Crank fillet radius

[DeNoZZo]

I had the rod journals on my crank cut .020. I noticed the other day while inspecting the crank that the rod journals fillets are not even all the way around. Some have really small fillet's and others have none. I know that when the fillet gets taken off, the crank's strength is significantly decreased. But how much, in #'s that is will I loose out on the potential of a crank with a proper fillet?

 

[huafist]

If the guy ground the crank with no fillet, then you need to find another machinist. Sounds like he put a new stone on the machine and didn't dress it properly. A properly cut crankshaft will still have a fillet just like it did before on it (except race prepped cranks, they have a different fillet - but still have a fillet).

 

[JacksonAuto]

+1

Find another machine shop and possibly a new crank.

 

[BogusSVO]

+3 Im with the 2 above


find another crank, unless this build is for grandma's sub 200hp build to get her to the market and church.

Call/Email Jackson Auto.. I bet he has one ground and ready to go!
(shamless plug for a supporting vendor)

 

[DeNoZZo]

Here are some pictures. I'm trying to avoid just tossing this crank out as a loss and forking out more $$ for a new one if I can. This will help you guys better judge my crank.

(rod journal that had suffered spun bearing)

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(another pic of rod journal that had suffered spun bearing)

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(spun rod bearing journal)

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I should have wiped some of the lint off the journal before I took the pics. I think that makes them look a little roughed up. Especially the one that used to be bad.

 

[huafist]

Without having the crank in-hand, it's really hard to tell what the radius looks like. You really need a tool to measure the radius with. On a crank like that, I like about a .0625" radius.

 

[defrag010]

What we have here is an example of what happens to most rolled fillet cranks when they are ground. Your crank is a 7 bolt which has rolled fillets, unlike 6 bolt cranks which come from the factory with a 1/16" ground radius. Depending on the depth of the rolled fillet (they can vary greatly from crankshaft to crankshaft), the OD of the journal after grinding can end up intersecting the thrust cheek at the valley in the radius of the rolled fillet. This means that you are left with effectively half of the radius (usually less than 1/32"). When this occurs, the proper procedure for shaping the radius is by blending. Once your ground journal OD reaches this point you increase the amount of radius left by increasing it by half of the oversize past this point. So if you need to go another .010" in size past this point then you increase your radius by .005". When you are at this point you will now have to grind the journal the same amount you previously did for the radius to equal to what came from the factory (ex.. if you reach this point at .030" under OD, then you will have to go to .060" under to achieve a factory level radius). It's actually not very hard to change the radius on a crank grinder stone, especially if it uses a table mounted radius dresser and hydraulic face dresser, so this should not be an excuse from whoever does it from you. This is where crank grinder experience and attitude comes into play.

I took an extremely close closeup of the rolled fillet on a main journal of a 7 bolt crank I have in my garage that has been ground .010" on that journal. The point I mention where your ground journal OD intersects at the valley of the rolled fillet is illustrated in blue. The red line is how you would visualize a blended radius. I also took a picture of an uncut 6 bolt from the same angle at the same distance so you can see what a factory radius looks like.

This is why you usually don't see oversized bearings larger than .020" for most engines with rolled fillet cranks. Some cranks have a deeper formed fillet than others, so they would naturally be okay at larger undersizes. I have personally found that the "point of no return" is around .040" undersize on both rods and mains for 7 bolt cranks. They make .040" rods but nobody makes over a .020" main.

As for a radius "magic minimum number", there is none. It depends on many things. Material of the crank, quality of the casting/forging, residual stresses left from machining (grinding too hard), weight of the rotating assembly, rpm, etc. I have personally seen a billet crank with a 1/8" radius break with a 1200gram bobweight but have seen a factory cast crank with 5/64" radius live at 7500rpm for a whole season with a 1800 gram bobweight. Assuming the worst, your crank (no matter what is done on the radius.. you can only mess it up so bad) will be fine for a fully balanced stock/street motor that doesn't exceed stock rpm levels and sees no detonation. If you are worried about it, take the crank to a better grinder and get him to turn your rods .040" and blend a 1/32" radius on it.

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[DeNoZZo]

What we have here is an example of what happens to most rolled fillet cranks when they are ground. Your crank is a 7 bolt which has rolled fillets, unlike 6 bolt cranks which come from the factory with a 1/16" ground radius. Depending on the depth of the rolled fillet (they can vary greatly from crankshaft to crankshaft), the OD of the journal after grinding can end up intersecting the thrust cheek at the valley in the radius of the rolled fillet. This means that you are left with effectively half of the radius (usually less than 1/32"). When this occurs, the proper procedure for shaping the radius is by blending. Once your ground journal OD reaches this point you increase the amount of radius left by increasing it by half of the oversize past this point. So if you need to go another .010" in size past this point then you increase your radius by .005". When you are at this point you will now have to grind the journal the same amount you previously did for the radius to equal to what came from the factory (ex.. if you reach this point at .030" under OD, then you will have to go to .060" under to achieve a factory level radius). It's actually not very hard to change the radius on a crank grinder stone, especially if it uses a table mounted radius dresser and hydraulic face dresser, so this should not be an excuse from whoever does it from you. This is where crank grinder experience and attitude comes into play.

I took an extremely close closeup of the rolled fillet on a main journal of a 7 bolt crank I have in my garage that has been ground .010" on that journal. The point I mention where your ground journal OD intersects at the valley of the rolled fillet is illustrated in blue. The red line is how you would visualize a blended radius. I also took a picture of an uncut 6 bolt from the same angle at the same distance so you can see what a factory radius looks like.

This is why you usually don't see oversized bearings larger than .020" for most engines with rolled fillet cranks. Some cranks have a deeper formed fillet than others, so they would naturally be okay at larger undersizes. I have personally found that the "point of no return" is around .040" undersize on both rods and mains for 7 bolt cranks. They make .040" rods but nobody makes over a .020" main.

As for a radius "magic minimum number", there is none. It depends on many things. Material of the crank, quality of the casting/forging, residual stresses left from machining (grinding too hard), weight of the rotating assembly, rpm, etc. I have personally seen a billet crank with a 1/8" radius break with a 1200gram bobweight but have seen a factory cast crank with 5/64" radius live at 7500rpm for a whole season with a 1800 gram bobweight. Assuming the worst, your crank (no matter what is done on the radius.. you can only mess it up so bad) will be fine for a fully balanced stock/street motor that doesn't exceed stock rpm levels and sees no detonation. If you are worried about it, take the crank to a better grinder and get him to turn your rods .040" and blend a 1/32" radius on it.

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Thank you very much for explaining that!