Piston Talk

[icanfixall]

Well all... I have been calling several shops asking how much cleanance we have between the piston and the head. Seems this question has not been addressed by any of the shops. At Promar I talked with John and Brian. Each piston is milled to fit. They warned me that these pistons will rock (roll over ATDC) about 12 thousands so make sure the piston has the proper cleanance or they will hit. Example. If the piston clears the top of the block by 30 thousands then add 12 thousands to that and you have 42 thousands of the piston above the block. If your head gasket is 70 thousands thick (Felpro are about 72 thousands umcompressed) you will have a clearance of 28 thousands before the piston hits the head. Diesel Direct will use 5 thousands above the deck. They mill each piston to fit also and the go the extra mile a ceramic coat the top of the milled pistons. I still want the factory specs for clearance of piston from head. The factory specs for piston above the block is 10 to 31 thousands and that was verified by our members here. Thanks. Chris at Diesel Direct told me the hard anodized tops on the Mahle pistons can easily be flycut using a 2 1/2 inch flycutter. All the anodized top is aluminum that has been traeted so it machines off fine. Looks like I will be flucutting my pistons soon. Chris also said they set up these 7.3 motors to run on 18 to 1 compression ratios. I don't feel to good about that number. I really question that.

 

[Diesel JD]

You think he is wrong about the 18:1 ratio, that it just sounds low, or do you think that will cause starting problems in an IDI? Interesting info. Keep it coming,
J.D.

 

[OkieGringo]

Wouldn't an 18:1 compression ratio give lower compression tester readings across all 8 cylinders? If you could still start it with that ratio, it would seem good for high boost turbos. You might need to add lotsa cetane boost to the fuel to fire it up. OkieGringo

 

[icanfixall]

My thoughts are this is too low for an IDI but I was told the chevy diesels use this 18 to 1 and are also an IDI motor. The psd motor uses an cr of 17 to 1 if I remember correctly. If we can start with a 18 to 1 cr that would sure make our batteries and starters last longer plus we would have more room for boost but with an added intercooler. Otherwise any boost over 12 will be lost because of the added heat the turbo does to the incomming air charge.

 

[towcat]

gary-
dropping the CR to 18:1 on the Ford IDI doesn't seem to be as feasible compared to the GM. Dunno why, but hearing the audio clip of one firing up is rather unerving. Btw, the chev 6.5 layout is the opposite compared to the ford. the reliefs in the piston is on the bottom compared to the top on the ford. personally, I don't know what the answer is for you idea, but you sure are putting alot of research into it

 

[icanfixall]

Well, I just got off the phone with DAS about the piston to head clearance. They don't measure it at all. They live by the piston protrusion out of the block and thats it. They don't go over 30 thousands out either. I'm at 46 thousands out so I guess its time to mill the piston tops or try to buy a set that has the pin machined 10 thousands higher in the piston. DAS said they were available from Mahle. That way you are not loosing any of the "meat" of the piston. So looking at my setup if I wanted everything to stick up out of the block equal amounts I need to mill this amount from each piston in each hole. The most would be 0.015 then 0.014 then 0.012 then 0.011 then 0.010 then 0.009 then 0.007 from the last 2 pistons. Geez... I wonder what that will do to the balance shot I have to do??? The Mahle engineers told me that these motors can run fine with pistons being up to 15 grams differance in weight??? I think 10 grams at 3000 rpm is 3 lbs differance in dianamic weight throwing around the stroke circle. 15 grams would be 50% more than that...

 

[Agnem]

One thing is for sure. If I ever rebuild one of my engines, I'll be looking for your phone number.

You da man on rebuilding!

