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+--- Thread: Con rod alignment (/showthread.php?tid=9391)
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Con rod alignment - leaf200 - 29-05-2024
I am building an engine and have found the pistons are not level indicating the big end and little end are not aligned. Normally I would assume bent rods but it’s a new crank and rods.
What would be an acceptable level of misalignment one piston is 10tho misaligned and two are 6tho misaligned
Many thanks
RE: Con rod alignment - Alan - 29-05-2024
Can you describe further I don’t understand what you mean by misaligned.
RE: Con rod alignment - leaf200 - 29-05-2024
Alan
The top of the piston is not level with the top of the block. I have rotated the rod confirming it’s the rod not the block causing the issue.
RE: Con rod alignment - dickie65 - 29-05-2024
Hi Leaf.
Is it the centre two pistons that are low in the block ?
It is common practice for high reving engines to have the centre two piston crowns machine so they do not hit the head at high revs.
Have your rods been white metalled ??
It is possible that the white metalling has been machined slightly off centre. so the centre to centre rod lengths need to be verified as well as the gudgeon pin to piston crown distance.
You can either take it all apart and do some accurate measurements if you are racing it.
Or make sure nothing is clashing and enjoy driving the car.
I am running an engine with 20 thou machined off the centre two pistons Its a 6500 rpm capable engine.
RE: Con rod alignment - Peter Johnson - 30-05-2024
The problem is that the front of the piston is .010” higher than the back of the piston with respect to the block face at TDC. The other pistons are less than .006” out of flat.
Is there a figure which is acceptable for running the engine before remedial work needs to be carried out? The crank is a new 1.5” phoenix splash. The rods are new (Barlow) 1.5” which have been white metalled. The block has been bored and lightly faced. Pistons are Tony Betts slipper pistons.
I’m hoping to have the car out at VSCC Prescott in August.
Many thanks
Peter
RE: Con rod alignment - John Cornforth - 30-05-2024
Hi
From your description it appears that one of your rods has the little end white metal bored in error at a slight angle (about a quarter of a degree if my maths is correct).
The resulting protrusion one side of the piston top face is probably not a problem if you have sufficient clearance to the head at TDC. However if the piston is canted in the bore you are likely to have reduced clearance or even contact with the cylinder side walls, near the top of the piston at one side and near the bottom at the other.
If it were me I would be unhappy to run in this condition. Something to take up with the rod metaller.
During a recent engine rebuild I discovered one rod had been metalled and bored with 10 thou greater distance between big and little end compared to the other three. This work was done by a well known remetaller based in London. Fortunately this error was small enough to be accommodated, but it does demonstrate that mistakes can happen.
RE: Con rod alignment - Terrytuned - 30-05-2024
Photo crank/conrod check.
RE: Con rod alignment - Alan - 30-05-2024
If you leave the rods in their current position fit the worst piston to the 3 other positions. This should tell you if the problem is piston or rod related. My guess would be that the piston crown is machined at an angle. There isn’t enough piston clearance in the bore for the piston to rock 10 thou, surely.
RE: Con rod alignment - Austin in the Shed - 30-05-2024
Or measure from the gudgeon pin to the top of piston with a digital vernier side to side.
RE: Con rod alignment - John Cornforth - 30-05-2024
In my experience, the slipper pistons supplied by Tony Betts are very accurately made. By all means check them though.