Making 18:1 pistons

[eracers999]

So if i cut a stock mahle piston down .028 to .030 i will have 18:1 comp??

The ceramic coating that was on the dome does that need to be replaced? It's been said that you could burn a hole in the top if you did'nt re coat. I couldent find a single post of a 6.5 burning a hole in a stock piston, they are uncoated.

I can realisticly wind up with a 18:1 piston set for about 425.00 what do the all knowings think of this??

Thanks/Kent

[rjschoolcraft]

While I have no experience directly, a cut down piston will leave a small portion of the cylinder wall exposed at TDC. A better approach, in my opinion, is to use a set that have a dished out volume in the top (designed for 18:1 compression) that still have full height side walls. Obviously, these will be more expensive...

[Billman]

I am unfamiliar with the 6.5 piston but I would not remove .030.

I have ordered many custom sets of pistons from JE & Wiseco. I think they will make you anything you want. Just give them dimensions. Style/Bore/Pin height etc...

Price could be another issue though.

[99gmccrew]

Would someone please explain the benefits of going to 18:1 pistons. My manual says my 99 L65
has 20.2:1 comp. ratio. I believe I've seen that the L56 has around 21.5:1. Is my comp ratio ok with sustained boost of 15psi on long grades while towing. Is there a point that should not be crossed on boost pressure for 20.2:1 cr's. I've read here that lower cr's allow you to have higher boost pressures, but why is this?
Thanks

[Peter J. Bierman]

I did cut 1mm off the standard 6.2 piston and have had no problems with it so far.
Official 18:1 pistons would be better though couse you alter the piston topshape too.
If you cut too much, it will be flat and I am not sure what that will do for gasflow and preformance.

Peter

[More Power]

Back in 1997, the folks at Peninsular Diesel told me that lowering the compression improves 6.5 durability. At the 2001 Rendezvous in Ohio, Matt Koning (Peninsular's owner) told our group during a seminar that they haven't had any 6.5 head gasket failures on any of their 18:1 marine 6.5 engines. Lastly, improving durability decreases risk when raising the power level.

In the winter 1999 issue of the Turbo Diesel Register, columnist and automotive journalist Kevin Cameron discussed what effect compression ratios have on diesel engine design and efficiency. Cameron's column focused on the "law of diminishing return" and how that is applied to engine design in general, and to compression ratios in particular. Cameron explained that efficiency increases as compression ratios increase, to a point (of diminishing return), then other less desirable side effects become an issue. One paragraph I found particularly interesting is where he said:

"But there's another effect to consider here, which redoubles the law of diminishing returns. That is overall heat loss (amount of heat that needs to be dissipated) during combustion. As we raise the compression ratio, we also raise the peak flame temperature, and that pushes heat through the head and piston crowns faster. A couple paragraphs ago, we made a curve of theoretical energy recovery versus compression ratio - and how this heat loss effect causes that curve to flatten out even faster. The result is, that for diesel engines, that peak efficiency comes somewhere pretty close to 17 to one compression." Kevin Cameron TDR

More Power...

[markrinker]

Interesting topic. What effect does lowering the compression ratio have on 'startability' in cold temperatures?

[tanker]

Ok, I have had both. My original engine with the 20.2:1 compression ratio had a lot of spunk at idle, but I also blew a head gasket and also cracked a head, all covered under warranty. When my crankshaft broke (out of warranty) I had a choice, go with a new GM replacement engine, or have one built. So I contacted "The Diesel Depot" and talked and thought about what I had learned at several of "The Diesel Page" Rendezvous'. I needed a reliable engine that would hold up. I went with the 18:1 compression ratio, and ceramic coated pistons along with the phaser gear drive, and had the complete rotating assembly balanced. What I first noticed was a little lag on initial acceleration, but as the turbo spooled up I could feel a differance in power. I increased my boost from about 10-12 psi, up to 15-17 psi. Towing was a big improvement, and on long hills it just got down and pulled steady without high exhaust temps. This also lowered the coolant temps as well. This engine runs smooth mainly because of the assembly being balanced eliminating vibrations. The down side is the slight lag off the line, and more white smoke (unburnt fuel) at cold starts. Starts ok even down at 10-12 degrees F. without being plugged in at work. At home its always garage kept so its never that cold inside. I do think that the lower compression helps with reliability. smile.gif

[More Power]

In our 18:1 6.5, I learned that increasing first-glow time from a normal 8 seconds (at +25 degrees F) to 12 seconds produced cold start-ability comparable to a typical 6.5 with factory pistons.

