Safe 200 cylinder wall thickness?

4lugnuts

4lugnuts

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I need to find out if anyone has any information on how thick the cylinder walls of the 200 I6 are. I am setting up a '66 engine for street use with forged pistons, aluminum head, weber carb, headers, etc.. The mods will later facilitate a Vortech V2 supercharger, with a fuel management system and direct fuel injection. I don't expect I will ever boost over 10psi..
My question is how much overbore can I do and safely retain enough strength? Sleveing is costly at about $100 per hole, so I hope to avoid that route.

While on the subject, any recommendations for a head gasket?

Thanks!
 
Howdy Lugnuts:

All I can tell you, with certainty, is more is better. For loPo builds I've heard of overbores of .060". One performance build, by Clifford I think, over bored .070" and used AMC 258 pistons. Given your planned level of performance, I'd overbore the least amount necessary to true the cylinders. There are two good reasons for less overbore: Strength to avoid catastrophic breaking of the block, and to help maintain structural rigidity. Even in basic street engine rebuilds, if a block requires more than .030" to true the cylinders, we recommend finding another block to build.

That's my two cents and I'm sticking to it.

Adios, David
 
I don't know the wall thickness of the small six, but the big six has a nominal wall thickness of .160 - .170", measured 2/3 of the way down from the deck. I would be surprised if the small six is thicker than that. That assumes no core shift of course. Core shift is a big reason racers have the walls sonic checked for thickness before building.

Keep the overbore to a minimum. Anything you gain from increased displacement is generally lost in increrased wall flex and a less-than-perfect seal.
 
Thanks everyone for your replies, I am currently building the engine and will let y'all know how things turn out.
 
The Clifford shop built 250 with an .070 overbore CZLN6 mentions is in my '61 Comet. NA Tri-Power setup uses fairly mild SCR of @ 9.5 with the AMC 258 3.75" pistons. The engine has all ARP fasteners, Balanced Shot Peened rods, custom balancer and a few other internal block mods and extensive head work.
Anecdotally: Has run great for over ten years without 'coddling' and occasional serial strip launches over 5K rpm .

Also anecdotally: Current small block six project is a Maverick 250 with a Vortech VII Supercharger. Stock block and cam with the head simply freshened but not milled yields @ 8.7:1 SCR with composite gasket. Pproject evolved from a Buick derived Turbocharger on the original 250 in the Maverick. With the T3/Draw-thru/Quadjet BOP setup, boosted incrementally until the piston lands broke off at @ 20PSI of boost. Current 250 SC blo-thru setup uses stock 250 internals with the freshened head and I keep boost to @ 10PSI with occasional runs to @ 15lbs. Uses' modified (inexpensive) Holley 2300-7448 350cfm carb with a BRPV operating with boost. Modified DSII ignition advance is controlled to @ 26 degrees max' and stock exhaust with sensor for mandatory AFR gauge (closely monitored) has kept detonation destruction at bay. Street use for past two years has yielded impressive performance with no problems. "Your mileage may vary"...


250 / Draw-Thru BOP TC setup
. . .

250/VII SC
. . .

have fun




250 CID inline Small Block Six - . Stock block and cam with 60 cc chamber head, V8 valve springs, ARP head bolts. Holley 2300 - 7448 2Bbl modified for blow-through/ boost, Aeromotive 1:1 boost referenced regulator and FI electric pump with return at tank. Flame Thrower rev-limited HEI ignition - DS II distributor modified for vac and centrifugal boost advance limiting. Vortech V-2 centrifugal supercharger with pulley ratio spec’d for inline six 250' powerband. Centerforce weighted clutch, T5-Z 'Cobra' 5 Speed, 3.80 rear. AM radio and bench seat.

.
 
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I dont think the original question ever got answered fully. But my experience working in a race shop showed that for most serious performance and racing builds, a minimum wall thickness of .125" was a pretty safe dimension to shoot for using a quality sonic checker. This would obviously be on the major thrust side of the cylinders and you need to check in about 8 to 10 spots vertically from top to bottom. You also need to check around the entire inside diameter of each cylinder at 90 degree increments. If we found spots thinner than .125", even if it wasn't on the major thrust side, we wouldn't consider using the block for any serious performance usage.
 
All Fords Michigan plant engines except the 240/300 in lines and the 370/429/460 and last 78 onwards 351M's were 130 thou thick at the thrust face according to Al Turner and Brad Girdwood. That can drop to 90 thou in service if there is core shift and a 30 thou overbore. For all other surfaces except the deck and bulkheads and caps, the target thickness was 187 thou according to Ak Miller.

Compared to brand C's and pre 1969 Michigan Fords, the average wall thickness was about 243 to 245 thou in head and block castings that used the small block style "thin-wall" green sand process.

see http://vb.foureyedpride.com/showthread.php?188347-Finally/page3
 
I had a fresh built 351C with forged pistions that was only .030 over that I installed in my 1963 Fairlane, it wasn't long before the block split one of the cylinder walls. But those Cleveland blocks were known to have core shift and some thin cylinder walls. X6 I like to keep the cylinder walls just as thick as possible though.
 
I'm just surprised Ford had 2 very good engines that had so much HP potential, and they made both of them with such a bad Achilles heel. The 351C and 427.....they still were great engines but could have been so much more if they'd had thicker cylinder walls!
 
powerband said:
The Clifford shop built 250 with an .070 overbore CZLN6 mentions is in my '61 Comet. NA Tri-Power setup uses fairly mild SCR of @ 9.5 with the AMC 258 3.75" pistons. The engine has all ARP fasteners, Balanced Shot Peened rods, custom balancer and a few other internal block mods and extensive head work.
Anecdotally: Has run great for over ten years without 'coddling' and occasional serial strip launches over 5K rpm .

Also anecdotally: Current small block six project is a Maverick 250 with a Vortech VII Supercharger. Stock block and cam with the head simply freshened but not milled yields @ 8.7:1 SCR with composite gasket. Pproject evolved from a Buick derived Turbocharger on the original 250 in the Maverick. With the T3/Draw-thru/Quadjet BOP setup, boosted incrementally until the piston lands broke off at @ 20PSI of boost. Current 250 SC blo-thru setup uses stock 250 internals with the freshened head and I keep boost to @ 10PSI with occasional runs to @ 15lbs. Uses' modified (inexpensive) Holley 2300-7448 350cfm carb with a BRPV operating with boost. Modified DSII ignition advance is controlled to @ 26 degrees max' and stock exhaust with sensor for mandatory AFR gauge (closely monitored) has kept detonation destruction at bay. Street use for past two years has yielded impressive performance with no problems. "Your mileage may vary"...


250 / Draw-Thru BOP TC setup
. . .

250/VII SC
. . .

have fun




250 CID inline Small Block Six - . Stock block and cam with 60 cc chamber head, V8 valve springs, ARP head bolts. Holley 2300 - 7448 2Bbl modified for blow-through/ boost, Aeromotive 1:1 boost referenced regulator and FI electric pump with return at tank. Flame Thrower rev-limited HEI ignition - DS II distributor modified for vac and centrifugal boost advance limiting. Vortech V-2 centrifugal supercharger with pulley ratio spec’d for inline six 250' powerband. Centerforce weighted clutch, T5-Z 'Cobra' 5 Speed, 3.80 rear. AM radio and bench seat.

.
Click to expand...
That is one awesome project! Sorry to take so long to reply, life just got in the way of my hot roding.. One question though, well, actually two.. how did you attach a ribbed pulley to the crank pulley, and how did you mount the s'charger?

ding...
 
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