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Discussion and questions related to the course How to Degree a Cam
Hello hpa community,
In our rallycar Abarth 500 equipped with the original 1,4l turbo engine the crankshaft is not in line with the cylinders. It is offset by at least 5mm, resulting in a weird motion / acceleration at tdc. In one instance the piston reaches tdc faster as it leaves again or vice versa.
This results in a unequal motion at tdc and therefore a shift in measured tdc when opting for the method you teach.
Wouldn't it be more sufficient to look for tdc with a dial gauge only, looking at the highest number in the gauge instead of taking two sides of equal angle out of tdc?
using the positive stop method works on all applications and combination known to me but I'm happy to be corrected
When using a dial gauge as you described it doesn't take in to account the rock from the bearing clearance whereas going past TDC and back again does but like Ross says using a positive stop is a pretty fool proof method.
Please let me know if my written words sound weird , my native language is german...
I'm absolutely with you: positive stop or using the degree wheel is a foolproof method. This is safe!
This is not correct (in my opinion) if the crankshaft is not in line with the cylinders.
I try to explain what i'm thinking:
There will be an error, if the crank is not centered because at that point where the piston is at tdc (crank throw, rod & piston are aligned), the line that these parts form is not parallel to the cylinder line. This makes the movement of the piston unequal when looking at the way to tdc upwards and from tdc downwards. Therefore in my opinion the degree wheel isn't helpful because the same number of degrees before and after tdc do not result in equal motion of the piston downwards either side . Also vice versa: the same distance of the piston down the bore measured with a dial gauge on either side of tdc will not result in the same angle of the crankshaft.
For this reason (and only when crank- and cylinderline are not in line) i would probably prefer to take a single point measurement only. Rocking of the piston is not touched anyway.
The error occuring gets bigger as the distance between crankshaft centerline and cylinderline grows. I did not do the math behind that problem because i have no cad system available. Furthermore i'm not sure if the error would be bigger than the uncertainty of the measurement per se, but i hope you can follow my thaughts and understand what i mean and the geometry i'm thinking of.
What he means, chaps, is that because of the connecting rod inclination around TDC, the true TDC of the crankshaft and the split measurement of the piston positions are going to be different.
Jochen, it's a simple triangulation problem and, if you can provide the stroke, connecting rod centre to centre length and actual crankshaft offset (to major thrust side?), gudgeon (AKA wrist or piston) offset if any, then I can give you the crankshaft postion at piston split lift at the point you want to know (it will vary) or piston positions at crankshaft true TDC.
You can figure it out yourself, if you like, with a simple sketch to guide you.