3.53" stroker, Dumb idea or Smart move.

[1990Z71Swede]

So, I've had this Idea for a while now.... others have too by the looks of what I find on the interwebs. The idea with this tiny amount of extra stroke is obviously not adding a few extra cubes. Instead it is an alternative to decking the block to get the quench right.

Now theoretically one could grind a 350 crank offset to a maximum of 3.68 stroke using 2" journal rods, but that is not needed.. 3.53" would be ideal to bring the pistons to zeroish deck on an untouched block. Even if they pop up ever so slightly, that is no biggie, worst case the pistons can be milled that tiny amount. And no need for new rods, as you can get to 3.53" with some margin with 0.060 undersize bearings.

The benefits of this somewhat different aproach is to bump CR a little bit, and hopefully offset most of the increased knock sensitivity by the much approved quench. And at the same time keeping the deck as thick as possible, head gaskets seem to be prone to go between the middle cylinders because of the added heat from the side by side exhaust ports, and even if most of this is probably because of the head castings, every little bit helps.

The engine is supposed to be a ~300hp sub 5000 Rpm build for our K1500 daily driver.

Anyone done something similar? My main concern would be taking a cast crank offset to 0.060 undersize, not so much for it becoming weak as some 0.060 rod bearings are nowdays sized for a larger fillet radius. (But you need a machine shop with matching grinding stones) https://www.summitracing.com/parts/cle-cb663c60/make/chevrolet/model/c1500/year/1992
I'm more concerned about the possibility of grinding through surface hardening, but maybe the stockers are not hardened?

 

[evilunclegrimace]

it seems like alot of extra work to achive something that could be done with a set of properly machined pistons.

 

[Hipster]

it seems like alot of extra work to achive something that could be done with a set of properly machined pistons.

Problem is most off the shelf pistons are .010 less compression height than stock to account for a deck cleanup. Order custom pistons you can get anything you want, one set is hella expensive.

 

[Hipster]

So, I've had this Idea for a while now.... others have too by the looks of what I find on the interwebs. The idea with this tiny amount of extra stroke is obviously not adding a few extra cubes. Instead it is an alternative to decking the block to get the quench right.

Now theoretically one could grind a 350 crank offset to a maximum of 3.68 stroke using 2" journal rods, but that is not needed.. 3.53" would be ideal to bring the pistons to zeroish deck on an untouched block. Even if they pop up ever so slightly, that is no biggie, worst case the pistons can be milled that tiny amount. And no need for new rods, as you can get to 3.53" with some margin with 0.060 undersize bearings.

The benefits of this somewhat different aproach is to bump CR a little bit, and hopefully offset most of the increased knock sensitivity by the much approved quench. And at the same time keeping the deck as thick as possible, head gaskets seem to be prone to go between the middle cylinders because of the added heat from the side by side exhaust ports, and even if most of this is probably because of the head castings, every little bit helps.

The engine is supposed to be a ~300hp sub 5000 Rpm build for our K1500 daily driver.

Anyone done something similar? My main concern would be taking a cast crank offset to 0.060 undersize, not so much for it becoming weak as some 0.060 rod bearings are nowdays sized for a larger fillet radius. (But you need a machine shop with matching grinding stones) https://www.summitracing.com/parts/cle-cb663c60/make/chevrolet/model/c1500/year/1992
I'm more concerned about the possibility of grinding through surface hardening, but maybe the stockers are not hardened?

a common thing used to be 3.5 cranks. Brings the piston up to .005-006 below deck(usually). They are still out there. If you're doing this yourself as a machinist, go for it but I can't see dumping the money in a cast crank and definitly not going .060 on one. They are only hardened but so deep. You want a performance build get the block decked. . It would be cheaper than the "custom" crank work you're talking about. No need to re-invent the wheel on a stockish daily driver build.

 

[1990Z71Swede]

Yeah, grinding the piss out of a cast crank is probably not a good idea.
So lets forget that part.

Going for a 3.50 crank seem to be a better option. At least as I'm trying to avoid taking more of the block than what is needed to square it. Also trying to avoid custom pistons for obvious reasons.

 

[pressureangle]

My memory bell is ringing- see if KB pistons aren't specifically addressing the deck height issue. You'll have to download the catalog.

KB Hyper Pistons | UEM Pistons

uempistons.com

 

[Schurkey]

Oh, C'mon. Guys used to take TWO HUNDRED THOUSANDTHS (0.200) off of stock cast-iron SBC 400 main journals to make 'em fit into 350 blocks, in the days before cheap Com_mie (Really? "Com_mie" is censored?) "383" cranks became popular.

