Forged 273 crank in an early 360 Block?

[VIOLENT\6]

I would like to build an internally ballanced 340 from a 360 block & forged 273 crank.

I know that the 273/318/340 cranks have a wider main bearing journal than the 360.

I have a source for a forged 273 crank that I can pick up cheap. Can the offending journal be welded & reground.

My father in law has an automotive machine shop, but he is more of a Chevy man so I doubt he has done this.

 

[C130 Chief]

Anything is possible, but sometimes you have to ask yourself why (especially if nobody else has done this before). First off ALL A series engines had the same width main bearing, the DIAMETER changed for the 360. This was to maintain crankshaft rigidity through journal overlap. A stock 360 or later 340 cast crank was not exactly a weak piece.

Welding up such a crank on all 5 main journals and then surfacing would cost quite a bit if you paid your friendly neighborhood machine shop to do it. Probably near the cost of a rebuildable 340. Plus, if you are starting out with a 360, why spend bucketloads of cash to reduce displacement?

 

[Locomotion]

You'll need to check the back of the 273 crank where the converter snout fits into. The very early 273's had a smaller register and the more common, newer converters won't fit.

But I agree with the above post. It's not cost-effective to weld up the journals or, I beleive someone makes spacers for it. The 360 cast crank is plenty strong for most applications and you'll have more cubic inches and torque.

 

[VIOLENT\6]

Anything is possible, but sometimes you have to ask yourself why (especially if nobody else has done this before). First off ALL A series engines had the same width main bearing, the DIAMETER changed for the 360. This was to maintain crankshaft rigidity through journal overlap. A stock 360 or later 340 cast crank was not exactly a weak piece.

Welding up such a crank on all 5 main journals and then surfacing would cost quite a bit if you paid your friendly neighborhood machine shop to do it. Probably near the cost of a rebuildable 340. Plus, if you are starting out with a 360, why spend bucketloads of cash to reduce displacement?

I misunderstood the information then.

I was not able to find 1 site that listed the dimensions of both the 318 & 360 bearing journals. Do you know of a link that will show that data?

As far as cost, the welding would be the only real expense as, like I said in the original post, there is a crank grinder in the family. This sort of thing is done all the time W/350 SB Chevy cranks that are installed into 400 SB Chevy blocks for the same reason, an internally balanced crank that will rev better.

What kind of $$$ is a rebuildable 340 available for? The few I found on E-bay were in the high price range.

That would be my 1st choice, a rebuildable 340, @ least the block. I have the 273 forged crank, the rest I can gather from readily available modern stock parts.

Barring that, a de-stroked 360 would be next choice. Readily avilable modern stock parts combined W/Keith Black pistons would make a destroked 360 bored .040" over (340) W/a 10.5:1 CR quite do-able. Forged 273 crank, early 360 block/59cc magnum heads/.060" thick headgasket/KB hypereutechtic 340 flat top pistons. The compression hieght of those particular pistons would put me about .048" from the head deck W/the .060" gasket.

If I go the full 3.58 stroke W/zero deck 360 flat top pistons & the 59 cc heads, the CR will be in the 11.3:1 range. Too much for pump gas. The alternative here would be to go 2.02 on the intake valves & unshroud the valves to increase the chamber size to about 66CCs.

Again, the welding/turning of the crank will not be that expensive in my particular case. I think far less than the price of a 340 block.

 

[VIOLENT\6]

You'll need to check the back of the 273 crank where the converter snout fits into. The very early 273's had a smaller register and the more common, newer converters won't fit.

But I agree with the above post. It's not cost-effective to weld up the journals or, I beleive someone makes spacers for it. The 360 cast crank is plenty strong for most applications and you'll have more cubic inches and torque.

Okay, another reason not to do it.

What about unshrouding the intake valves on the magnum heads & going to 2.02" intake valves then?

If I can get the 59cc chambers of the Magnum heads to 66cc I could build a budget 10.5:1 360 similar to this 318 build up.

http://www.moparmusclemagazine.com/techarticles/mopp_0409_318_engine_build/

 

[Locomotion]

The compression numbers you have are based on published blueprint specs. Actual OEM deck heights and chamber volumes are usually more so that would lower compression a bit.

Using Magnum heads, you could also use Magnum style pistons to get the compression down. Just another option you would have to do the math on after measuring everything. Reducing quench can promote detonation in some cases even if compression is reduced. Quench promotes turbulence which helps combustion efficiency.

I don't know it going to bigger valves will result in much chamber volume difference. But I just don't understand giving up cubic inces. An efficient way to make more horsepower per cubic inch is to destroke, which racers do in Competition Eliminator classes that have a weight-to-cubic inch rule. At 10lbs per cubic inch, a 280 cubic inch engine in a 2,800lb car will be faster that a 300 cubic inch engine in a 3,000lb car if both engines are built with the same expertise. But when there are no rules or limits, more cubic inches will be faster at the same weight, with very few exceptions.

 

[ValiantMike]

From 'How to Hotrod Small Block Mopar'
CRANKSHAFT SPECS
The general specifications for A engine crankshafts are as follows:

CRANKSHAFT SPIN DIA.--2.125 in.
MAIN DIA. 273/318/340--2.50 in.
MAIN DIA. 360--2.81 in.

 

[VIOLENT\6]

The compression numbers you have are based on Just another option you would have to do the math on after measuring everything. Reducing quench can promote detonation in some cases even if compression is reduced. Quench promotes turbulence which helps combustion efficiency.

I don't know it going to bigger valves will result in much chamber volume difference.

I fully understand "quench" effects on detonation & the fact that stock deck hieght will probably be more than published specs.

The .048 effective piston/head deck clearance I cited was what I thought to be a good compromise of adequate clearance, good quench distance & compression ratio. If the deck hieght of the block was sufficient (eccessive) after squaring, the head gasket thickness could be reduced to yield the the desired .048" clearance & 10.5:1 compression ratio.

As far as bigger valves reducing CR, of course it wouldn't. My thoughts on that were if the valves were unshrouded, thereby increasing combustion chamber volume & decreasing CR, why not take advantage of the unshroiuding by opening up the bowl & utilizing a larger valve.

EDIT: THe .048" quench distance/10.5:1 CR was computed for the de-troked 360 using KB 340 flat toped pistons @ + .012" .+ .060" head gasket.

A 360 utilizing KB flat top pistons @ zero deck & .039 head gasket would net 10.4:1 W 66cc chamber volume while the next step lower CR W/the KB piston (18cc head volume) would be 9.8:1 W/the stock 59cc chamber volume.
I am not new @ building/modifying engines. Most of my experience has been W/SB Chevies & Harley big twin strokers. I just purcahsed a "65" Valient for my son & I am trying to help him build a budget screamer.

He wants to make it faster than his Mom & Dad's highly modified Daytona Charger.

Well quicker maybe, @ 175 MPH, faster in that little Valiant would take a lot of $$$$$..