A brief history:

1992 LT1 with LT4 heads, supercharged, CNC'ed and ported. A tapping at idle developed which gradually got louder. It was subsequently determined to be a failed cam lobe (#6 intake). No evidence of knocking, and good oil pressure.

As I disassembled the motor, no other problems have been found (except below) and everything looks clean.

While I have the heads off, I decided to pop the pistons to replace the low tension oil rings to fix my oil consumption issues. I checked all the rod ends and everything seems really tight and solid. However I am seeing discoloration on the rod bearings, although they are very smooth, not scored and the crank journals are bright and shiny.

I'm thinking that I will measure existing clearances with plastigauge and then decide on simply to replace the bearings or maybe go under/oversize by .001 or .002 depending what I find when I measure the clearances.

I am on a limited budget until next summer when I probably plan to drop a crate motor into the car, and the car is not my DD, so I am not necessarily looking for a 50,000 mile fix.

See attached picture for the view of #6 and associated bearings..

What would the guru's do?

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  piston small.jpg (64.93 KB)

I like your current approach. Microgauge it, and put in new rod bearings. They are cheap enough...

The discolouration is a bit odd... so I wonder if they were the right fit initially?

Quote:

My thoughts exactly.

just going to through this out there for you as this is what we have to do with our zr1 main bearings.if your rod bearing gauge out a little loose you could send them off and have them coated with a thermal coating .

A smidge loose is better than too tight!
I'd replace them while you are there.

So how'd they check out? Any news yet?

Jake

I will make it a unanimous 'Andy agreement'. But why the crate engine next year?

The black discoloration on the backside of the upper brg shell is fairly normal. It's from small amounts of oil being trapped between the brg and rod bore when the brg transferred heat to the big end of the rod. It's nothing to be worried about.

On the lower brg I see some signs of preignition/detonation. That is the shiny wear located dead center in the bottom of the brg shell. This is from the rod oblonging at the parting line (gaining clearance in that area) and of course squeezing out clearance at the top and bottom... Again common. You will often see that wear pattern in engines that use stock rods in high HP or stress applications.

It can be a problem in the long run. To reduce this you can open up your clearance on the rods a bit. In a supercharged street engine with those heavy pistons would like to see .0025 on the rods.... More in a race car.

Of course it really just lets you know that your approaching the limit on stock rods or you tune isn't nearly as good you thought. I've been on both sides of that coin in the past and neither worked out well when I ignored the warning.
Will

Just my .02 guys, but plasti guage is not the real true way to measure the journals. Micrometers are the only way to get a true reading

I would just run with it... looks like you are already planning on replacing the engine and spring is less than 6 months away.

DG

Need a better pic. But from far away those look better than my mains and I slapped them back together. You asked what I'd do, not what I should do...

Thanks guys for all the posts. After working frantically on my Delta Medallion status I finally got around to doing some basic measurements with plastigage. The clearance with the existing engine parts is .0015" (I measured both #6 and #5).

The engine had experienced detonation due to a poor tune a couple of years ago.

So - I am simply going to replace all the bearings and rings. I will measure all clearances while I disassemble; if they are all at .0015 I will go .0025 - this engine will always be boosted.

Oh - Andy; I have given up the crate engine idea, I am enjoying tinkering too much :-) Will you be at T4T next week?

A few more questions:

1. The cylinder bores are very smooth, with some slight scoring on piston skirts and lower bores. I cannot feel the scoring with my fingers, its a visual effect. I'm thinking of a light DIY hone - anyone else done this?

2. Does anyone have recommendations on rings? I want to fix my oil consumption problems so need normal tension oil control rings.

3. Should I replace the rod bolts?

Cheers!

.0015 brg clearance is way too tight for a boosted engine.... Really it's too tight for anything but a stock daily driver.

The cyl wall scoring is because of the detonation. Those pistons look like SRP??

I can see some flecking on the top ring land face... That's where detonation is melting your piston. It's harder to see in your pic - but the skirt looks pretty bad comming down from the oil ring. Thats from too little bore clearance or the piston getting over heated and expanding too much (causing too too little bore cleanace)...

1. It won't hurt to run a 280grit ball hone in it. You won't gain much clearance and maybe it'll get the bad marks out. I would say best case is you take it to a machinest and have them hone it right but your looking at close to a $100 to do that.

2. Without a doubt I like the Perfect Circle "Fire Power" rings. This is the Speed Pro Hell Fire top ring (repackaged) with Perfect Circles own Napier second ring and a Hastings CP20 standard tension oil ring. Be aware that the top ring is bit hard on cyl walls in regular street cars... However if your boosting it hard you got little choice but to run it.

http://www.cnc-motorsports.com/produc ... ?ProdID=17384&CtgID=16857


3. Absolutely NOT. If you replace the rod bolts you will have to rebuild the big end of the rods. Waste of $$$ in a stock rod. If you want better rods buy a set of Eagle SIR rods or the SCAT rods with 3/8 APR 8740 cap screws... both are less than $300 and you'll nearly have that in a set of stock rods after you buy bolts and pay a shop to rebuild them.
Will