whats the biggest bore you could go in an lt1 i heard there are some guys out there running 4.100" bores in their lt1 witha sonic check of coarse, how about sleeving? has anybody here actually done a 4.100 or bigger bore? this is what i have in mind for my next engine

 

Mark Montalvo had some big bores...can't recall if he was at more than 90 over, though. Trying to bore that big and sleeve all 8 cylinders is prolly more trouble than it it worth. You are taking a hell of a lot of the structural integrity out of the block, and may make it a candidate for the big BOOM BOOM syndrome.

 

There's no definite answer to your question, because you haven't specified how you plan to use the motor and what kind of a budget you have. Will it be expected to last for a long time in a street environment, will it be a race engine, is it going to be NA or forced induction. etc.? In terms of maximum overbore, an IT1 is no different from any contemporary conventional (gen I) small block. The cylinder walls are thin right from the start. And like any other block, core shift may occur and result in thin (and thick) spots. A skilled (and expensive) engine builder can check and correct for these things and the bores can be completely recentered, if desired. But no matter what, there is simply not a lot of meat in those cylinders.

Let me give an example, my own block. It is curently at 0.040" over and that is causing problems. I have having difficulty getting a good cylinder seal with high boost (22psi) and am reasonably certain this is related to bore distortion at high boost/high revs. Yet I don't suppose 0.040" on an NA application would be a major problem. Mark Montalvo went huge on a couple of LT1's a while back. They were NA high-end motors, and I doubt they were expected to last very long. I have seen an NA LT1 split a cylinder at 0.060" over.

If you really want a big-bore small block, it's more cost effective to simply switch over to a gen I style setup with a Bowtie or aftermarket block. But doing either makes no sense until you have spent the long buck on top end components. The bang for the buck just isn't there in trying to get another few hundreths out of the bore compared to cylinder heads, intake, valve train, etc.

Rich Krause

 

Just for the record, Mark's 4.00" x 4.090" 421ci LT1 was an 800HP nitrous motor. It lasted long enough to get the numbers on the engine dyno. I never heard any more about it after that.

 

Apparently its sitting at B&B, why i have no clue.

 

why go that route with such a big bore instead of a 4.125" crank? I know its more rotating mass but with 4 bolt mains you should be able to make the same power and have it last longer, right?

 

i want to run a n/a street motor (not a daily driver though), big solid roller and my canfield heads (311cfm@650), i was thinking maybee a 4.100 bore and 3.875 stroke,

 

Now that's funny!

Sonic check your block that will tell ya. In the mid 80's I worked for a machine shop that built race motors for all different applications and I remember the Buick Grand National guy's bringing in maultiple blocks to have sonic checked because there was variations in bore thickness from casting to casting and these guy's where trying to find that one special block that would allow them to go large over bore. The GN guy's would sonic check as many blocks as they could get there hands on in order to find that one casting that would allow them to go to that large bore.

I do remember TPS Specialties selling a 409ci LT1 that supposedly was a reliable street application? But double check on that.

 

Combination Motorsports sells a crate 409ci LT1. I remember when Mark was building the 421, he had to find the right block, and then recenter the bores to put the meat on the thrust side. Sounds too "chancey" for me.

 

What about a 400block? Siamese bores cant be THAT bad now can they? Circle track guys love those motors