 

[icanfixall]

Thanks Mell. I'm flattered by your comment but.... What I do is not so special. I try to look at every possible measurement and differant way something can be assembled. In the machine shop I worked in you should have seen the work we had to do on nuclear stop and throttle valves that weighed over 15,000 lbs. The steps needed to be followed and doubled checked by many others. The 20 ft long by 6 ft diameter spent fuel storage cans would have 10,000 inspections before they were certified for the spent fuel storage. They were build to withstand 500 degrees for 10,000 years... We knew they wouldn't be asked to last that long but they were designed for just that. On my motor I just want the best I'm capable of. Some things like the piston to head clearance seems to be a number nobody has. I will find this number but how close it is may be off by a few thousands. About the easiest way I have found will be placing clay on top of the piston and torqueing down the head, then removing it and mearureing the thickness of the clay. Even that may not be as accurate as I want. These motors are not NASCAR motors that are assembled in clean rooms like an operating room. We only turn about 3400 rpm max. Sure some have run them more but I personally try to stay below 3000 rpm at all times. Its just where I'm happy. I am starting to get concerned about getting my pistons back from Diesel Direct. They are trying to even out the balance differances of 2 pistons. Looks like I would be better off just buying another set but with 10 thousands milled off. Even that seems like there is 2 differant ways to drop the piston down 10 thousands. One is milling the top and the other is raising the pin in the piston. I'll be calling Mahle today about that information.

 

[icanfixall]

Just got off the phone with Mahle. They have low compression pistons in both large and small pin types. They mill the crowns off 10 thousands and that does not compromise the strength any. My question is why does the piston have this "extra" meat that could be romoved to save material in the making of these pistons??? Anyway I think I'll justmill my pistons after they fly back to me from Florida. I will be installing them just to recheck the amount they are out of the block. Somehow my readings of as much as 46 thousands out seem a little suspect. Maybe its time to buy more Starret measureing tools.... I do like their tools... Not much better than that anymore....

 

[typ4]

Hey there, I found the build sheet when I assembled my engine and I came out with 22 thou of protrusion.
I wonder if your rods were rebuilt by a shop that just hones the bushings. Diesel rods have to be bored so they are the same length. I bet they were honed because Mahle pistons don't vary much.
That is how I got my rods a little shorter,6 thou, from the machine shop. There isnt much meat to offset bore the bushings but a little helped since the block was decked .005.
I didnt check piston to head clearance but when you do let us know. I would mill the pistons off the same amount rather than have the balance job disrupted. Smooth is better that exact compression I think.

It is hell to be machinist oriented and building an engine ,isnt it??
They dont have close tolerances on anything.
Have a good day

 

[icanfixall]

Russ, I bought a set of turbo rods directly from Ford. They were the remanufatured type. I'm begining to think my readings were suspect. So when the pistons get back from Florida I will recheck the readings I have.

 

[icanfixall]

Still no pistons in the drop off location (my porch) yet.. So I started checking the dome thickness of the "other" set of Mahle turbo big pin pistons and compared that to the stock set I pulled out of my 2 spare motors. Funny thing the stock non turbo motors had Mahle pistons in them from the factory. Anyways, the new set of turbo pistons dome thickness is 0.580 thick and the non turbo pistons are 0.560 thick. I wonder how much I can mill off of them and still have a piston??? I'm thinking 0.025 would be the limit but I don't know where to find out. The Mahle one and only rep is "out of the office" till the 28th..... Ask me how I know this......

 

[typ4]

Well my 30 year experienced engine parts man, that I grew up with, said Mahle is oem in those engines. That is what I have found in all the ones I have done. He says DO NOT use anything else..
I measured mine with a dial indicator zeroed on the deck and then slid over to the piston top then brought it up, not perfect but pretty dang close.

 

[icanfixall]

I did a similar indicator deck reading. I used a Starratt 196 with a button pad. Thats why I'm doughting my readings. Just for comparisons I will be using a 1 inch dial indicator this time. Sometimes I'm just a little over the top trying to verify "things".

 

[sle2115]

I did a similar indicator deck reading. I used a Starratt 196 with a button pad. Thats why I'm doughting my readings. Just for comparisons I will be using a 1 inch dial indicator this time. Sometimes I'm just a little over the top trying to verify "things".

Can't ever be over correct!