing the gray hairs... or what's left of them anyway... has taught me is that sometimes it's best to walk away from a problem for a bit and give it some thought. It was very easy to talk myself out of another attempt at those nuts yesterday since I was having a hard time getting stood up straight anyway. Threw the tarp on it and my dog Bubba and I spent the day eating and watching the final round of the Masters. Course my dog didn't have a clue, but he was enjoying the enthusiastic mention of his name when Bubba became a potential for the win.

    The solution to the nuts is simple enough but somehow it eluded me Saturday? Gonna see if a pair of swivel adapters... possibly with a short extension in between will hold up. Forgot about that handy-dandy tool option. Doh...! If that doesn't get it thinking I can buy a cheap-o breaker bar and bend it to suit.

    So what did I see in there so far? Surprisingly good looking bores for the parts of them I could see. Looked like a good burn was going on in the holes too. Few tiny little hard flecks of white in the one hole, (burnt aluminum) but not a major meltdown. There's enough clean bore for good measurements anyway. The question is what do the bores look like under the pistons? Hoping to get the stuff out of there after work today unless it's raining by then.

    Kelly you were close... The tractor is a 1957 John Deere 820.

    Dave that blue thing is what I use to keep my small garage floor oiled up. It's my 1948 FL Panhead. One modern improvement on it is an automatic advance distributor. Also put new Kevlar clutch kit and chains and sprockets etc. in it a couple years ago. Other than that it's pretty much stock. It's a lot of fun for the occasional putt but I'm not hardcore enough to consider it for a long haul. Here's a couple pictures of it. (side note the tractor in the background is a J.D. model R that I sold last year... not the 820...

    Copy of 48Left.jpgCopy of 48Right.jpg)

  19. #19
    Administrator Kelly Bramble's Avatar
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    Leaping forward to the year 2001 - John Deere coaxed me into buying a 790 with the 419 front end loader.

    It is my opinion that John Deere built better tractors in the past. Though my 790 has been reasonable things like the battery installation and front seals could be improved.

    Think I have a little more than 200 hours – bought new.

    My ride has the Yanmar Diesel 30 horse.

    790-John-Deere.jpg

  20. #20
    Administrator Kelly Bramble's Avatar
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    Nice…. (Wipes drool from chin).

    Copy of 48Left.jpgCopy of 48Right.jpg)

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