Machine shop says the decks on my 60k mile LY6 are flat, but the deck heights are mismatched by 0.002in from one side of the engine to the other (passenger vs. driver). I threw this into a compression ratio calculator assuming flat top pistons and saw a 0.05pt difference in static compression ratio (10.18 vs 10.23). Is this anything to worry about on a mild street engine? The shop is recommending I deck the block.

 

I would have it decked if it were mine, then you would know everything is in spec, especially if your building a higher HP motor. If stock, you could probably get away with it, might even be able to run a slightly thicker head gasket that matched the difference on the one side.

 

I'd deck it...

 

Unless you check all the crank throws for stroke variation and find them all within a thou, con rods for length variation and find them all within a thou and pistons for compression height variation and find them all within a thou, I would say that .002" side to side on block deck height measurement is within acceptable limits. Plus or minus a thou is the spec for deck height. That means one deck can be a thou low, the other a thou high resulting in a two thou variation from side to side which is within limits.

Steve

 

HKE decked my block and when I measured they were off by .007". I'm running .045" and .051" custom cometics to account for it. I would suggest a similar approach instead of re-machining.

 

What i gathered from the initial post was the block has not been decked at all. In which case, decking wouldn't hurt. I could see if it had already been decked.

 

This block has not been decked since it came from the factory. I will ask today what the deck heights measured at exactly. I am not opposed to resurfacing them, but at about $150 I need a good reason. Knowing they are in spec is a good thing, but like Steve said they may already be in spec at 0.002 difference, not to mention once the pistons, rods and crank are in there, depending on how their tolerances stack, I may see additional variation that helps or hurts the 0.002 mismatch.

 

Holy *****...my shop charged me 100 to deck the block....and heads.

 

These are the machining prices I was quoted prior to dropping the block off:
clean & mag block $75.00
polish crank $35.00
hone w/ torqueplate $200.00
bore $80.00
deck $148.00
balance $175.00
line hone $135.00
file fit rings $30.00
clean heads $60.00
R&R cam bearings $75.00

They seemed to be in line with what I found researching.

 

That's cheap you guys in the USA are lucky, Just be happy you don't have Canadian prices and our TAX.

 

I talked to the shop. The deck heights are at 9.2375 and 9.2395 (0.002in difference). They are flat (no problems front to rear). Their opinion was that I could get away without decking it, but still recommended I match the two sides. Assuming a 0.051 gasket, 6.125 rod, 1.304 compression height and 3.622 stroke, the current dimensions give 0.0485 and 0.0505 quench, so I should be able to safely shave the deck some and improve quench while matching the two sides.

They are also recommending line honing the mains, but didn't have any measurements to say why - just that in their experience they tend not to be straight on these blocks from the factory. I asked if they could measure, but they said by the time they set it up and measure they may as well hone it. I am not too keen on machining for the sake of machining. Any thoughts on this? In my reading I haven't heard of this being a problem.

 

Line hone first. Then worry about the deck. Once you line hone, the decks will change since the deck height is based off the crank centerline.

 

We have had as much as .007" from the GM blocks off the shelf before surfacing or blueprinting. We have also had cores come in from good running vehicles that were off even more. It should be acceptable at that point (.002) but if it is out why not go ahead and correct the problem?

 

Bandit it sounds to Me like They are just after the sales increase in money rather than line honing because They have knowledge that They need it . If it were Mine I would mock it up and see if there is a larger than .002" discrepency in where the pistons are at TDC . You're pistons must be ending up right at flush with the deck from what You said You're quench will be the way it is now . Use a straight edge across each piston top and compare all 8 and if none are more than .002" difference ,,,,,,,,,,run it . I seriously doubt there will be from everything You've said ,,,,,,,,,good luck .