05-09-2022, 05:29 PM
Hi Denis
Regular engine noises can be more or less divided into two types, those that only depend on RPM regardless of load, and those that also depend on load i.e. throttle opening / combustion pressure in the cylinders. The first category can be heard if you gradually rev up with the car stationary.
From your descriptions, the noise would seem to be in the second category. This points towards big or little ends. The ignition timing will affect combustion pressure, with more advance leading to higher peak pressure and more loading on the big and little ends. An HC head will also increase pressure. Pressure is highest at around the torque maximum at roughly 2500 to 3000 RPM, but will drop away at higher RPM for a standard inlet/carb due to restricted breathing. The general racket made by an A7 engine at high RPM may also mask noises to some extent ! Colder/thicker oil before the engine is warmed up can also temporarily quieten this sort of noise.
If you have any slack in the distributor or the various gear pairs between crankshaft and distributor, this can cause the timing to have scatter, with some firings having slightly more advance than others. I suspect this gives an uneven slightly "clattery" characteristic. A timing strobe light shone on a mark added to the camshaft pulley can reveal this as a blurred rather than a clear mark. The only cure other than eliminating wear is to back off the advance a little. Bear in mind that to retard the timing by 4 degrees you need to rotate the distributor clockwise by 2 degrees, which isn't very much. The DK4A body has a circumference of about 225 mm, so 2 degrees shift is 1.2 mm. If you still have the little plate and pointer, this is one division.
The only other thing I can think of is that the HC head has a flat area that overlaps with the piston crown. The clearance is very small, I have no exact data but I believe the pistons are very slightly below the top of the block at TDC, so it's not much more than the thickness of a compressed HC gasket, perhaps 30 thou. This assumes the top face of the block hasn't been lowered by refacing. Clearly, if you have any extra clearance in the big/little ends or any crank whip at high RPM, it might just be possible to run out of clearance especially on cylinders 2 and 3. I don't know if there is a safe and reliable way of checking clearance without dismantling, e.g by putting soft solder wire into the gap and seeing how far it is squashed down - maybe others could comment ? If the clearance is too small, a metal spacer shim between crankcase and block could be useful but would of course require dismantling.
Regular engine noises can be more or less divided into two types, those that only depend on RPM regardless of load, and those that also depend on load i.e. throttle opening / combustion pressure in the cylinders. The first category can be heard if you gradually rev up with the car stationary.
From your descriptions, the noise would seem to be in the second category. This points towards big or little ends. The ignition timing will affect combustion pressure, with more advance leading to higher peak pressure and more loading on the big and little ends. An HC head will also increase pressure. Pressure is highest at around the torque maximum at roughly 2500 to 3000 RPM, but will drop away at higher RPM for a standard inlet/carb due to restricted breathing. The general racket made by an A7 engine at high RPM may also mask noises to some extent ! Colder/thicker oil before the engine is warmed up can also temporarily quieten this sort of noise.
If you have any slack in the distributor or the various gear pairs between crankshaft and distributor, this can cause the timing to have scatter, with some firings having slightly more advance than others. I suspect this gives an uneven slightly "clattery" characteristic. A timing strobe light shone on a mark added to the camshaft pulley can reveal this as a blurred rather than a clear mark. The only cure other than eliminating wear is to back off the advance a little. Bear in mind that to retard the timing by 4 degrees you need to rotate the distributor clockwise by 2 degrees, which isn't very much. The DK4A body has a circumference of about 225 mm, so 2 degrees shift is 1.2 mm. If you still have the little plate and pointer, this is one division.
The only other thing I can think of is that the HC head has a flat area that overlaps with the piston crown. The clearance is very small, I have no exact data but I believe the pistons are very slightly below the top of the block at TDC, so it's not much more than the thickness of a compressed HC gasket, perhaps 30 thou. This assumes the top face of the block hasn't been lowered by refacing. Clearly, if you have any extra clearance in the big/little ends or any crank whip at high RPM, it might just be possible to run out of clearance especially on cylinders 2 and 3. I don't know if there is a safe and reliable way of checking clearance without dismantling, e.g by putting soft solder wire into the gap and seeing how far it is squashed down - maybe others could comment ? If the clearance is too small, a metal spacer shim between crankcase and block could be useful but would of course require dismantling.