Stroker power expectations

[JT68]

Pat, I thought we had it cleared up over the phone? See the reply from Bill (Datrock) in the other thread. A new metric keyway has to be cut to drive the 1600 lower chain sprocket just like the 1600 crank has-and in the same location, that is the specific answer. There are various methods to do it, and it requires care and precision as we discussed. (If they mess it up your cam timing will be advanced or retarded and the engine will be mis-timed)

In addition to that, you either cut off the crank nose and retap it - OR - don’t cut the crank nose and make a spacer as previously described by Peter and Erik.

[unklpat]

Jim, how much do you extend the keyway? It seems that this info would be good for those who are interested in this process. Pat

[iloveredmeat]

Hey Pat, I have an alternative theory for using a U20 crankshaft in an R16 stroker.

And coming from someone firmly seated at the Kid’s Table, this post should provide much levity for the Elder Statesmen.

But… my feeling is that different opinions, and questioning the status quo, aren’t bad things.

So, here goes nothing:

You DO NOT have to modify a U20 crank AT ALL to work in an R16 Stroker.

There is no need to cut or shorten a U20 crank, just use a spacer. We’ve clearly shown that you can use a spacer to accommodate the longer nose of the U20 crank. This seems to be widely accepted and is settled law.

But nothing else has to be done. Period.

What?!!?

That’s Blasphemy!

Let me explain…

Regarding the keyway - some/most/all? elders insist that the keyway on the U20 needs to be extended/added/modified so a longer or additional key can be added to ‘drive’ the R16 timing gear.

And I respectfully disagree - a U20 Crankshaft keyway does NOT need to be extended, nor does a new keyway need to be added.

This is probably the point in reading when most of you are doing one of 3 things:

1. Spitting out your coffee laughing in disbelief.

2. Throwing your beer stein across the room in disgust.

Or

3. Quietly nodding in agreement.

How could I say such a thing?

Read on.

First off, I spoke to Dave Rebello a couple of times the other day (who has built his fair share of race-winning engines over the past several decades) and one item we talked about was whether a keyway needed to be added to a U20 crankshaft for a Stroker build. In a nutshell, he said that he has added them in the past, and he will add one if asked, but that a new keyway is just not necessary.

His explanation, as I understand it, is that the keys / keyways DO NOT “drive” the timing gears. The keys are too weak to actually drive a gear and would simply shear off if it came down to it.

The keys are there strictly to index the gears for alignment and timing, so if you’re relying on the keys for structural integrity, you’re doing something wrong.

For you and your friend’s stroker scenario, the key furthest from the nose on the U20 crank interfaces enough with the R16 timing gears to engage for a perfect index. When the crankshaft bolt is tightened properly and the entire setup is clamped down, all the compression, friction, and tolerances keep anything from slipping. Zero timing issues.

I’m going to admit that when I heard this, I kinda winced and wished I had asked for a keyway. But, on the other hand it makes perfect sense. All that twisting force on those little removable keys?

But, I actually made calls for a second and third opinion. I talked to my brother (former racer) and another Very Elder Statesmen… and they both 100% agreed with Rebello. Bottom line - the keys are not for strength, they are only for indexing, and all of the strength is from the crankshaft bolt compression.

These three people are enough to convince me outright, but while waiting for call-backs I did some reading and uncovered more supporting evidence.

There are many posts out there that support this idea… a quick search yields stuff like this:

“A keyway is only used for alignment, it isn't intended to withstand the twisting forces a running engine would subject it to. Once the bolt is tightened (properly) then nothing should change position at all (pulley to crank alignment). The key could be removed (if it could be removed) and nothing would change because the clamping force of the bolt holds it all in place.”

https://www.x-h2o.com/threads/purpose-o ... eys.42860/

Compelling, eh?

And I suspect a few of the experienced guys actually know this in their gut.

Sure, it might look nicer and ‘feel’ like it would work better, but I contend that adding/extending a key is NOT a required modification to use a U20 crankshaft in an R16 Stroker.

So, this alternative idea means you and your friend can basically ‘drop in’ a U20 crank and save some machining cost on the crank. Though you will need to have a spacer machined.

I hope that makes sense and helps, keep us posted. Good luck.
Peter

[unklpat]

Wow, Peter, and I thought you were just a dumb hunter. It's killer, when the actual truth comes out, and those whoe care, appreciate it. Pat

[unklpat]

So, lets explain, what is the actual truth? Pat

[JT68]

Hi Peter, you are exactly correct, the job of the keyway is primarily to locate the sprocket rotationally.