MP

[tanker]

To Kent. I don't think cutting material from a piston top is a good idea. Yes it would cost less than a set of 18:1 pistons, but not being an engineer or metalurist (Sorry Dr. Lee :rolleyes: ) I'm not sure about reliability. That is what you want in a new engine, but that might be cutting into the reliability of what you want to achieve. I would spend the money inside the engine, and later on external accessories that are an easy bolt on, when you can afford it.
I to was unprepared for buying a new engine when mine went, so I went with as much reliabilty as I could swing at the time. I could have gone crazy with many other improvements, but my wallet was talking to me as well. I hope all works out with your project. smile.gif

[dieseldummy]

It seems like a while ago Grape posted that a 6.5 piston is only .049" thick to begin with. I was going to turn my pistons down .030" until I read that. The Diesel Depots 18:1 pistons are comparably priced to stock pistons I believe that the set I bought cost $695 w/anodized coating like stock. To get them ceramic coated was another $100. You can bore a new set of wristpin bushings offset .010", shave a little off of the pistons, and do things like that I guess, but after labor is factored in I reccomend going with a regular set of 18:1's. Just my .02.
Justin

[Tough Guy]

Two years ago a set of 18:1 pistons with a ceramic topcoat with rings was only $975 from the Diesel Depot, an advertiser here...

You get what you pay for, I would maybe try and cut corners else where...

Cheers

[grape]

I'm holding a stock 6.5 piston in my hand right now that I cut in half on my band saw. It is .490".......the fricken thing is 1/2" thick at the very thinnest point right in the center, I'm gonna gamble on .020" off of the 1/2 inch. You guys can continue to be scared, but I think I'll live. We are also leaving a 1/4" inch band around the deck of the piston and just turning the center on my lathe.
As far as getting what I paid for......I damn sure didn't get $2000 worth of performance from a banks turbo kit.

[99gmccrew]

Thanks for the info all. I assume if you want 18:1 pistons that you must pull the engine for the installation?

[eracers999]

Great responces,

Well there is 2 ways to get 18:1 that i know of.

1. You buy the piston cut .030 and it"s already recoated, common practice from what i have learned.

2. The piston has a raised pin height of .030 and the deck has not been cut.

Either way you wind up .030 below deck unless you buy a JK piston or order a custom.

Grape said it, when you look at the thickness of the dome removing .030 isnt going to do a thing to the strength of the piston.

At the engine shop where i am getting the block work done i asked them the very question of removing the coating on top of the piston to remove .030 and they said that they take .010 off the tops of 7.3 ford pistons all the time to compensate for decking the block and they couldent see where burning a hole in a piston would even be a issue.

Is a stock 6.5 piston coated on the dome in any way???

If they have to be recoated, they will, other wise there goin in.

Kent

[grape]

i'd deffinately re-coat them, usually around $150 to have the thermal ceramic stuff coated for a set of 8. The only pistons I know of with the wrist pin moved, Jamie sells and it is only moved .010".

[david]

I have the maley pistons with .030 cut off!!!!!
They made it that way.
I have a small puff of blue at start up but it runs very well and starts 1 or 2 sec. cold or hot!!!!! tongue.gif

[Cowracer]

Originally posted by grape:
We are also leaving a 1/4" inch band around the deck of the piston and just turning the center on my lathe.
So let me get this straight...

You are cutting at .30 dish into a piston with a diameter 1/2 inch less than piston diameter?

Hmmm.

Sounds worth a try, now where do I go to get them recoated?

