Hello everyone,
Have any of you ever installed QED 420 camshafts and can you tell me if you had to mill the piston tops?
I am reassembling an engine with Q420 cams, Q55 springs, Sprint cylinder head and standard QED forged pistons and the clearance between the intake valve and the piston is close to 0.00mm! Measured with an MOP of 106° at both intake and exhaust.
According to QED, this shouldn’t have been necessary!
Regards
Stéphane
Please can you post photos of your piston tops?
Hi Stephane.
I have also responded separately to your email but posted here as worhtwhile discussion to the forum on this type of issue
The problem with valve to piston clearance is indeed unusual with forged pistons and large cutouts and QED 420 cams that you have I believe. Have you checked the actual cam lift is 0.420 inch perhaps QED supplied the wrong cams with higher lift ?
When you say the plasticene was very thin and close to zero clearance was this even across the inlet valve pocket or in one specific location like at the edges of the pocket. I have had to mill larger diameter pockets to clear the sides of the valve on a couple of occasions but that has been with 1.625 inch diameter race valves and pistons designed for the sprint size 1.53 inch valves.
A photo of the piston top and distance of the piston deck below the block deck would be useful in diagnosis
Also a picture of the cylinder head showing the location of the inlet valve edge versus the face of the head.
Also what is the lift on the inlet valve that you are measuring at TDC compared to what’s on the QED specification sheet.
In the end you may need to mill the inlet valve pockets deeper but I would check everything above before thinking of doing that.
Cheers
Rohan
Is head gasket thickness of concern?
Hi all,
Thank you for your reply.
Regards
Stéphane
Are you 100% sure the cam timing is correct?
How did you measure TDC?
What are your valve clearances set to?
Piston deck to block deck is OK and cutouts OK also.
The inlet valves lower edge appears to be very close to the head face when normally they are recessed around 2 mm. If lift at TDC is correct which confirms you have the timing correct then I supect you need to seat the inlet valve a little deeper into the head. How hard this is and what else may need work to achieve that depends on the shim you currently need versus how much it needs to be recessed
The QED quoted lift at TDC is valve lift as i understand it and thus includes the take up of valve clearance.
cheers
Rohan
Regards
Stéphane
Did you measure TDC by reading and noting the points on the degree wheel when the dial indicator starts to rise and when it starts to fall? The reference point for TDC is half way between the two. There is a dwell period when the piston reaches the top of the bore so it’s easy to miss true TDC if you don’t do the procedure correctly.
Twincam pistons have an offset gudgeon pin therefore max piston height is not the average of the two readings. Max piston height is on the way up only.
I use the method of zeroing the dial indicator at the highest point, which lasts several degrees, and marking on my graduated wheel the angles 0.10mm before TDC and 0.10mm after TDC, before taking the middle.
Is this accurate enough?
A good cam manufacturer will give a tappet clearance used when specifying their timing; many manufacturers use a standard 1mm. Check with QED.
If you know of a better method to measure TDC appreciate if you could advise further. Logically TDC is half way between the point when the piston stops rising and the point when it starts to fall as you rotate the engine in a clockwise direction. Piston pin offset should not change that.
Q420 spec. attached including valve clearances
Q420 (2).pdf (271 KB)
Crudely from pic the offset pin produces an asymmetric broadened curve at the max piston height. TDC of the crank occurs only when the piston first reaches max height on up stroke.
Yes but what then is the practical procedure for determining TDC on an engine if you consider this effect important?
Simple, Crank shaft will be at TDC when the piston is lifted to it’s max height on the up stroke, that is when engine turned in operating direction.
At, TDC the instantaneous change in the position of the piston relative to angular rotation of the crank is zero. In practical terms, this means the piston doesn’t move for small movements in the crank making an accurate location of TDC difficult to find. All the timing books I have read suggest that taking two measurements of the crank angle with the piston the same distance down the bore either side of TDC and finding the midpoint is the most accurate way to establish TDC.
No one is considering a dynamic situation here, there is a fixed distance between the crank journal and piston crown, when the crank TDC is reached the piston will have reached it’s max height in the bore. A piston with an offset pin does not follow the normal sinusoidal motion at the change of direction point.
I can understand how the geometry is not symetrical around TDC for an offet pin piston. I have never done the calculations does anyone know how significant it is before I pull out my calculator. 
cheers
Rohan
Please pull your calculator out Rohan
