IDI piston height

[Mikey89014]

Hi,

Can someone tell me why my new pistons come out of the block .040 ? The book calls for .031 max. These are .010 destroked pistons, i was told. They are part number H729CP30.. They say Mahle on the piston as well.. I also had the block decked .010, according to the machine shop.. Thanks everyone..

 

[PwrSmoke]

Either:

A: Block was decked more than you were told (or it was decked previously), so you need to check.

B: They are not 0.010" destroked pistons. Check with Mahle and have the machine shop check pin height.

Have your machine shop look up AERA bulletin TB-535 (dated 1988). It may confuse things more but says:

"The recommended maximum piston protrusion specification fro Navistar 6.9L (420 CID) diesel engines is 0.055" +/- 0.002. There seems to be some confusion as to the proper piston protrusion specification for this engine because Navistar International is not including the information as part of their service publications. The measurement of 0.055 +/- 0,002" was calculated using the compressed thickness of the cylinder head gasket of 0.062" +/- 0.002". AERA members have reported measuring actual piston protrusion from 0.020" through 0.035". Increased piston protrusion beyond the recommended specification may result in contact betweem piston and valves or improper combustion. reduced protrusion can cause hard starting and white smoke during warmup on cold mornings."

The AERA Technical Committee

 

[Mikey89014]

Thanks a lot, appreciate it.

I just found out my new pistons are not .010 destroked. I used an old piston from a previous .030 over and .010 decked reman engine to check the height, and it's .028. So, this tells me i need at least 3 new .010 destroked pistons, as only 5 of the old set are not beat up from broken glow plugs. I dont think i want to use the new set being they are .040 out, This is going to be a turbo engine, and .040 out may be too much compression, i would think. Back to the drawing board

 

[IDIBRONCO]

It seems like someone used to make a thicker head gasket for this problem. Maybe it was just for the 6.2/6.5. Don't remember for sure.

 

[icanfixall]

The felpro head gasket is about 10 thous thicker than all the others. I have not verified this yet. How did you determine the piston height.... I have the special piston deck height tool with a dial indicater on it... If you want to borrow it please pm me a name and an address. Sadly this tool was with Jasom Tompson from Diesel Power Magazine for about a year. I have been helping him with his 7.3 rebuild. He has written some really great articles in the magazine titled.. Project 300. Its about the 6.9 truck he bought for $300.00 and is fixing it up with details about cost and the how toos. My navistar service shop manual quotes the max piston out of the block is 31 thousands. You can mill 14 thousands off the piston tops too but don't do any more than that. The heat will be too close to the top ring and this information came from Eric at Mahle pistons. I talked with him 6 years ago about milling pistons. My thinking the reason your pistons are out that far is because the block has been machined to straighten the deck once or twice. Milling the pistons is simple in a lathe or in a bridgeport mill flycutting them. They will need to be balanced after milling. The pistons that have some damage might clean up from milling them too so you wont really need 3 new pistons.

 

[typ4]

and it will cause piston to valve intimacy with a cam spec change when they are that far out.

 

[Mikey89014]

I have a depth mic tool that I used , and they are definitely out by . 040 with the new ones , and .028 with the old ones . I may see if I can find a thicker gasket first, and maybe 3 pistons second. I don't think I want to mill the new pistons , as I will have to balance everything. I will look today .

I'm also thinking the machine shop took more than .010 off the deck ,that's what he stamped on it , but I'm not so sure at this point.

 

[icanfixall]

Something isn't making sence here. You say the old pistons are 28 out but the new pistons that are 10 thous lower are 40 out... Thats does not add up. What I'm saying is the old pistons are lower than the new pistons. You may have a set of old pistons that were low comp by 10 thous and someone has milled then even more because someone decked the block. These engines have a measurement from the top of the deck to the center of the crank. Thats very difficult to measure because you need to "find" the centerline of the crank by fabricating a dummy shaft and bolting it in the main bearing. Then measure down to the edge of that tool and subtract half the tool diameter to find the centerline measurement. BTW the specs for this measurement is 11.137 to 11.141... Thats only a 4 thousands tollerance and you have at least 10 taken off the block... This may well be where you problems are. Now you will be required to mill the pistons to make things fit properly.

 

[Mikey89014]

The old pistons are the original .030 over pistons that are .010 destroked, they were in a reman engine that was decked .010 for sure, they say Mahle inside. The new ones are Clevite "224-2190" , that also say Mahle inside. The new ones are .040 out because i guess they are not destroked like the old ones, they are the stock 1.945 height that aren't designed for a decked block. I measured them many times with the pistons as far TDC as i can get. I measured this with a dial indicator on the piston, and rotated the crank until the indicator stops moving either way at TDC, then measured the piston height with a depth mic, measuring from the top of the piston to the deck. Thanks

 

[Mikey89014]

Anyways, i just measured the new pistons with some calipers, and it's 1.945 in height. They are labeled wrong. Old ones are 1.937 in height, just measured them as well.

 

[Mikey89014]

I guess im going to try to find a thicker gasket for now.

 

[Mikey89014]

Also, does anyone sell this offset small end pin bushing that I have been reading about here? Thanks

 

[icanfixall]

You will probably have to buy new and machine them offcenter. Thats what typ4 did...

 

[Mikey89014]

I just found out that there is conflicting info on the piston height. I have 2 pistons that have the same number , but different heights . I also saw online that an H729cp clevite piston has a 1.937 compression height, according to the site, however mine are 1.945, i measured them 5 times at least.. I also saw summit has an H729cp with 1.945 height and northern auto parts along with federal mogul sites with a 1.937 height. *** is going on with the specs? Do manufacturers have the same part number for 2 different height pistons? thanks

 

[Mikey89014]

I also just found out by measuring , the ring lands for both the standard height and .010 destroked height pistons have the exact same ring land heights, and i can actually see where the portion of just the head itself , above the lands , is lower or higher on each piston. This maybe tells me that the manufacturer themselves maybe just shaves the head on a piston to achieve the desired height, which might also explain the same part numbers for each. I can also tell these pistons were not shaved by the engine builder because of the head number stampings, etc.