The clamping force from the pulley/balancer provides a huge amount of friction which will certainly prevent the sprocket from turning under normal circumstances. No doubt.

The copied internet information is also correct that the key could be removed after the pulley is tightened. Would anyone do that? probably not.


Of course there are lots of ways to build engines, cheap, expensive, 100%used parts, 100%new. Cast parts, forged parts. jury rigged parts.

Yes, you could literally locate the chain sprocket with superglue and a toothpick if you wanted to.

On the other hand, the shear strength of a hardened 5mm square key is substantial-actually quite high.

Recently (CBEZ's R16cam sprocket adventure), the only thing that saved that engine from WAY more destruction was a much smaller 4mm woodruff key on the cam. That sound he heard was the 4mmkey saving his engine with every click.

While Dave is certainly correct that the crank keystock wasn't intended to carry the load of the valvetrain for years, I think Dave and virtually every pro builder would all agree the most robust solution by far is to add a crank keyway.

Personally, I'd say if you are going to spend thousands on a stroker, I’d recommend a couple hundred bucks to add the keyway no matter who does the work. Can you cut corners? Of course.

[cbez]

Haha, great I'm going down in history as a warning case

[JT68]

Haha, great I'm going down in history as a warning case

Yep, you were a lucky pup! A very good save.

For a stroker, if one were to assemble the U20 crank and R16 lower timing sprocket without cutting a keyway, the 1600 sprocket would engage about 3 mm of the very rear of the existing 2L key, so about 20% of it's total shear strength.

If CBEZ's camshaft had only be been driven by 20% of the key, I have no doubt it would have sheared.

[JT68]

Here are 2 approaches:

Here is what Peter is describing:

image0 (9).jpeg

about 3 mm of unsupported woodruff key engagement. (just the very tip).


The way we currently do it:

image1 (1).jpeg

We used to add a woodruff key just like Nissan used, but the advantage of the keystock is the ability to index off the existing slot. If the crank is correctly positioned in X,Y,Z axis on the mill, it works well. Turned steel spacer for the pulley is hanging on the nose.

You guys decide what you would rather have....

(Nissan would never set up an engine like the first photo)

[GeoffM]

To say that the key isn't the torque carrier on a non-interference hub would be counter to any engineering/machinist/millwright training I've heard. I'm not any of them but I've been in the business of machinery and machine shops for 30 years. In this case, you may get lucky with the clamp force and the coefficient of friction between the hub shoulders and the shaft shoulder and spacer but that's a risk based on a lot of assumptions and variables....unless you have the actual torque values calculated. I've seen bearing races, which have a .0015-.002" shrink fit, spun on shafts many times...and the clamp force of a two thou shrink fit is many times what you could reliably get from stack-up clamp force.

I'd only rely on that 3mm of key for indexing if I had a .002" shrink fit. But that's just my opinion.

[unklpat]

One more maybe stupid question. Since the crank pully is only driving the rotational mass of the cam, distributor, pushrods, rockers, etc, wouldn't that be significantly less than the rotational mass of the U20 upper timing chain, cam, etc? Jim those pictures are very clear, and perfectly show what you are describing. Thank you. Pat

[iloveredmeat]

Wow, Peter, and I thought you were just a dumb hunter. It's killer, when the actual truth comes out, and those whoe care, appreciate it. Pat

“’Tis the smart hunter who fools his prey into thinking he is a dumb hunter.”
A.J. Parkinson

Here are 2 approaches:

Hey JT, those are great images, they clearly show what’s happening. Your elongated key looks really nice, can’t ever argue the quality of your work.

But – and I promise I’m not saying this just to be a contrarian – I’m eager to learn, and to understand, so I’m (over?) thinking critically about this whole crank thing.

In my opinion your photos actually serve to reaffirm my contention that zero modifications need to be done to safely use a U20 crank in an R16 stroker.

What you show bolsters the argument that the Murphy keys are not intended for strength, but are present strictly to index the crank gear.

Why? Because your very nice elongated key will undoubtedly have greater shear strength, just by virtue of mass. So… if the original designers intended for the Murphy keys to provide strength, then why wouldn’t they have simply made a nice long key like you did? Why would they use a very short little rounded key, when they could have just as easily made them full length, and/or made them thicker/taller, or made them out of Kryptonite?

I believe this illustrates what I’ve regurgitated (gleaned from others) - that the Murphy keys are present strictly for indexing.

I also feel like if it fails, it fails… meaning if the key is going to shear, then isn’t a slip a slip? With or without a key?

