Lmm pistons hit my lml crank

[Jakezed22]

So before I started chucking pistons in I talked to wagler and they assured me lmm pistons and rods would go in my lml easy.

So I put one in and it hits the crank, tried a few more and most of them do except for 5/6.

The thing i don't get is, everyone I talk to and every forum I read, sounds like guys have no issues with the crank hitting other rpo pistons.

Anyone got some real world experience?

 

[bimmer95]

I would not have known this.Stock lmm pistons?Hitting right under the wrist pin I assume?

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[Chevy1925]

So before I started chucking pistons in I talked to wagler and they assured me lmm pistons and rods would go in my lml easy.

So I put one in and it hits the crank, tried a few more and most of them do except for 5/6.

The thing i don't get is, everyone I talk to and every forum I read, sounds like guys have no issues with the crank hitting other rpo pistons.

Anyone got some real world experience?

Do you have the pistons installed correctly? Assuming you are using stock pistons.

 

[clrussell]

Dumb question.. why are you using Lmm pistons?

 

[bimmer95]

Lol that's what I'm wondering as well..they may be hitting because nobody ever tried it before

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[Jakezed22]

Was just trying to save money. Yes installed correctly.

Some hit under wrist pin. Some hit on the skirt.

 

[clrussell]

If you’re planning on making any power you’re taking a big chance with Lmm as they are notorious to crack, just like lbz

 

[LBZ]

Saving money in a diesel rebuild are not words you should be using. At the very least find a set of lb7/lly pistons and de-lip them if you can’t afford new Mahle or similar ones. lbz/lmm piston are an awful choice.

As to why they hit are the rods new?

 

[PureHybrid]

Was just trying to save money. Yes installed correctly.

Some hit under wrist pin. Some hit on the skirt.

Arrows up AND left / right side?

Rods look like noodles?

 

[Chevy1925]

Based on tech from Guy many years ago, looks like it’s due to how gm setup the pistons and crank counter weights. Pistons will need machine work or change them out to lb7/lly which should clear/fit as guys have ran them before on lml cranks

From Guy

The LML rods are considerably narrower at the wrist pin bushing. .708 (top of bushing) and 1.260 (bottom of bushing) for the LML vs .955 and 1.475 for all previous. This reduces the load capability of the wrist pin bushing. Although provides more structure to the piston. We had some of these in the shop in early 2010.

Perhaps GM learned from their mistake with the LBZ/LMM piston with integral wrist pin bushing (read 100 grams heavier than LB7/LLY). This design required a smaller dia. counter weight on the crank and more weight in the flywheel and damper to get it to balance properly. Not only were the pistons a disaster but the extra weight caused more bearing and crank failures.

Now they seem to have gone in the opposite direction in an attempt to lighten everything up.

It would be cool if they would hold up to some power, but I'm not going to hold my breath.

 

[Jakezed22]

Arrows up AND left / right side?

Rods look like noodles?

Yup 100 percent installed right.

No rods are straight. This truck was stock, with a blown head gasket. Ran perfect otherwise.

I appreciate the advice but this is stock, no towing, going in a light weight car. We've got a few lmm's in the family with over 200 000km on them and deleted/tuned with canned tuners. Lmm pistons would've been fine.

But my machine shop can get me a set of new lml pistons for 700 bucks so ill go that route.

The point of this thread was to point out that they hit even though I read and talked to lots of people who said they would work.

 

[Chevy1925]

Yup 100 percent installed right.

No rods are straight. This truck was stock, with a blown head gasket. Ran perfect otherwise.

I appreciate the advice but this is stock, no towing, going in a light weight car. We've got a few lmm's in the family with over 200 000km on them and deleted/tuned with canned tuners. Lmm pistons would've been fine.

But my machine shop can get me a set of new lml pistons for 700 bucks so ill go that route.

The point of this thread was to point out that they hit even though I read and talked to lots of people who said they would work.

As I stated above.

Also, you can’t put lml pistons on lmm rods. You will need lml rods as well or machine the rods to fit the pistons

 

[JoshH]

I'm a little surprised Wagler would tell you this would work. Your best bet on a budget would be to buy a set of LB7/LLY pistons to go in the motor.

 

[Bdsankey]

Personally, if I am going to take the time to assemble an engine of some type the last thing I would do is reuse LBZ/LMM pistons with their track record of cracking at all HP ranges (stock to modified). 200k km is only ~120k miles so that is realistically nothing in terms of miles.


I agree with Josh and your best option would be to pickup some LB7/LLY pistons to use with your setup. You'll have a stronger/more reliable piston that isn't going to break the bank nor leave you stranded just because they feel like it.

 

[Jakezed22]

I'm a little surprised Wagler would tell you this would work. Your best bet on a budget would be to buy a set of LB7/LLY pistons to go in the motor.

Exactly, I even said "so they will work no problem at all? " and the guy was like yup 100 percent. So maybe he was just fucking with me cause I wanted free tech support.

I don't know but after hearing that and then getting them all ready and in only to find out they hit, I was pretty disappointed in them.

This thread will be good for some one searching the same. When I searched I couldn't find anything on swapping pistons / cranks.