4.0 rebuild questions

[GravyDavey]

Hey guys,
Last winter I had the 4.0 from my 88 rebuilt by a recommended machine shop. At the time, the rod bearings were shot and I was chasing a mystery knock. It wasn't a flexplate knock.

Within a 30 minutes of run time I could hear that the rebuilt engine had a knock. Actually, I think it's piston slap and I was able to isolate it to cylinder 6. Noise went go away if you got it nice and hot. Engine ran well otherwise.

The noise still bothered me though because its a fresh rebuild and I was told it would belike new. Went back and forth with the builder even had them tear it down. They told me that everything is in spec and they won't warranty it. Even charged me for the tear down.

I took it to a 2nd shop a couple of weeks ago for another opinion. They just got back to me and told me that the piston to cylinder wall clearance is 3 1/2 thousands. He said that's loose but he couldn't say for sure that it would cause piston slap. Can I get some opinions on that? FSM says .0009-.0017 is spec. How does "3 1/2 thousands" convert into a decimal point number? .0035?

I think I'm going to have the 2nd shop rebuild it again, but I want to have a good feeling that its going to finally come out right.

 

[RCP Phx]

Those numbers sound reasonable but would actually depend on the pistons that were used?

 

[GravyDavey]

I'm not sure what brand, but I know they're cast not forged. I'm really at crossroads here. If I'm right on my decimal conversion (not sure I am), then it seems like the clearance is double what the FSM says it should be.

Here's a video of what it sounds like: https://www.youtube.com/watch?v=qG2X5zVEEsg

I pull the fuel injector on 6 and the sound dies way down. Piston slap?

 

[Anak]

You are correct in your math.

My guess is that the first shop is trying to claim that is acceptable on account of other engines which run such loose tolerances. I don't know what the tolerances are for the 8.1L in my Suburban, but that thing sounds like a diesel on start-up. Let it warm up and it will finally act like a gas engine. It bugs me. I don't think it is right, and if the day ever comes that I can justify rebuilding that thing I have every intention of tightening up the tolerances. They call it "cold start knock" and claim it is no problem. I guess if you don't care what the customer thinks then maybe it isn't a problem.

Best of luck.

 

[RCP Phx]

It definitely doesn't sound good! Was it bored with new properly sized pistons installed?

 

[GravyDavey]

It was overbored .040 and new pistons were installed.

The 2nd shop guy said that the problem could have been that the 1st shop bored according to the spec provided by the piston manufacturer, but they didn't actually measure the pistons before boring. He said his procedure is to measure the pistons after he as them in the shop then bore according to his measurement of the pistons.

You think .0035 is loose enough to get piston slap?

 

[RCP Phx]

That is correct on the measuring. My stroker with Autotec forged pistons requires .003" clearance measured at a "specific" point on the piston.

 

[GravyDavey]

Forged needs room for expansion right? Out of curiosity, do you get any slap with that clearance?

 

[RCP Phx]

Forged pistons are usually considered to expand less under the same conditions. I have no slap of any type but I also run "floating pin rods".

 

[GravyDavey]

Thanks, I'm at a loss as to what I should do. Crate motor or another rebuild.

I thought maybe it was a wrist pin noise at one time.

 

[RCP Phx]

You have a "pressed-in" wrist pin which can cause more side to side movement force from the rod so if there is excessive rod side clearance it will exacerbate the issue.

 

[RCP Phx]

Forged pistons are usually considered to expand less under the same conditions. I have no slap of any type but I also run "floating pin rods".

Sorry,I should have said "My" pistons, for the same material composition cast pistons will expand less than forged pistons!

 

[90xj06]

i got a rebuilt engine from autozone i primed the oil pump for a while before starting it and it ran fine for 5 minutes started knocking and ticking. lost compression on 3 cylinders thought it was hosed. checked all the push rods and everything looked fine. so i slept on it. got it started the next day and it slowly grabbed all the cylinders and now runs fine.


it took a while for it to come around. i think the lifters got air in them or the rings got suck or something.


have you driven it around any?

