LLY - Cracked piston

NevadaLLY

NevadaLLY

Adam B
Feb 24, 2019
311
39
28
Reno, NV

Piston to wall clearance is stated by Mahle as .0016-.0022. My nominal bore size is 4.0755-4.0760. I think my machine shop went a little strong on the .020 overbore. My micrometer (.0001) measured the pistons at 4.0727-4.0730. Any way you look at it, even putting the 4.0730 in the 4.0755 my tolerance is .0025. So my question, do I need to remedy these piston clearances? Looks like I’ll throw the 4.0727 in the 4.0755 for .0028 clearance, and the 4.0730 in the 4.0760 for .0030 clearance. It’s the closest I can get with the parts I have. Also, even if the bore was dead nuts 4.0750, the largest piston I have at 4.0730 which is on the high side of the tolerance. The shop had the pistons and measured, so not sure why it ended up slightly big. Thoughts??
 
PureHybrid

PureHybrid

Isuzu Shakes IT
Feb 15, 2012
3,310
352
83
Central OH
How hard do you want to push it? And how many miles are you going to drive it?

I wouldn't be scared to put it together with that piston clearance and ring gap on the high side unless it was a stock rebuild. Usually .0045" per bore inch is fine (top ring)
 
NevadaLLY

NevadaLLY

Adam B
Feb 24, 2019
311
39
28
Reno, NV
The build is for typical street use/ heavy towing. I have a 19k toy hauler and travel around the mountainous west coast. It’ll see 7% grades for 8+ miles, so that’s where all the hard work is. There will obviously be some spirited, max effort pulls when empty, but that’s not really where I’ll build the heat I’m worried about.
Miles will be in the tens of thousands, if not going for another 100k
 
NevadaLLY

NevadaLLY

Adam B
Feb 24, 2019
311
39
28
Reno, NV
bimmer95

bimmer95

Member
Mar 16, 2019
82
7
8
Morgantown Pennsylvania
www.duramaxdiesels.com

Cylinder wall honing surface finish & ring gap

What is the preferred surface finish for seating rings on Mahle's race cast pistons/rings? I have 240 and 320 grit hones available. Also what ring gaps are preferred (compression and second ring)? Running small twins and maybe a small nitrous jet to clean up as needed.
www.duramaxdiesels.com www.duramaxdiesels.com

See post 3.If your bore is on the high side of spec ,you probably won't have an issue with tight ring gaps.Ive only modified the ring gap on one set of duramax mahle rings ever.And that was for hotrodding purposes

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Fingers

Fingers

Village Idiot
Vendor/Sponsor
Apr 1, 2008
1,711
82
48
White Oak, PA
Letters up. Race cast can be installed either side. The difference with the OEM is the pin hole was offset. Not so on the Race case.
 
NevadaLLY

NevadaLLY

Adam B
Feb 24, 2019
311
39
28
Reno, NV
Fingers said:
Letters up. Race cast can be installed either side. The difference with the OEM is the pin hole was offset. Not so on the Race case.
Click to expand...
Interesting. Do you know how far offset? I still have my old pistons and held the new up to both RH and LH and couldn’t spot the difference. Thanks for the intel!
 
S

S Phinney

Active member
Aug 15, 2008
4,011
18
28
Quncy, Fl
I have ran cast piston top end gaps anywhere from .018-.025 and haven’t had a issue. I like to see .018-.020 myself. Depending on the rings manufacturer top and second rings have been all over the place with end gap measurements. You can gap them if needed but are setup up as ready to run in most cases. Measure and reference your application in their information package.


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NevadaLLY

NevadaLLY

Adam B
Feb 24, 2019
311
39
28
Reno, NV
Small changeup in the build plan. Comp cams kept giving me the 2 week story for delivery since FEB and just told me "maybe June" LOL. I've changed over to the SoCal #3388 AF cam which will be here Wednesday! I hope to have this thing back on the road within the month of April.
 
NevadaLLY

NevadaLLY

Adam B
Feb 24, 2019
311
39
28
Reno, NV
When are billet injector hold downs necessary? Never saw any issues on old injectors but wasn’t running big fuel. New pistons are lower compression so I’m wondering what the real line in the sand is - if there is one? Forum is all over the place from do it with stock, do it after 500hp, and not needed until 1000+.
 
DAVe3283

DAVe3283

Heavy & Slow
Sep 3, 2009
3,679
235
63
Boise, ID, USA
For what it's worth, I tow on a 600 wheel HP tune with stock holddowns, and run 700+ at the track. So I'd say at least 600 HP before you need them.

Though I do have the Wagler aluminum heads with the cast-in injector cups, but don't know if that matters.

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Bdsankey

Bdsankey

Vendor
Vendor/Sponsor
Feb 1, 2018
3,905
1,073
113
Neenah, Wisconsin
There is no firm number. It purely depends on cylinder pressure and the condition of the hold down and bolt.

I personally start recommending them around the 800whp mark. With that, I have pushed my stock hold downs (brand new) to 1300whp.
 
NevadaLLY

NevadaLLY

Adam B
Feb 24, 2019
311
39
28
Reno, NV
I think I’ll keep the stock. Only have been out twice. Once for head gaskets, and this second time for the build. They’ve never been over torqued. It won’t see very many WOT, max effort runs and if it does, the truck will be empty.
 
B

buick455

Member
Sep 26, 2010
55
2
8
I am still running the 326,000mi original ones on my LB7. Been 750hp, 840hp, 960hp, and now over 1200hp. They've been removed and installed a lot of times. I have been running screw in cups and ARP hold down studs though since the engine was built the first time. I think for an LB7 the key is having screw in cups or heads with "cup less" injector bores. With an LLY up, I presume the oem hold downs should go a long ways with studs, they are pretty stout.
 
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