Fiecos Dan

Had my cossie crank ground, after No4 big end knocked out.

The machinest told me it was slightly bent, but could get it true again on a -0.25mm big ends, -0.50mm mains.


But last week i CC checked the piston and bores, and today due to the cc's being out, i checked the stroke of the crank.

No 1 = 77.15mm stroke = 8.33 c/r
No 2 = 76.81mm stroke = 8.16 c/r
No 3 = 76.68mm stroke = 8.13 c/r
No 4 = 77.34mm stroke = 8.52 c/r



I've now got to machine the pistons of 1 and 4 to get the c/r correct.



After a crank grind is this usual, and would the machinest notice this whilist grinding the crank?
As i've never had a crank stroke change after a crank grind before.

RWD_cossie_wil

iTrader: (9)

Regrind should make NO difference at all, the size of the journal is taken up by the bearing. To get a stroke difference across the 4 big ends would indicate to me that the crank is still bent. Did you check the crank in the block torqued down, or on an engineers flat table in V blocks?

Fiecos Dan

Crank in engine torque'd down.

RWD_cossie_wil

iTrader: (9)

did you plasti-gauge all the bearings before fitting them? I can't see how you would get a different stoke unless the block is not straight or the crank is not straight....

I take it you used a DTI from the top of the block to measure the crank throw?

Fiecos Dan

i measured the Stroke from TDC to BDC with dial gauge on each test, and digital verniers in a jig bolt to the top of the block.

And each test was carried out twice on each bore with identical results to 0.01mm.


Whats plasti-gauge?

all i done was build the bottomend with gearoil.





Also checked TDC and BDC on different bores, and they are true to each bore, meaning TDC @ 1 is TDC @ 4, and BDC @ 2 and 3.

RWD_cossie_wil

iTrader: (9)

Plasti-gauge is a tool for measuring your bearing clearences/tolerence. You buy a kit, which comes with some thin plastic type strips that you put into your bearing gaps (say the main caps) in place of your bearing, then torque it down to the usual torque. You then take it back out, and micrometer the plastic. This will tell you how thick your bearings need to be .

From what you have written, I would say the crank is still bent.... . I am not an expert by any means, so I might be wrong, but personally I would take the crank back out and measure it again on its own.

Fiecos Dan

Cheers,




How would you measure a crank out of the engine?

Fiecos Dan

Just checked No2 and No4 again.


and pushing the piston down whilst @ BDC and pushing the rod up @ TDC.


and it added 0.04mm onto No2 stroke, and similar to No4's stroke.

leecavturbo

it just doesn't sound right does it dan!
i.e don't build it yet

Fiecos Dan

Last thing i wont to do is machine the pistons to suit, then find out the cranks bent.

glancy2081

then will it not be out of balance if you were to do this

melzieann

i would agree with RWD_cossie_wil it does sound like possible crank bent

Fiecos Dan

After the machineing for C/R

i would have to balance them all again.

Butch.

iTrader: (1)

does sound like a bent crank or maybe a conrod out of shape slightly try swapping a piston and rod into another bore and see if you get the same measurment that you did on the bore before you swapped them over...

Alg1k

iTrader: (2)

Sounds like the crank is a bit bannnana like
Have you put a dti on the fly wheel end and checked for runout.
Also on removing the crank inspect the bearings for bruising on there sides.

Fiecos Dan

Hi, A bent conrod will only alter the TDC and BDC hieght, not the stroke.

i might swap No1+2 and No3+4 around to see if Rods are the same length.



I havent checked flywheel trueness, but will do in abit.

I think the crank bent, and to straighten it with a grind, has put the stroke out.???

focusv8

If the crank has been machined to correct for the bend, then it would appear that the journals have been machined slightly off true centre for each journal, that's the only way you could have unequal stroke.

No amount of machining of pistons / rods will ever get it back in true balance, you may get equal C/R but you won't get equal stroke.

New crank required.

Maybe the machinist didn't know you would be checking stroke and maybe the tolerance is acceptable for a standard engine.

.

Fiecos Dan

Cheers,

i know machineing the pistons will only do C/R.

The machinest was told before hand its for a 500 bhp cossie, so needs to be good.

Fiecos Dan

Cheers,


i've just checked the flywheel.

it runs +/- 0.1mm out on the diameter, and +/- 0.30 on the back and front faces.

At the point its -0.1mm on the hieght its +0.3mm on front face, and -0.30mm on back face
At the point its +0.1mm on the hieght its -0.3mm on the front, and + 0.3mm on the back face


Which shows it has bent, and even if the crank has been straightern'd on the grind, the flywheel face hasnt.

Alg1k

iTrader: (2)

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