 

[CJR]

My personal comments on rebuilding/modifying internal combustion engines;
1. Many different brands of pistons use many different types of alloys; each alloy having different thermal expansions.
2. I NEVER BORE a block to some fixed increment; +0.040" etc. and then try to fit a piston to it. I only use forged pistons and the piston manufacturer,who knows what his particular alloy's thermal expansion is, and he tells me what bore diameter I need for his forged pistons.
3. My actual experience with cast piston failures and the damage they cause more then justifies the cost of forged pistons. Many cast pistons are cast with internal steel re-inforcing plates to strengthen them; so with different thermal coefficients/expansions for steel and aluminum , the high thermal stresses eventually cause aluminum fatigue and piston failure.
4. ALL my pistons are machined so that the piston pins are "free-floated" and shortened(lighter/higher revving) and machined with grooves for double TruArc locks in the piston to lock the shorter lighter piston pin on each end.
5. ALL my modified engines are balanced.

Why do I do this? Because that's how Mechanical Engineers are trained in Engineering school to design internal combustion engines. A little racing experience also helps to see actual engine failures and what they cost $$$$$$$.

Best regards,

CJR

 

[GravyDavey]

I ran the motor for about 2k miles. All the while communicating with the engine shop trying to figure out why it makes such a racket. 2nd shop said it should be ready by the end of the week. Hopefully, I have better luck with this second build. I spoke for a while this morning with the guy about piston choices in particular.

 

[JonnyCat63]

I'm located in San Diego, as well. Out of curiosity, which shops have you used?

 

[RCP Phx]

1. Many different brands of pistons use many different types of alloys; each alloy having different thermal expansions.
2. I NEVER BORE a block to some fixed increment; +0.040" etc. and then try to fit a piston to it. I only use forged pistons and the piston manufacturer,who knows what his particular alloy's thermal expansion is, and he tells me what bore diameter I need for his forged pistons.
3. My actual experience with cast piston failures and the damage they cause more then justifies the cost of forged pistons. Many cast pistons are cast with internal steel re-inforcing plates to strengthen them; so with different thermal coefficients/expansions for steel and aluminum , the high thermal stresses eventually cause aluminum fatigue and piston failure.
4. ALL my pistons are machined so that the piston pins are "free-floated" and shortened(lighter/higher revving) and machined with grooves for double TruArc locks in the piston to lock the shorter lighter piston pin on each end.

That's were Russ P takes things even further by using Scat rods that are "bushed" for a "free-floating" .927" tapered wrist pins! The Autotec pistons he uses are about the best using "4032" which has a very low expansion.

 

[CJR]

After my last post I decided to elaborate further on the process required to bore an engine block;
1. When a new/"green engine block" is bored, the bores are round and straight but still have some residual stresses.
2. As the engine runs/thermally cycles, some of the residual stresses "relax" and the bores become somewhat oval and the bore centerlines(C/L) bend slightly over time.
3. When the "seasoned engine" is ready to be re-bored, NO ONE knows what distortion exists in EACH bore or how much boring will be required to clean-up each bore.
4. So the boring procedure typically goes like this; the oval/worn/distorted bores are opened-up gradually until each bore is ROUND and STRAIGHT. Then typically, for a six(6) cylinder engine, we have six(6) round/straight bores each one having a DIFFERENT DIAMETER. Then we do a FINAL BORE/HONE so that all bores are now the SAME DIAMETER.
5. At this point, we are now ready to order our pistons based on our FINAL bore diameter.

As you can see, we don't buy pistons before the engine block is bored/honed because NO ONE knows how much each bore needs to be cleaned-up or what will be the final bore diameter for all cylinder bores.. Likewise, savvy engine builders look for WELL SEASONED engine blocks that are stable, after numerous thermal cycles, OR they have the engine block annealed to remove residual stresses. If you build an engine for high horsepower output for long periods of time, with a new/green engine block, the engine block may stress-relieve during the race and distort the bores and catastrophically "crunch" the pistons. Racers are in the business($$$$$) to finish races and WIN. "Crunched engines" DON"T finish or win races.

Best regards,

CJR

 

[GravyDavey]

Thanks, CJR and RCP. Great info here! Some day I hope to have a garage where I can build a stroker myself.

PM sent to JohnyCat re engine shops.

 

[GravyDavey]

Rebuilt motor installed and it sounds great. That noise was certainly piston slap from it being too loose.