I concede I can’t think of how a nice long key like yours can hurt, and as Geoff says, there has to be some torque carrying going on, but is that what is being relied on? I imagine there is a shrink fit, and anecdotally I heard of guys racing no keys at all. No idea why they would do that. Old skool crazies!

But, again, if strength were the purpose, why wouldn’t it be a more stout or full length key? I still think it’s a valid alternative notion that no extra key work is needed for a U20 crank to work in an R16 stroker.

Obviously, it’s Pat and his friend’s decision whether or not they want to add a key or go without, and I’ve exhausted my brain power thinking about an alternative to modifying the crank.

Thanks for sharing your knowledge.
Peter

[JT68]

Peter, the Nissan woodruff keys are 5mm cross section and quite strong. You might want to check the shear strength before you consider them superfluous. Again, Nissan, us, and most pro engine builders would not set it up the way you are suggesting.

No, the lower timing gear is not a shrink or press fit, it’s a slip fit. It will spin if allowed to and there would be much destruction.

As I said, as long as the gear never slips all is well. You are betting the whole farm on it.
CBEZ demonstrated for us that a 4 mm key saved his engine. (The 4mm key kept the valve train in time with the crank preventing significant damage)

One could use bubble gum instead of loctite during engine assembly too. Do we want to see how well that works either? Nope, That would not meet our standards either.

I’d still maintain the better, most solid, most robust way to do it is cut the keyway. It’s your engine, your money. I’d say not machining the keyway in an expensive stroker is definitely cutting a corner. You can rationalize it any way you like.

[iloveredmeat]

Peter, the Nissan woodruff keys are 5mm cross section and quite strong. You might want to check the shear strength before you consider them superfluous. Again, Nissan, us, and most pro engine builders would not set it up the way you are suggesting.

No, the lower timing gear is not a shrink or press fit, it’s a slip fit. It will spin if allowed to and there would be much destruction.

As I said, as long as the gear never slips all is well. You are betting the whole farm on it.
CBEZ demonstrated for us that a 4 mm key saved his engine. (The 4mm key kept the valve train in time with the crank preventing significant damage)

One could use bubble gum instead of loctite during engine assembly too. Do we want to see how well that works either? Nope, That would not meet our standards either.

I’d still maintain the better, most solid, most robust way to do it is cut the keyway. It’s your engine, your money. I’d say not machining the keyway in an expensive stroker is definitely cutting a corner. You can rationalize it any way you like.

Good info, totally agree, and fingers crossed!

And please believe me, the tone of the written word can belie my intended tone - my questions really honestly truly aren't meant to be argumentative or dismissive... they really are questions I'm looking for the answers to.

Like: if the keys were intended to provide torque strength, then why wouldn't they over-build them like your long key? That's along the lines of your bubble gum versus loctite question... no you'd never use bubble gum. So, why would you use a 5mm key when there is plenty of room for a significantly larger key? I wish I knew.

And if the bolt goes loose and a key does in fact shear (I believe it's settled that these keys will usually shear without bolt compression?), how does that help to reduce damage over no key at all (or a smaller bit of key)? Isn't a spin a spin? And if it's just a 'safety valve' back to the isn't bigger better question?

I'm sure you're exhausted by this thread, I'm sorry if you ever thought I was doing anything other than positing an alternative.

And if you or anyone else knows the answers I'd appreciate it, and I promise no more posts or questions on this topic.

Thanks. pm

[JT68]

No problem Peter, and no tone interpreted. No tone here either.

The reason WE stay with 5mm stock is geometry/fit.

The Nissan keyway is 5mm, so using 5mm keystock makes the machining easier, but more importantly the gear has a 5mm keyway.
With anything larger you’d have to broach the sprocket too, much harder to do.

I don’t think it’s concluded that the 5mm key would shear immediately. It may live long enough to survive a while. CBEZ’s smaller 4mm key survived long enough to save his engine.

Yes, the bubble gum was a joke of course, but it actually would work too under some conditions-any goo on the threads might.

So would used bearings, pistons, rings, chains etc.

If you re-used everything (in decent condition) measured and assembled carefully. It would work just fine for an unknown amount of time, maybe go forever…. Am I suggesting you build an engine this way? Of course not. Re-using bearings, pistons and rings is certainly cutting corners too. Can they work? Of course.

The machined keyway is simply a better/more robust way of building it. Not sure how else I can say it.

Why use new, top quality parts ever? Same reason.

I really doubt the 5mm solid keystock would shear at reasonable rpm, but it sure would offer more protection than no key or the little nubbin of the stock key.

If I had paid Dave $$$$ for a dyno’d stroker, I would want the keyway. That’s me.