Cross drilling crank

[Walt Fricke]

I talked with a local crank grinder about doing this for the middle bearing so #2 and #5 rod bearings would be better fed. He thought he could do it (though he thought 911 cranks were made of very hard steel), but wondered what else he ought to know. I figured he could copy the "teardrop" from what Porsche did, and copy the hole diameter as well. And that the protocol is, in fact, to drill one hole all the way through the journal, intersecting the crank's internal oil passage half-way through. And that it ought not to matter at what "degree" on the crank this hole was drilled.

Have I got anything wrong here? Left anything off? My race rods have lived just fine up to 8,000 rpm with stock cranks plus the turbo oil pump (so the GT3 oil pump ought to do even better), but people are telling me I need to cross drill for extra insurance, and it seems simple (and thus inexpensive) enough.

Walt Fricke

(I spotted a post here on how to groove the bearings, of which I have a plentiful supply of partly used ones for practice)

[Diesel_mp5]

Hey . Get in contact with this guy he is supposed to be the WHIZ! Hope that helps.

http://www.competitioneng.com/ Only Porsches

[TimT]

To complement the cross-drilled crank, you should modify the middle main bearing

Made a lil fixture you chuck in a lathe, thats cuts a groove in the bearing...



finished bearing

[TimT]

Oops missed the part mentioning your aware of completing the mod by grooving the bearing..

And yes he should generously radius the cross-drilling

does your local guy have the facilities to re treat the crank?

ie tenifer, nitriding etc?

[Walt Fricke]

Tim

Thanks

I think it may be your fixture I saw pictures of earlier.

But why would one need to reharden (nitride) a crank after drilling two holes? This is a standard 66mm forged crank I just had polished and balanced (which it didn't much need). Does the drilling operation create that much heat?

Walt

[TimT]

Walt, you are probably right on the issue of rehardening the crank, typically when we have cranks cross drilled, they get the full treatment, ie polishing and balancing, so rehardening is just another line on the bill.

now that you mention it Im not sure it would ( the tenifer etc) be necessary simply because you drill a hole through the crank.

[Rob 930]

I just had my 930 crankshaft cross drilled by a machinist in Portland. I et the impression this man has done hundreds of them in this way, so I deferred entirely to his experience. It seems like a reasonable mod to do, especially for a high revving (7-8K+ rpm) engine (which mine is not). But even for my application, the concept of providing another oil path to the center rods appealed to me. For those who don't already know, cross drilling provides a third inlet for oil to the crankshaft. The crank is normally fed oil only from each end, and the oil must make its way through the length of the crankshaft to each of the rod bearings, with a resultant loss in pressure by the time it reaches the center rods. Cross drilling alleviates the long, circuitous path formerly required to feed the center rods. The only real downside for center drilling is the risk of having it done wrong. I understand that if done right, it won't weaken the crank to any appreciable degree. The machinest I went to had never seen one break at the drilled hole, and he does this on many racing engines from small 911s up to 935s.

There are at least two ways to do it that I'm aware. One is to drill the crank and put a groove in the bearing shells, as shown by TimT. I have also seen the crank drilled and then grooved. My machinist preferred the method of grooving the bearing in order to preserve crankshaft strength. In my case he did radius or "spoon" the hole on each side of the crank. I'm not aware of how he chose the circumferential indexed position for the hole, or if it mattered. I know that there's a trick to drilling so that chips don't get inside the crank. He's apparently perfected this. Otherwise, it is necessary to remove the plugs in the crank, flush it, and replace the plugs, which I understand is a tricky and rather painful job. I believe he uses air pressure somehow to keep chips from getting in the crank, but I'm not sure. There should be no need to re-nitride the crank after this operation.

He also made a mod to the case. He enlarged the oil passageway to the center main. This requires a very long drill, and it required that he pull the plugs out of the oil galleries in the case (two of them, I think) to clean out the chips (and then, of course, replace the plugs). Then, he enlarged the oil feed hole in the bearing shell -- the one that already had a groove in it. He believes that a larger supply of oil is necessary to feed that center journal, now that it's feeding more than just the main bearing. This makes sense to me.

Here are some pictures:

Enlarged oil feed hole in center main bearing web



Grooved center main bearing shell, showing enlarged oil hole in bearing. Note that the edge of the enlarged hole in the bearing looks a little gnarly. I was assured that it had been properly de-burred and is not a performance factor, though it isn't terribly pretty. According to the machinest, it's tricky to make this hole look neat, for reasons I didn't explore. I don't mind, as long as the bearing contact surface is smooth and no particles can get loose.



