I bought a 5.3 lm7 and I need to change out the pistons. Can I change to lm4 pistons? What change will they have for compression? Just looking for a little info from someone who knows better then me.

Thanks

 

Well, the LM7 and the LM4 pistons are the same. I think you are talking about the L33 pistons, which were flat tops. Here is a problem you have to make sure you avoid. Those newer motors use floating wrist pins, and the LM7 uses a press fit piston. You can put flat tops in that motor though. You can just purchase a set of 4.8 pistons for your year of motor and it will fit right up. Factory compression height on 4.8 and 5.3 pistons is pretty close to the same. Here is the factory Mahle replacement catalog. http://www.development.itskentodd.co...cat_piston.pdf If you look at part # 224-3424 that the flat top 4.8 piston for the year motor you have. If you compare that to the actual 5.3 pistons (part number 224-3425) for that motor and you'll see the compression height relatively close. The 4.8 piston has alittle lower compression height (only .003 or so out of the hole vs .008 of the 5.3) being they had to keep the compression down around 9.5:1 on the factory 4.8 to make it 87 octane friendly.

In reality that's all they did in the later motors that had the LS6 heads or were the HO versions is they just put the flatop pistons from the 4.8 on the 5.3 rods with the LS6 heads to make up for the additional volume of the LS6 heads.

So swapping out the pistons and running 61 cc 4.8/5.3 heads will put you somewhere around 10.3:1 compression on paper anyways.

But I ask this question, why are you wanting to switch them out? If you are tearing down an AOK engine to basically install hypereutic pistons to only bump the compression I feel it's not 100% the best thing to do. There is alot of better things to do with your time and money. Now if it needs a rebuild that's another thing.

If it needs a rebuild first things first get the block to a machine shop and have them sonic check it. It might cost you more, but if you can overbore it to 5.7 your off the shelf options for pistons and cylinder heads open up greatly.

Be advised most iron block motors have a 9.230" deck vs the 9.240 deck of the aluminum 5.7 blocks so you would need to know what the deck height of the block is for sure before really doing anything. The good news is that the aftermarket has off the shelf forged 5.7 slugs for a 9.230" deck using factory rods. http://www.summitracing.com/parts/WIS-K366A05/

This probiably raises more questions than it answers but unless the motor needs rebuilt I wouldn't be changing

Wanted to add, educate yourself alittle on the desired quench of these motors and how to compression height is calculated and what it really means. I'd start reading here. http://www.lunatipower.com/Tech/Pist...ionHeight.aspx

 

My lm7 pistons are **** and I have a chance to buy lm4 flat top pistons that came out of an ssr from what I am being told.

 

Not sure if those would fit on your rods or not. GM made a change in 04 / 05 where they went to floating pin pistons vs press pin pistons that your factory rods use. You should rebalance the entire assembly. Look for new stock 4.8 replacement pistons like those in the Mahle catalog I showed above. You can pickup a set of those stock 224-3424 pistons with pins from a Mahle distributor for roughly $300 and then you would get another $100 or so in rings. Might not be a bad option if you are looking to do it on the cheap. You can get a set of Enginetech pistons even cheaper for about ($250 a set) I don't know how good/bad their track record is though.

Used pistons might be cheaper but for how cheap the stock replacements are it's hard not to pay that if you would pay for the used ones considering you need to purchase new rings regardless.

 

 

If I remember right the ssr had an l33 flat top pistons, all aluminum 5.3...if so those would work...I don't remember them putting a low compression motor (under9.75:1) in an ssr

 

Thanka for the info!

 

I would not assume that they are L33. Find out what year SSR they are out of. If its a 2003 low production year only 3416 made they are probably press fit pins .If they are floating get the rods also, and balance the assembly.

 

I have a 2000 5.3 and im using trick flows 205 heads with there 58cc chambers and wiseco junkyard dogs forged flat top pistons with a stock bore and stock stroke, any idead where I would be compression wise with a stock width head gasket?