.060 off of a 2.1" rod journal means nothing, when NAPCAR was using Honda-size (1.88?) journals, (granted, in high-dollar forged or billet cranks, but also at a gazillion RPM for 500 miles at a time.) and the early SBCs were using 2" journals in both forged and cast cranks. A .060-under 2.10 rod journal becomes 2.040, still .040 more diameter than the early "small-journal" cranks had when new.

A cast crank has no surface hardening other than whatever might accrue due to work-hardening. Forged cranks may have deliberately hardened journals, via heat-treatment including cryogenics, or chemical processes.

My most-recent SBC project involves a plain ol' ordinary cast-iron one-piece rear main seal "350" crank, offset ground by .020, leading to .040 additional stroke. (.020 "up" and .020 "down".) I'm thinking this required .050-under rod bearings. Stroke changed from 3.48 to 3.52. SOMEDAY, I'll get off my lazy asp and actually put the thing together.

This allows me to use off-the-shelf (cheap) pistons and merely "square-decking" the block--a quick shave to true-up the gasket surface. Otherwise, I'd need to carve ~.030 from the decks like I did with my K1500 engine.

Potential Downsides:
Offset grinding the crank reduces the clearance between bottom-of-piston and crank counterweights. Clearly, we don't want the pistons to rub the crankshaft.

Offset grinding will get you at, or at least towards the maximum "undersize" bearings available. If the crank gets wounded, there may not be enough material to regrind and still be able to get a bearing that fits. That's why I didn't start out at .060-under. At .050 under, I've still got the potential for .060-under bearings if it needs another .010 cut off later.

Raising the piston in the bore requires boring the cylinders unless there's ZERO ridge. The rings will be raised higher in the bore as the pistons go higher. If the top ring collides with any amount of "ridge", they'll be damaged, and maybe wreck the ring grooves in the process..

 

[L31MaxExpress]

My buddy Kurt built a L31 that had spun rod bearings using a 3.50" stroke off the shelf crank. Adds compression and quench without decking the block.

 

[Hipster]

Oh, C'mon. Guys used to take TWO HUNDRED THOUSANDTHS (0.200) off of stock cast-iron SBC 400 main journals to make 'em fit into 350 blocks, in the days before cheap Com_mie (Really? "Com_mie" is censored?) "383" cranks became popular.

.060 off of a 2.1" rod journal means nothing, when NAPCAR was using Honda-size (1.88?) journals, (granted, in high-dollar forged or billet cranks, but also at a gazillion RPM for 500 miles at a time.) and the early SBCs were using 2" journals in both forged and cast cranks. A .060-under 2.10 rod journal becomes 2.040, still .040 more diameter than the early "small-journal" cranks had when new.

A cast crank has no surface hardening other than whatever might accrue due to work-hardening. Forged cranks may have deliberately hardened journals, via heat-treatment including cryogenics, or chemical processes.

My most-recent SBC project involves a plain ol' ordinary cast-iron one-piece rear main seal "350" crank, offset ground by .020, leading to .040 additional stroke. (.020 "up" and .020 "down".) I'm thinking this required .050-under rod bearings. Stroke changed from 3.48 to 3.52. SOMEDAY, I'll get off my lazy asp and actually put the thing together.

This allows me to use off-the-shelf (cheap) pistons and merely "square-decking" the block--a quick shave to true-up the gasket surface. Otherwise, I'd need to carve ~.030 from the decks like I did with my K1500 engine. Many guys won't run an .020 crank. I'm one of them.

Potential Downsides:
Offset grinding the crank reduces the clearance between bottom-of-piston and crank counterweights. Clearly, we don't want the pistons to rub the crankshaft.

Offset grinding will get you at, or at least towards the maximum "undersize" bearings available. If the crank gets wounded, there may not be enough material to regrind and still be able to get a bearing that fits. That's why I didn't start out at .060-under. At .050 under, I've still got the potential for .060-under bearings if it needs another .010 cut off later.

Raising the piston in the bore requires boring the cylinders unless there's ZERO ridge. The rings will be raised higher in the bore as the pistons go higher. If the top ring collides with any amount of "ridge", they'll be damaged, and maybe wreck the ring grooves in the process..

Mopar guys used to do it plenty as well, but they didn't last. Soft journals. I'm under the impression most cranks are heat treated at least is some fashion post-casting/forging. quenching, nitriding, chemically, induction, etc. Some processes may not even be .020 deep.

 

[Hipster]

Crankshafts:Stock and Performance - Engine Builder Magazine

The crankshaft is the working arm of the engine. All the force generated by combustion and the downward motion of the pistons is focused on the crank throws. Related Articles - The Road to AAPEX Season 2, Ep 8 - The Road to AAPEX Season 2, Ep 7 - The Road to AAPEX Season 2,

www.enginebuildermag.com