offset grinding stock 4.0 crank

[jason]

well i have been reading all of these stroker threads and picking up a lot of good ideas my motor is in need of a rebuild and i was thinking stroker but to get any of the benefits i would have to do to many mods to the 89 renix motor so my idea now is to offset grind the stock crank and over bore it and use a 91 cylinder head bored out throttle body and header i think this will give me noticeable gains without dealing with milling the block or head or any of the other headaches i talked to a machinist who said the max he could offset the crank would be .050 for $150 he also said the stock motors deck height was close to that so i went home and checked a 2000 motor i had on the stand and i got .029 roughly with my digital dial caliper not very accurate but close so with the stock gasket .051 i dont think think it will be a problem this is theoretical at this point so i ask for input am i way off thinking this will work.

 

[j99xj]

In theory it will work. With a stroke offset of 0.05" and 0.06" overbore you are only ahead about 10 cubic inches.

Certainly this increase in engine size will improve horsepower/torque at all engine speeds but I've heard that every cubic inch adds about 1 hp and 1 lb-ft of torque.

That being said, you are going to a lot of trouble for 10 hp.

If I were you I would spend my money on higher quality stock parts and port the cylinder heads instead.

 

[jason]

Thanks for your reply I was thinking it would be close to 19 cubic inches gain between the stroke and bore yesterday but I was probally doing something wrong I have been on dinos site and he says the 5.0 is an offset ground 4.2 crank stroked to 4.06 i used that number subtracted the 4.2 stroke of 3.895 and got a difference of .17 witch i added to the stock stroke of the 4.0 of 3.44 and ended up with a stroke of 3.61 this is almost a third the stroke gained from the 4.2 crank so if the 4.2 crank will gain 32 cubic inches stroking my stock crank should produce at least 10 add the 8 or so from boring the cylinders 60 over and i got around 19 like i said i probally got something wrong but this is pulling info from the web magazines and whatever else i can find i dont think it will work now because the piston will travel above the deck and i dont see a way to bring it lower without using 4.2 rods and milling the block and it all gets compicated again

 

[5-90]

If you're dead set on offset grinding a crank, why not try to locate a crank for a 258 anyhow? Get a junkyard core, and have that ground - which would allow you to achieve the same displacement with less welding/grinding, or give you a potential for more displacement (if the rods don't "self-clearance" on the oil sump rails of the block.)

Also, bear in mind the RENIX block has the potential to go to 4.000" - if you have the block checked ultrasonically, and it passes. That's a full eighth inch (.125") over - and that's what I'm thinking for a project I've got - 4.000" x 4.000" x 6 = 301.6ci.

How much a shop is willing to offset grind a crankshaft depends on the shop - if you're going to get silly, get a shop that specialises in crankshaft work (since they'll have more practise at weld/grind work.) Typically, an offset ground crankshaft has material welded onto the crankpins first - the idea to be to allow the greater stroke without having to use a custom rod (since the big end would otherwise get much smaller.)

Meanwhile, here's the same advice I give others doing performance buildups - what are you trying to do with it? If you don't know where you're going, how will you know when you finally get there? Adding displacement has a certain "cool" factor, but what do you plan to do with it?

 

[jason]

yea i give up it was just a thought it seemed to be a practical way to gain a little without having to modify everything to make it work by using mostly stock parts i wasn't looking for big horsepower just what i could get why the work was going to be done dont want to put a lot of money and work into a renix motor it will be cheaper and easier on my other truck when its ready then i will do a real stroker with the 4.2 crank thanks for your replys i put it out there for some outside ideas or reasoning

 

[privatejoker]

If you're serious about making more power, grind the 4.2 crank

 

[5-90]

If you're serious about making more power, grind the 4.2 crank

Isn't that what I started off saying?:read: Offset grinding with a starting stroke of 3.895" is going to allow you to net more than a starting stroke of 3.44"...