4.2 Crankshafts

CastleMGBV8 · Post by **CastleMGBV8** » Mon Dec 24, 2007 11:11 am

I had been looking for a 4.2 crank and noticed on JE's site that he had one on offer, reground and Tuftrided for a £100.00 + p&p seemsd a bargain to me so ordered one this morning.

It appears he has a few available so if anyone wants to upgrade a 3.9 grab yourself a bargain.

Kevin

CastleMGBV8 · Post by **CastleMGBV8** » Mon Dec 24, 2007 11:30 am

Correction, that should read £120.00 + p&p

Kevin.

ian.stewart · Post by **ian.stewart** » Mon Dec 24, 2007 1:05 pm

It would cost you that to get a crank reground, Just been curious, how much more stroke has the 4.2 over the 3.9?? the mind is wandering --yet again

Wotland · Post by **Wotland** » Mon Dec 24, 2007 1:49 pm

ian.stewart wrote:It would cost you that to get a crank reground, Just been curious, how much more stroke has the 4.2 over the 3.9?? the mind is wandering --yet again

4.2 crank has 77mm stroke VS 71.12mm on standard 3.5/3.9 crank.
4.2 crank uses same rods as 3.9 but pistons with different compression height.

ian.stewart · Post by **ian.stewart** » Mon Dec 24, 2007 2:45 pm

Intresting, 5.88mm stroke increase, I wonder how much meat my Omegas have on the crown, probably if I use comp gaskets at 1,15mm less the tins I was using the comps will allow me to run the pistons 1mm out the top of the bore so if my crowns will allow me to remove 5mm I could be in for a 4.2
OOOOH the brain is working overtime now.

CastleMGBV8 · Post by **CastleMGBV8** » Mon Dec 24, 2007 3:13 pm

The 4.2 crank has according to my land rover data sheet a stroke of between 76.222mm and 76.759mm! which is a big variation, taking the average of 76.5 say and with chevy 305 pistons at 94.89mm give a capacity of approx 4326cc.

I will need to use the 5.85" rods to get the pistons to the top of the bores and with the Buick 300 heads should make a nice engine for not to much money.

Kevin

ian.stewart · Post by **ian.stewart** » Mon Dec 24, 2007 3:35 pm

I have some SBC rods in a 3.5 engine, they, I think are 327 rods with and stock pistons with a nip removed from the tops, I will need to have a good look round to see what I need for this,
Ian

ian.stewart · Post by **ian.stewart** » Mon Dec 24, 2007 3:57 pm

Just had a thought, are these ground down damaged serp cranks or Icebergs???? im assuming they are Icebergs.

Post by **katanaman** » Mon Dec 24, 2007 4:23 pm

ian.stewart wrote:Just had a thought, are these ground down damaged serp cranks or Icebergs???? im assuming they are Icebergs.

Why cant they be normal re-ground 4.2 cranks?

CastleMGBV8 · Post by **CastleMGBV8** » Mon Dec 24, 2007 4:23 pm

Ian,

I don't know all John said is they were 10thou undersize on mains and big end journals and I can't find out now until after the holidays, is there anything I should be concerned about?

They could well be ground down iceberg cranks as john said he bought a batch of cranks some time ago, and I remember reading that he had bought up the stock of redundant iceberg cranks from Land Rover.

Kevin.

HairbearTE · Post by **HairbearTE** » Mon Dec 24, 2007 6:25 pm

The 4.2 crank is in some ways the strongest factory crank on paper but it is also technically the most brittle. I think the key to using one in a tuned application is getting your clearances and balancing spot on. If you do then I think the crank will be very strong. There are stories of early tuned 4.2's self-destructing but I think that probably had something to do with the offset piston design of the 4.2 that is not really ideal for a racing engine. I bought one last year NOS from a guy who took some stock off a Land Rover main dealer branch that closed. It appears to be nitrided and crossdrilled from the factory! Does anyone know if this was done to all 4.2 cranks?

HairbearTE · Post by **HairbearTE** » Mon Dec 24, 2007 6:33 pm

katanaman wrote:
ian.stewart wrote:Just had a thought, are these ground down damaged serp cranks or Icebergs???? im assuming they are Icebergs.
Why cant they be normal re-ground 4.2 cranks?

I think all 4.2 cranks were iceberg castings Marki.

ian.stewart · Post by **ian.stewart** » Mon Dec 24, 2007 6:48 pm

HairbearTE wrote:The 4.2 crank is in some ways the strongest factory crank on paper but it is also technically the most brittle. I think the key to using one in a tuned application is getting your clearances and balancing spot on. If you do then I think the crank will be very strong. There are stories of early tuned 4.2's self-destructing but I think that probably had something to do with the offset piston design of the 4.2 that is not really ideal for a racing engine. I bought one last year NOS from a guy who took some stock off a Land Rover main dealer branch that closed. It appears to be nitrided and crossdrilled from the factory! Does anyone know if this was done to all 4.2 cranks?

Thats an intresting perspective, I think the Icebergs did get a bad press, were the cranks not made for the still born Diesel engine, and one of the reason they were dropped was the breakage of the cranks, but a Diesel has far more compression loads imparted upon it. I could quite easilly see a quick spin up on the lateh to reduce the rotating mass a bit and possibly a bit of Knife edging as well,

HairbearTE · Post by **HairbearTE** » Mon Dec 24, 2007 7:05 pm

For interest I shall weigh it vs a 3.9 crank. If I start weighing cranks on the bathroom scales on xmas day then there is a good chance that i will be booted out the house so i'll wait for a quiet moment and report back! My one concern for Kevin would be if they were all nitrided from the factory then the .010" undersize will have affected that.

ian.stewart · Post by **ian.stewart** » Mon Dec 24, 2007 7:53 pm

If I remember from my apprenticeship days the surface treatment is only a few thou thick, Between 0.0025" and 0,004" and the 10 thou regrind I am fairly sure will have removed it from the journals, The only advantage that Nitriding/Tuffriding does is create a hard surface, it does nothing to stregthen the metal, other than stopping grooves, scuffs and scratches which in turn may prevent problems due to stress risers at these points.
I remember on some of our MOD and LLoyds jobs that all the stressed corners had generous Radii and were rolled under extreme pressure with a steel roller to consolidate the grain and pollish in that area, not my favorate job.