Big end failure

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rigsby
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Big end failure

Post by rigsby » Tue Oct 02, 2018 10:39 pm

I have stripped three nc 30 engines in the last 12 months my own engine and 2 of my friends engines. two of the three engines both had big end shell failure, both with back right cylinder big end failure rendering both crank shafts unusable. I have read that the bottom ends are solid but with my past experience im not so sure, is this common on these engines and in particular that big end or is it just coincidence.

Anybody got a usable crankshaft they want to sell!

Thanks

Dynamohum
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Re: Big end failure

Post by Dynamohum » Wed Oct 03, 2018 12:12 am

The more learned may give you the definative answer to your question regarding big end failure ive always thought it's the result of oil being contaminated by fuel either by faulty float valve hrc tap mod or knackered tap diaphram along with a dodgy float valve as i have one engine sitting out of an ex track bike that developed a knock which suffered the above fate after being run with a sump full of petroil mix.

rigsby
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Re: Big end failure

Post by rigsby » Wed Oct 03, 2018 8:27 am

Two out of the three are race engines and as you would expect its those that have the big end issue, the road engine i stripped was ok so i placed that crank and rods in one of the other cases with new shells etc. When i stripped both race engines neither of them had contaminated oil didn't overely smell of fuel etc and all other big end shells were ok and mains shells too. So constant thrashing seems to result in premature big end failure on that back right cylinder, problem is cranks are becoming hard to find especially a good one and i dont know of any ways or people who can service the cranks, you cant have them re ground as you have no thickness options available apart from green brown yellow etc.
If any body knows of a company or a process to breathe life back into a crank with a scoured journal please let me know and also anybody with a decent crank to sell.

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Re: Big end failure

Post by moforockband » Wed Oct 03, 2018 8:57 am

Hi Rigsby,
What are you revving the engines to when you are racing? Std redline or higher? Sometimes just knocking 500 rpm off will keep them together (in case you are revving higher than std).
You can still make race winning power at 14500.
Forget undergrinding the crank though. Firms like Chr*s Apple*ee will not be able to give you a crank job that will last. They use non specific shell bearings which quickly die.
I know SEP can reweld/regrind that journal back to std. But you would have to check that the surface is OEM perfect and the whole crank is straight after welding. None of these companies will guarantee their work though!
A snippet of wisdom I have is check your piston to head clearance. If it is too tight, and the pistons kiss the head during a missed gear change etc, you will kill big end bearings really fast. Keep this clearance close to std also.

rigsby
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Re: Big end failure

Post by rigsby » Wed Oct 03, 2018 12:40 pm

Thanks for info , to be fair i thought that having the crank serviced was going to be a ball ache. Its interesting to hear about piston to head clearance one of the race engines i did somebody had removed the middle layer from the 3 ply head gasket to raise compression, they had then made spacers from the removed gasket layer and placed them under the rocker assemblies, very crude and i know the guy had said it had 2 cranks with shells put in by someone else and both times had failed with in one race meeting and he always insisted on this raised compression codge so that makes sense, but the other race engine had never been apart so maybe just coincidence that they both have failed on the same big end. In terms of over revving as far as i am aware the CDI that were being used were the standard items with no removed rev limiter, although through the life of an engine thats been raced often will hit the rev limiter etc so it looks to me that the engine been subject to racing and alot of abuse big end failure is a problem with these v 4 motors.
I will have to try and find a usable crank or the engine is parts only. Bummer!

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gavins
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Re: Big end failure

Post by gavins » Thu Oct 04, 2018 8:58 am

I have had two standard race motors blow with big end failure. They usually last about 2 seasons racing if you don't touch them and just change the oil/filter every other meeting.

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Re: Big end failure

Post by moforockband » Thu Oct 04, 2018 9:13 pm

If one of these engines has been rebuilt with a new big end bearing on a slightly scored journal, it will die super fast also due to oil pressure loss.
I think you've proved that the rear RH big end is the 1st to go. It may be worth looking at why if you have time. The FZR400 has the same issue. That was rectified with drilling the oil orifice out.
It doesn't really matter if you use fully synth or even change the oil every meeting, if the journal loses pressure for a second or 2 at high rpm it's on the way out. The big end should never touch the crank, it floats on a constant high pressure film of oil.
I'd definitely build your next engine with std squish/gaskets and clearances, and work on cramming more air/fuel in there to get the power. ie ram air.

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Re: Big end failure

Post by newtothis » Thu Oct 04, 2018 11:57 pm

If you can find a firm that metal spray and regrind cranks they ought to be able to get the journal back to OE size and finish. Many years ago I had an MGB crank done by the firm that refurbed london double decker bus cranks and it lasted 3 years on the road and then 2 seasons as a stop-gap race engine when somone needed one in a hurry. So there's nothing wrong with a repaired crank...

Bradford grinders might be worth a look

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Re: Big end failure

Post by moforockband » Sat Oct 06, 2018 1:23 pm

newtothis wrote:
Thu Oct 04, 2018 11:57 pm
So there's nothing wrong with a repaired crank...
Bike cranks spin 3 times faster than a mbg crank, use tri metal bearings and use much tighter tolerances. Considering the extortionate cost of rebuilding a v4 engine, with the disappointment and extra expense of it failing at the circuit, you can't compare a car engine to a bike one.

Apart from 2T cranks I've never heard of anyone with luck from a re-worked 4t race bike crank. All of them went bang on exactly the same big end pin.
I would like to see it done though.

rigsby
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Re: Big end failure

Post by rigsby » Mon Oct 08, 2018 7:32 am

Thanks for info,

I have managed to find a set of crank cases crank rods pistons off ebay for £35 i checked photo's and it sold as usable looks ok and for 35 quid delivered worth the gamble, fingers crossed. I have also heard that g force now produce a range of thicker shells so traditional crank journal grinding can be done, thats my next port of call if this ebay purchase isn't any good. As with premature big end failure i dont think there is any solution the big end failures havent been due to an internal problem blocked oil passage etc it's purely the result of high revving abuse that all race engines are subjected too and iv raced a lot of different 400 's and 600's and i think the nc30 is prone to this failure as a v4 set up compared to the inline 4 's i have owned that seem to be able to take more abuse. So i agree with the 2 season rule , change the big ends before the crank journals get damaged so thats what i will do an expensive solution but better than trying to repeatedly find good cranks i.e road engine cranks which are becoming v difficult to find and this will only get worse.
Thanks for posts on this topic
Rigsby


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