Best w&d grit grade to polish up crank journal
Discussion
Pumaracing said:
The depth of the hard case on a nitrided crank is the unknown variable here. It's unlikely to be more than about 20 thou on radius unless the crank spent a hell of a long time in the nitrogen. It takes like 2 or 3 days at 550C just to get 20 thou. Then some of that will get used up during the final grind to size, usually about 5 thou leaving maybe 15.
So hopefully enough to get a 10 thou diameter undersize out of it without renitriding or maybe even two grinds of 10 thou.
But, the next issue is how much of a problem is it really if you use up the case depth? Cast iron cranks are soft as muck. EN19B cranks aren't hardened at all. Only EN40B nitriding steel is.
So in conclusion I'd say it's ok either way.
I'd pretty much agree. I did have one re-nitrided once - it came back bent!So hopefully enough to get a 10 thou diameter undersize out of it without renitriding or maybe even two grinds of 10 thou.
But, the next issue is how much of a problem is it really if you use up the case depth? Cast iron cranks are soft as muck. EN19B cranks aren't hardened at all. Only EN40B nitriding steel is.
So in conclusion I'd say it's ok either way.
You need to check whether the bearing clearance is sufficient on the big ends and whether the bearing material is sufficiently compliant in a touch condition. The latest aluminium bearings are very hard wearing but will wipe out with the slightest amount of contamination or excessive pressure from an uncontrolled spark event.
A bit off topic but in away sim 20+ years ago I used to help out with a mate with an Avenger stage car , we fitted a 1760 cross flow ex local grasser install ok ran lovely ,I took it out to settle things in 3 miles up the road no oil press so stopped it ,towed back stripped minimal damage [shells and crank polish ] fitted new oil pump and rebuilt it went down again in exactly the same place , time for deep investigation! complete strip ,jury rigged pressure test and sat block on a coke stove revealed a cracked block it only opened when block was warmed through !!new /replacement block rebuilt with a softer cam and put in works van !problems are not always easily seen
PeterBurgess said:
Internal Dave. One bloke with a 2.4 Pinto lost his engine three times, he had a miniscule leak on a joint in the scavenge pipe in the sump which just sucked air on long right hand bends, was perfect on engine dyno and on our rolling road.It was 25 years ago but I remember it cos the bloke was so gutted each time it failed 
We have seen problem about half a dozen times altogether. Heartbreaking as engines so expensive and all seemed fine just to be dashed on the track. Have even seen hairline cracks in pipes. Not saying it is so in your mate's case. Maybe problem with the new dry sump set up though? Seems to have the familiar ring to the failure, interesting to know if they had just come out of a long bend or in a long bend when it failed and any similarities with driving for both failures? Shame anyhow.
Peter
Thank you for that info Peter. They're reading this thread so hopefully someone will check all this out properly if I don't get asked to be involved. I've already told them it's vanishingly unlikely to be a fault in the dimensions of either the conrods or crank journals and 99% sure to be a problem with either oil pressure, oil flow, oil aeration or oil temperature. I now find out there was no oil temperature gauge fitted which is a bit of a false economy with so many expensive bits in the engine. We have seen problem about half a dozen times altogether. Heartbreaking as engines so expensive and all seemed fine just to be dashed on the track. Have even seen hairline cracks in pipes. Not saying it is so in your mate's case. Maybe problem with the new dry sump set up though? Seems to have the familiar ring to the failure, interesting to know if they had just come out of a long bend or in a long bend when it failed and any similarities with driving for both failures? Shame anyhow.Peter
james7 said:
I would start off with 1200 wet and dry and work down to 2000. Should only take 20mins. It may need something sticking over the hole where it "poked a leg out of bed" 
Seems like a lot of bloody hard work to me! I'd go for a 50/50 mix of engine & gearbox oil with some sawdust for good measure but I fully agree with rape tape over the hole in the crankcase...Another question which I have no personal experience of answering. I'm told the last failure was only 3 or 4 miles from the start of the first stage. How long would it take a 250 bhp engine to bring the oil in a dry sump system up to normal temperature and maybe well above that? I assume the engine was fully warmed up before the start at least as far as water temp was concerned.
Bits from a Renault Clio 197 that went bang at the same event. The two things at the bottom are gudgeon pins I'm told. I've never seen gudgeon pins flattened and split right open before. Mind you I tried to build engines that didn't blow up rather than vice versa so I don't have much experience in this area. 
Pumaracing said:
Another question which I have no personal experience of answering. I'm told the last failure was only 3 or 4 miles from the start of the first stage. How long would it take a 250 bhp engine to bring the oil in a dry sump system up to normal temperature and maybe well above that? I assume the engine was fully warmed up before the start at least as far as water temp was concerned.
My old XE engine had dry sump, no oil cooler, finned sump pan which hung out the bottom of the car and the tank was next to the exhaust headers. It held iro 7.5ltrs of oil. Had a bit more bhp too. Never had any overheating issues at all. In fact it took a while to get it hot enough to give it some stick. The only time it got a bit hot (iirc 110c+) was on the rolling road on a hot day and a fan blown underneath fixed that. On a hot trackday etc it was usually below 85-90.But some of the XE dry sump systems hold as little as 4 ltrs of oil, have a tiny tank which in an escort could be quite close to the exhaust and dont have the pan in the air stream. And if it did not many are heavily finned for cooling.
So I guess it would depend on what was fitted at the time but I would be surprised if it had overheated massively in that time. Also worth bearing in mind that some of the small tank set ups can have low pressure with 1/2ltr of oil missing from the tank. So whoever does the checks has to be 100% on the case with it.
Update.
With only a week to go after the blowup until another rally last w/e the owner and my mate the co-driver frantically bolted the old bits from the various donor engines back into the car while an autopsy on the broken bits is planned for the future. The original bored out block that came with the car, the old dry sump system of identical make to the new one in the two blow ups and whichever head hadn't been used in the new rebuild.
It all ran like a champ all w/e with sustained 8500 rpm no problem. It points to something very obvious in either the block or dry sump system that let go twice and hopefully someone will discover what that was eventually.
With only a week to go after the blowup until another rally last w/e the owner and my mate the co-driver frantically bolted the old bits from the various donor engines back into the car while an autopsy on the broken bits is planned for the future. The original bored out block that came with the car, the old dry sump system of identical make to the new one in the two blow ups and whichever head hadn't been used in the new rebuild.
It all ran like a champ all w/e with sustained 8500 rpm no problem. It points to something very obvious in either the block or dry sump system that let go twice and hopefully someone will discover what that was eventually.
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