Low end in my 94 started knocking last week, decided to rebuild and while I’ve been through every bit of this car I’ve not rebuilt a low end before so I’m going to have some questions.

Pulled the ZF and engine on Saturday. Will hopefully get into the engine this weekend to confirm the low end issue.

Some opening questions…
no doubt age/miles/abuse caused it, 122k almost 30 yrs and heads/cam. I’d like to future proof the low end so it can rev without worry if I want to go bigger on the cam. What do I need to consider? Hoping to reuse crank/rods/pistons if they are undamaged.

ARP rod bolts still the go to?

any favorite bearing sets? Rings?

Do main bolts need to be replaced?

align bore and block decking… necessary? Recommended? Optional?

what does a machine shop expect to receive, just block with rotating assembly and otherwise stripped down?

oil pump, timing set, rockers, cam, etc are all practically new. Are they typically safe to clean and reuse?

I’m hoping to be able to get away with a hone and crank polish but understand that may be an unknown until it’s in the hands of the machine shop. Open to you advice.

as it sits now…

 

So if it's got a knock.... Usually that means spun rod bearing. Crank and rods aren't going to be reusable without major machine work after such an event. Oiling galleys are going to be full of metal too... FYI, that's a trip to the machine shop to get all the galley plugs removed and hot tanked, new cam bearingscat a minimum. Pistons and rings probably going to be junk as well.... bore... who knows.

There's no reason to install ARP rod bolts because that requires machine work on the stock rods, which is not cost effective. After the cost of the bolts and the required cap cat and hone of the big ends... you could have bought a brand new set of Scat rods.

Factory main bolts are fine for 2x the power a heads/cam LT1 can make... and again would require additional machine work to line hone the block.

Best to get it apart and see what's hurt.... I'll check back and be happy to advise you on what your options are to fix it.
Will

 

Thank you appreciate the reply and any advice/experience sharing. I keep hoping to have time after work to start getting into it but it’s still sitting on the stand exactly as pictured above.

I’m sure this weekend I can tear it down.

 

Pulled the pan this afternoon. There was definitely some bearing material in the oil filter housing but couldn’t find it much anywhere else.

I wiggled each con rod and far as I can tell it is just #8. Quite literally the definition of a “spun” bearing.

I’m going to pop the cap back on, right side the engine and get the heads off before I go any further into the bottom end.

but here is a pic of the bearing halves, obviously spun by their position. Presuming then might be stuck to the rod journal. Won’t know what that looks like till I get this really torn down.

 

Tore it all the way down - just crank, rods, pistons remain.

Can anyone advise me - what does a machine shop normally expect to receive? Can I bring them this engine as is with the lower rotating assembly installed? Would it be more cost effective for me to have it fully disassembled?

I didn’t take additional pics, however as far as I can tell it’s still just the #8 rod that spun the bearing. I did take the bearing halves off and the crank doesn’t look too badly beat up but I can feel grooves in the journal wit a fingernail. I do expect it needs to be ground.

The block looks great, there is still cross hatch in each cylinder. For a basic, cost effective rebuild, is it safe to clean/hone the block - keeping it a 350 and the. Rebuild with new bearings, rings? Rods I need to decide to upgrade the entire rod set or ARP/resize the stockers. Is decking and boring a must?

I have LE heads/intake, a small cam CC502, and bolt ons. Looking for a ~400 crank hp motor that revs to 6k and has a reliable bottom.

 

As for what the machine shop expects.... I have received it all - from complete with accessories to bare blocks. Just remember they do this for a living and charge for their time. To save as much as possible, I suggest you bring them the bare block , crank and the spun rod.

The rod can be fixed if it didn't get hot enough to turn the entire big end blue or black. They will tell you... if it needs a new rod... you are probably going to buy 8 and 8 new pistons to go with them, as I highly doubt that they will want to press off a stock hyper piston and reinstall it on a new rod... I know that I won't. Way too easy for them to crack and way too hard to see that they did...

At a minimum I would suggest getting the block hot tanked, new cam bearings, rings, rod/main bearings and cylinders honed. You will also want to be very careful cleaning the lifters and pushrods.

Crank can be turned or you should be able to find a good standard LT1 crank easy enough if you don't want to turn the original or it needs to go something excessive like .030 to fix... if you do decide to turn the original crank ask him to put it on the small side of the spec... this will help with reliability considering the added power. GEN2 LTX engines are notoriously tight on rod and main bearing clearance and why so many spin bearings once people start modding them. If you get a replacement crank have them measure it after polishing and help you decide if you need "X" bearings or not.... probably will because GM has those blocks and rods on the minimum for the housing bore spec...

IMO you want a minimum of .002 clearance on the rods and .0025 on the mains.

