First problem is you are using a micrometer which won't give you the most accurate reading. It will come very close IF you have it perfectly strait and center in the bore.

Secondly, are you sure you have used that 4" ring compressor before because I am willing to bet money on it that if you flip it over 180 degrees, it will fit perfectly! Those compressors are designed with a tapered inside diameter so one side is larger than 4".

 

First three bank Matco





Second three bank Matco and these are just at home not at the shop





Ring compressor lip that goes in the cylinder to along the piston and rod assembly!





Lip clearly doesn't fit the damn bore!

 

Did you try measuring 90 degrees apart?

 

Yes sir its just a small bore .018 below 4.00 is what i come up with and i measured three different bores the largest was still .012 shy of the 4.00 mark.

 

Within factory specs, nothing to see here.

3.98 is the listed factory bore on the LQ4/9.

 

Hey DietCoke where did you fine specs showing the undersize bore being within tolerances? I have always seen slightly over but not under.

 

3.98 is the listed lq4/9 bore in my gmpp catalog

 

Page 266

LQ9 (LQ4)
12572808
Iron
9.240
3.98"


http://www.pirate4x4.com/tech/billav...08_Catalog.pdf

 

Gm service manual i have seen say 4.0007-4.0017 I know at one time the early LQ engines were shown to have a 3.98 bore however everybody who tore one down measured it to a 4.00 I am guessing I just have one of the actual 3.98 bores.

 

3.98 is the rough bore, which is honed/finished to spec from there. If it's straight, there wouldnt be a need to finish it out to 4.

It's also possible it wasnt a production engine and someone built it from a gmpp block as-is without doing a finish hone

 

i plan on pulling it down this weekend to take a good luck at everything and run part numbers! Thanks DietCoke btw you wouldnt be interested in a trade for my 317 heads for your 853 heads would you? I am going to go n/a with this engine!

 

no i already have enough crap laying around, lol. good luck

 

Lol thinks what all you got laying around i might could use it!

 

I wouldn't be too surprised as the later LS2 block used in the L76 is said to have a 3.988 bore. same casting as the regular LS2 but the finished bore wasn't brought out to a true 4" I guess that means you have more room to clean up any cylinders

 

I had heard of them just never ran into one so I always kinda thought it was a myth or GM
Printed the smaller bore for insurance reasons or something because displacement actually plays as role in insurance cost. So glad to be getting some
Good information from others who have seen this before!