Tim

[grape]

we aren't sure how much we have to cut yet. I'm going to try to cut as little as possible and do as much with the rod as I can. I think we can go .020" down on the rod at the most. I forgot all of my math for the 6.2.....but I'm going to have to first lay the crank in the engine and figure out where the top of the piston is with a stock rod, then do the whole cc thing with the head gasket, head, etc.

[Jim P]

I don't know about a 6.2 but on a 6.5, the piston sticks up .010" past the block when the piston is at tdc.


I installed new wrist pin bushings in my rods and then bored them .010" off center to shorten the rod up .010" The new bushings only have about .025" of stock so that is about all you can go. You could also have a little machined off the split at the bottom of the rod and then have it bored. This is what they usually do to resize a rod except they hone it out to size instead of boring it.. The problem is that the hone will take equal amounts off both the rod and the cap which will keep the rod length the same. So you need to bore the rod which will remove only material from the rod and none from the cap. After boring, the rod will have just a little stock left to hone to size.


I also used the pistons that are .010" shorter from the wrist pin to the top. With my rods shortened .010" and using these pistons, my pistons are now .010" below the block when the piston is at tdc.

[BuffaloGuy]

Any known problems with the piston being below the deck at TDC?
Ken

[eracers999]

I have been in touch with Mahle and they dont make the 6.5 piston with .030 off. Talked to jamie at diesel depot and they have been trying to get Mahle to produce a 18:1 piston, no luck yet. They make one .010 down for those that have cut the deck. It is very common from what ive learned to cut the piston down and recoat the tops, ceramic or anodized. I talked to jamie at diesel depot and theirs are down .035 and they are a sterling piston. Mine are commin down .032 and then being sent back out to be ceramic coated. The main thing to pay attention to is keeping the ricardo cup. You must be able to retain that cup and not make it flat.

Kent

[Kennedy]

Make sure that they have the steel plate and keystone ring.

My mod 18:1 pistons are dished enough to get the CR down, yet protrude .005" above the deck. I haven't done any in a while, but the ones out running are going strong.

[tom.mcinerney]

For a practical , economic rebuild i have a hard time imagining that a ceramic or anodized surface would be warranted. But I have no knowledge or experience to support my suspicion.
Kent/Grape: Please keep us posted.
John Kennedy's post above (need keystone & plate) suggests that he (JK) has encountered some substandard pistons . These would be a greater threat, i'd bet, than skimmed crowns.

[CareyWeber]

Originally posted by kennedy:
Make sure that they have the steel plate and keystone ring.

My mod 18:1 pistons are dished enough to get the CR down, yet protrude .005" above the deck. I haven't done any in a while, but the ones out running are going strong. John,

Is the stock 6.5 piston keystoned?

The steel plate is it for where the keystone ring rides?

Is there a Total Seal ring that is keystoned?

Carey

[Kennedy]

Originally posted by CareyWeber:
</font><blockquote>quote:</font><hr />Originally posted by kennedy:
Make sure that they have the steel plate and keystone ring.

My mod 18:1 pistons are dished enough to get the CR down, yet protrude .005" above the deck. I haven't done any in a while, but the ones out running are going strong. John,

Is the stock 6.5 piston keystoned?

The steel plate is it for where the keystone ring rides?

Is there a Total Seal ring that is keystoned?

Carey </font>[/QUOTE]The top ring is Keystone and rides in the steel plate. The second ring is the one that you want gapless.

I've had excellent success with the gapless, but know of one party who says he has worn out his bore with them. My guess is that the installer did not set the end gaps properly or something else was installed incorrectly.

Most of my 18:1 pistons are running gapless.

[CareyWeber]

Thanks John

[Old6.2]

Have you looked at Silvolite pistons? The 6.5s are hyperutectic with a cast iron top land for the Keystone ring set and are available in standard pin location #3436d or "destroked" #3449d with the pin located .010 higher. These might be a good starting point for your mods. I used these to rebuild my 6.2 three years ago and have no problems. They were under $300 per set but the keystone rings were $165!!