As for what the minminimum of what you can get away with... Machine shop that's looking at it should be able to make that determination with a little measuring.... like I said before about the rod bolts... waste of money on stock PM rods. If your trying to do this as cheaply as possible, just know the factory bolts are not a weak point.... same stuff we have to use on 602 and 604 oval track crate engines.... they stand alot of abuse once the bearing clearances are sorted... as delivered not so much.... The PM rods break in the middle of the beam all the time... I've never seen a bolt let go and I have some cheated up 604s that top 550hp.
Will

 

Thank you for all the above info. I’ve read that the PM rod are robust but that the factory bolts stretch at high rpm, distorting the big end and hence rod bearing failure. I’ve also read that LT1s are built tight, so the extra abuse on the aged assembly makes sense too. ARP rod bolts were supposedly the cure for the bolt stretch issue, no merit to this? Keep in mind pretty mild motor.

New rods/pistons would certainly up the cost. And I get it, going cheap isn’t going to get me far, but between daycare, mortgage and all the rest I do need to be budget minded about a car that is nothing more than a toy. So I’m trying to make economical decisions. I’ll need to decide on rods.

For the time being, I’ll continue the tear down. If pulling crank and rod/pistons saves me a few bucks I’m all for it. I ordered a # punch set so I can indicate which hole the rods come from. Not wanting to damage things I YouTubed a video…

if you have a moment to spare, is this fellows method ok? I was hesitant to smack things. Skip to 4:45

 

JMO, I know money is different for people but if you swing it, I'd do the following:

-You obviously know the crank will need to be serviced.
-Might need a cylinder bore depending on the taper. I'd expect to be doing this regardless. (Which also equates to new pistons). Don't need anything fancy for pistons. Last set I bough was about $200 for some hypereutectics.
- Have the rods (all of them) serviced.
- Have them install the pistons on the new rods.
- ARP rod bolts. Weak point on these motors are the rods. These are what tend to break. Ask the shop to get the rough torque numbers for these as ARP does by measured stretch, not ft-lbs. (Or just buy a dial indicator if you don't have one)
- You could do a line bore and for the cost of it, I'd personally do it, but if you're tight on budget, usually you can probably skip. But also talk to the shop and get their opinion as they can give you the definitive measurements.
-Get the heads serviced since they're off. If you're going bigger cam, you can also address valvetrain at this time.

I take it you're going to be assembling yourself. Get a NICE dial bore gauge and be meticulous in your measurements. Get two sizes of crank/rod bearings. You can mix and match sizes within .001 of eachother when you need to chase that 5 tenths. Me mindful that when you mix/match, the smaller of the two needs to be on top (closer to the piston). Google this for sanity check to make sure I'm not telling you backwards. I personally like to build it so my crank journals are in the middle of the tolerance and my rods are towards the top to promote oil flow to the rods.
-Tons of assembly lube.
-Check thrust. Should be .005 to .008 IIRC.
-High pressure lube for the pushrods.

Good luck!

 

Smacking it like that is completely fine. Doesnt take much to mark tge rods.

 

Won't hurt anything to use the stamp set and hammer on the rods. People have been doing this for 50+yrs... even the factory... the mains were marked with a hammer and stamp at GM.

If you're going to keep stock rods and pistons, I would not spend money to upgrade the bolts.
Will

 

Thanks all… sounds like a lot of votes for new pistons and rods! Which is I presume how many people end up with 355 ci. I haven’t priced this out but I’m assuming given the guts of the LT1 is SBC that there’s a million options across a large price range.

I do want to do the final assembly myself, mostly as a learning experience. This is my first rebuild of a car engine and I know there will be much learning to do.

Where does the dial bore gauge come in? I presumed I could use plastigage to validate tolerances on rods/mains.

 

Dial bore gauges are typically used to measure cylinder bores for size and taper. Due to their high degree of accuracy (good ones measure to 1 ten thousandths, or .0001 of an inch) many machinist use them to measure bearing clearance as well... its an industrial tool and can measure the inside of any bore practically. There are other way to do these things like inside micrometers.... while non-critical dimension checks on some things can be made with snap gauges and plastic gauges.

What you do with the rods and pistons depends.... Eagle SIR rods or the Scat Pro Stock rod are right at/just over $300-$350. Hyper replacement pistons are $200-$350 (Silvolite, KB claimer or Speed Pro)... Forged pistons would be about-$550 (Wiesco Pro Tru)... alot of these new pistons will come with new rings so there is some cost offset there... Going with new pistons opens up option for much better/stronger parts and the ability to use 6.0" rods which will lighten the Bob weight considerably and longer rods do offer some minimal performance gain with limited induction combinations. Also careful selection of the pistons can add some compression as the valve reliefs are smaller and reduce parasitic drag as the rings can be smaller and skirt design better.

Keep in mind that you will have to rebalance the crankshaft if you change from the factory rods and pistons... that's a minimum of $200.
Will

 

Appreciate all the info here. I received my punch set and need to punch each rod/cap and main cap. Then I’ll do the final tear down of the rotating assembly.

I think after that it’s off to the machine shop, it really seems I need their evaluation of things before I can make next step decisions. I still need to find a machine shop, I have a few references.