Cross drilled crankshaft, showing "spooning" on the main journal

[PFM]

I am in the midst of my first rebuild and did the crank cross drill, would anyone want to share the bearing groove dimensions? I built the tool to cut the groove now just wonder what size of the recommended groove is?

Thanks in advance,

PFM

[Walt Fricke]

Alas, mine is in the short block on the engine stand so I can't measure. Rudtners cut it based on their specs, so I didn't have to figure what it should be (not that I could). However, I think you could come close by scaling from these (and other) pictures if none of the machinists who have done this chime in.

So far I have not seen a post by anyone who has grooved the crank journal and regretted it. Henry Schmidt certainly thinks it is the way to go. The cranks I have seen break have done so at the radius. And on the 2.8 RSR cranks Porsche increased this fillet radius. It seems that machinists who do cranks are a conservative lot. Mine would only drill the "crossdrill" hole half way (opening only on one side of journal) despite my assertions that I thought normally one drilled all the way through.

Henry recommends drilling the oil passage in the case to (I think I remember) 6mm to help supply the oil. I cheated and used 1/4" because you need a long drill bit for clearance and such and I had one of those. We'll see if I come to regret the extra 0.014" of diameter.

An unexpected complication to this has been getting replacement plugs for the main oil gallery: my local Porsche dealer got the only one in the inventory in the USA for me. I had planned on putting in 1/2" pipe plugs, but found that on the 930 case the hole is too large for this (unlike the 2.7's case), and 3/4" NPT taps are expensive. So I'll find some aluminum stock and turn a duplicate based on the 964 part I bought.



Walt

[Henry Schmidt]

We cross drill every crank on every engine. Even the GT3 cranks.
We groove the bearing but when we designed the Super Crank we grooved the crank rather than the bearing. Note that every main journal is cross drilled.

[kenikh]

As an added data point, I had my crank cross drilled by Marine and without my prompting, grooved the crank, too. This was initially a concern, as I'd already grooved my bearing and was worried if these mods were incompatible. Asking the experts, this was not a concern and will work fine. More oil in this case is preferable to less.

[917guy]

A while ago I pondered the idea of using a drilled hole in a 911 crank for better rod bearing oil supply. I had a 1972 911S engine with #2&5 rod bearing failures a while back. Are 2 such drillings not much better? In my latest Newsletter, I suggest that they are. I also go into how to get more oil out of your early (pre SC) 911 oil pump. In 2009, I (was) retired from putting together a monthly PCA newsletter (after winning 3rd place in the PCA competition). I have carried on independently, as this is so much fun. OK maybe it is lots of work. On the free-yudu.com website, you can search for my past issues under RED RIVER SPORTS CAR ENTHUSIAST. Below is a link to my yudu library.

The January 2012 issue, linked below, has some pics and things to ponder on early engine mods. Plugging crank holes. Oil pump stuff. Crank mods.

RED RIVER SPORTS CAR ENTHUSIAST JANUARY 2012


Read Publications from 917guy's Library | YUDUfree for a link to past issues.

David

[Walt Fricke]

David - thank you for sharing this.

A friend, who worked his way through law school as a machinist, succeeded in tapping the oil plug passages in his 911 crankshaft. He said it was difficult, and required many many taps.

It sounds like your aluminum plug method might be easier overall, though once the threads are cut there should be no temptation to skip pulling the plugs any time the crank was out of the motor.

[917guy]

Walt,

Interesting that a law student found fun/ profit in machine shop work. I have had fun with it as a recreation since I was a teen. I have been a chemical engineer and materials researcher since the 60s. It seems we all slide down similar 'slippery slopes' to this point, but from somewhat different places.

The ease of redoing it is why I went with grub screws so long ago. A single tap did all the cranks I had back 40 years ago, and is still flawless today. Now that I have a good soft-plug method, I will not be afraid to deplug a long-used 911 engine in future. If it has had no crank or oil system related trauma, I might not bother, as I would with the threaded plugs. In all the cranks I have 'opened' I have found lots of gray gunk, but no chunks. I am still paranoid about bits of a previously failed rod bearing getting into my new bearings, after a careful rebuild.

What do you think of 2 drilled crank holes versus 1? I have only done one crank that way, a 1972 S. I hope to fire it up later in 2012. I have a 1982 3.0 engine apart also, but have not drilled its crank. I am not sure which cam/ FI system I will use with that engine, so RPM may not be an issue. I twin plugged the 1982 heads, but if not using high RPM, this will not be of much benefit.

David

[Weissach911]

Originally Posted by Walt Fricke

Tim

Thanks

I think it may be your fixture I saw pictures of earlier.

But why would one need to reharden (nitride) a crank after drilling two holes? This is a standard 66mm forged crank I just had polished and balanced (which it didn't much need). Does the drilling operation create that much heat?

Walt

Cranks were not 'nitrided' they are a medium Carbon Steel and were Tenifer treated - this is ferritic Carbo-nitriding which is very different to more traditional Gas Nitriding.

It has been commercially described as 'Soft Nitriding' or 'Salt bath' nitriding but produces very different results to the more traditional nitriding we normally discuss with regard to crankshafts. Gas nitriding would need a steel such as an EN40B which has relatively high levels of Aluminium added to from the corrrect type of nitride.

It does provide some scuff resistance to the journals but it doesn't really produce the depth of hardening or the level of residual surface compressive stresses needed to produce real improvements in fatigue life. Case depths are only a few microns.

If you try to gas nitride the stock crankshaft you could end up with a brittle surface layer which could crack and spall.

The US trade name for Tenifer treatment is Tuftriding and now a slight variation called Melonite treatment is also available.

These processes, like gas nitriding, are relatively low temperature but you would need to take the bungs out and thoroughly clean after treatment. It is very unlikley to cause any thermally induced distortion.

If you cross drill with sufficient cooling then heat is unlikely to be an issue but you could produce some residual stress in the surface of the material around the hole and this could lead to the formation of short fatigue cracks.

The temperatures involved in Tuftriding will virtually eliminate these residual stresses and is, I think, a reasonably good idea.

[nismo silvia]

I would like to see how long these engines last since cross drilling is a big no no also so is having a grove all the way around the crank shaft.

I am not sure what benefit you think you are adding but I have a feeling all of your cranks are blue by now.

[Henry Schmidt]

Originally Posted by nismo silvia

I would like to see how long these engines last since cross drilling is a big no no also so is having a grove all the way around the crank shaft.

I am not sure what benefit you think you are adding but I have a feeling all of your cranks are blue by now.

I always appreciate the input from "experts". lol
We've been cross drilling Porsche race cranks since the late 70s IMSA engines.
For the last 8+ years we've been cross drilling every crank we use (street or track).
As for "blue cranks" that's just plain hilarious......keep the comments coming...laughter is a great way to start the day.

[nismo silvia]

I would like to see one of these setups after a race or even 10-20 thousand miles, post up some pics. Also just because it has been done since the 70's doesn't make it right.

Originally Posted by Henry Schmidt

I always appreciate the input from "experts". lol
We've been cross drilling Porsche race cranks since the late 70s IMSA engines.
For the last 8+ years we've been cross drilling every crank we use (street or track).
As for "blue cranks" that's just plain hilarious......keep the comments coming...laughter is a great way to start the day.

[Rob 930]

Originally Posted by nismo silvia

I would like to see one of these setups after a race or even 10-20 thousand miles, post up some pics. Also just because it has been done since the 70's doesn't make it right.

Wow, just speechless here. When I was alerted to a post to this thread, I figured it might be one that was interesting -- that the awakening of this old thread might actually provide new, good information or commentary, or maybe an intelligent question.

Nismo Silvia, please note that my post in this thread was from 11 years ago. That engine, and many like it, built by top engine builders and machinists, are running well on racetracks and roadways around the globe. I have no idea if its a good idea to cross drill the crankshaft on a Nismo or a Silvia. But I can assure you, as can many people here with a great deal more expertise than me -- or you -- that a cross drilled crank in a Porsche 911 series engine is a sound, proven, modification that does nothing but improve the life of internal engine components. But it needs to be done by an expert, so I suggest you don't try it at home.

[Henry Schmidt]

Originally Posted by nismo silvia

....edit... Also just because it has been done since the 70's doesn't make it right.

No it doesn't make it right but it does offer a huge data base on what does and does not work.
If we built one engine in the 70s the antidote evidence would be minimal but because we have hundreds of data points (engines) to reference, I feel confident that we were and are using the best methods available.