 

Here’s my 2 cents. For anything other than a crude refresh (original equipment and machining was indeed crude)… use a machine shop that does a lot of dirt late model or circle track engines, otherwise you take a chance for your goal of revving without worry (that’s all I got to go on here, apologize if it’s not the case). RPM’s demand better components, so be specific about what you want. Your current components for an LT1 are no more than 6,000RPM and that’s above the design spec. 7,500RPM for example needs special high end hydraulic lifters at the very least, and upgraded springs, rockers, more flow and fuel to overcome the dramatically increased level of friction and load, tighter main clearances, better windage control, stronger rods and pistons and pins, stronger cam chain. The machine shop will want the pistons you’re using and the bores should be honed (or bored) with a torque plate. The finish will also depend on the ring package. At or above 7,500 you will need shaft mounted rockers and solid roller lifters, and billet core cam if you’re cam spec follows, forged rotating assembly, and a lot of detail work. So the point is develop your specific goals and go from there.

 

Thanks for this info, what you describe sounds far and above my plans and goals.

It has mild Comp cam (which looked perfect when I pulled it out last weekend) and with that cam it received a complete upgrade to the top end, springs, rockers, etc. I run an electric pump and a 95 timing cover clearanced for a double roller. All of this barely has a few thousand miles and really just needs a cleaning. I’m planning to reuse those components - maybe not the lifters which are stock originals.

The goal remains ~400hp but I’m ceetainly not hung up on an ho number. No more than 6k rpm. Prior to spinning the rod bearing, I had my fuel cut at 6100 but in light of this experience I think I’ll be bringing that down and keeping shifts around 5500. I really miss driving the car and it’s made me realize that I miss cruising more than power so I think the experience is also going to shift how I drive the car.

Haven’t found a machine shop yet, mostly because no time, likely won’t get to that until end of month when I have some time off.

 

Curious if anyone can answer this question about engine balancing and the dual mass flywheel.

My understanding is that LT1 are factory externally balanced with weights on the flywheel. In the event I go with new rods/pistons the rotating assembly will need to be rebalanced. I’d think an internal balance would be done and that means the DMF would have to be rebalanced to neutral.

I’m struggling to find the “weights” on my dual mass flywheel that would accomplish the external balance. I don’t see anything bolted to it. There is a section, circled in yellow below, that is raised. Does that mass accomplish the balance? To neutral balance is mass removed from that raised area?

 

You are correct about the weight, spec for the OEM crank is zero balance at the front and 23.4 in/oz balance at the flywheel.

This means if you buy an aftermarket flywheel, get the balance weight kit. Not sure how the DM flywheel was resurfaced as it’s way to heavy for anything that I like to do, but what I understand is some have carefully spot welded the two masses together, resurfaced, balanced, then cut the spot welds off. Problem I see with that is possible break down of the elastomer.

Note that if you go to a SMFW especially a lighter one, install a better/heavier harmonic balancer at the front (ATI or Fluid Damper etc) as the heavy DMFW helps dampen out torsional vibration and leaves the crank more vulnerable with a light weight SMFW

 

You do not have to remove the weight from the flywheel if you have the rotating assembly rebalanced due to new rods and pistons. You can.. but you don't have to. If you decide to remove the weight, I have seen and know a member local to me that had his dual mass flywheel weight removed on a lathe, its not a big deal at all hasn't caused him a problem and is how I'd remove one in my shop. If you need a new flywheel most aftermarket ones have a bolt on weight so that you can choose between external or internal balance on the rear.

Of all the LTXs I have built with 6spds everyone has elected to use a new single mass flywheel... and I went internal balance to minimize the amount of drilling I would have to do on the rear counterweight of their brand new 4340 forged crankshafts.... I did one 91 L98 into a 383 LPE SR combo for a member here that kept the dual mass flywheel and we just left it external balance on the rear as it was a cast crank and 5.7 rods... which would have needed Mallory to balance internally... this was advantageous for him financially as it was cheaper for me to balance it but with your combo anything you buy aftermarket rods and pistons wise is going to be lighter than the stock parts so it doesn't matter what you do internal or external balance on the rear.
Will

 

Thanks fellas.

I know the DMF isn’t for everyone. I do like it for a few reason, primarily noise reduction in the ZF. I don’t like loud. I have stock mufflers even. I know a gurgling ZF would drive me nuts.

It’s also just a street car/toy and heavy flywheels do make driving a little more relaxed - in my experience.

Seems if I stick with stock on the crank I don’t have much to worry over.

I do need to put it through the FSM check. It passes that test a few years back when I had it apart last.

Apprecaite all the responses!

 

I have some feelers out to local machine shops - waiting to hear back. I almost feel it’s “if” I hear back as I’ve found that you often have to go there to talk to someone.

anyway, I’m trying to plan that it’s going to need a bore. So I’m looking at pistons/rods for a 355. There’s an infinite number of combos.

Some have mentioned Scat rods. Any p/n? I’m thinking pressed pins, hyper pistons and I’d like to keep the compression similar to stock. So a stock-type piston. That wouldn’t change compression, quench, all those factors that I’d like to leave well enough alone. Basically, upgrade rods and pistons that are like stock for the 30 over bore.