What's going on with my valve clearances? (sanity check)
Posted: Thu Apr 23, 2020 3:43 am
In getting the engine back together after having the head, pump, sump and other associated bits off, I'm having a weird problem with valve clearances. If it makes any difference, this is a Stromberg head that has been modified to Dave Bean Stage II specs.
Before taking it all apart I checked the clearances and found all 4 intakes to be at .004" and only one exhaust was correct (exact figures are on a scrap of paper down in the garage). I measured the shims, did the math, ordered some new shims and carried on with the other work.
After re-assembling everything I found the clearances way off. Fair enough, since everything had been disturbed and I wasn't the last person to have it apart so I can't account for anything in the 'as found' condition. I took the measurements, took the valvetrain apart again, re-measured the shims (they were all what I thought they were), ordered a few more.
When that order of shims arrived I verified their sizes, installed them, re-assembled the valvetrain again, checked the clearances and found many off again, but to the extreme in the opposite direction. Some that had been very tight were now very loose. Pull it apart again and do some math, swap some shims around between various positions and my growing collection, put it back together and one more is in tolerance, but the very loose ones are now very tight again. The excursion of these changes have been much greater than the change in the sizes of the shims. Bigger shim to reduce clearance, smaller shim to enlarge it, right? And it's only five of the eight that are being troublesome like this, the ones on the exhaust side being the worst.
Additional details:
-When I reassembled, I used the same type head gasket as before but converted from head bolts to studs since DBE was out of the bolts.
-Head torque has not been disturbed since the head was re-installed.
-Every shim has been verified (some more than once) with a Mitutoyo digital micrometer than has been calibrated by an A2LA-accredited vendor within the last 60 days (borrowed that from the tools I use in my job ).
-Everything was well-oiled during reassembly.
-I verified during each reassembly that the camshafts had not been swapped and that their bearings were fully seated.
-Upon each reassembly I verified that the timing was all still lining-up after pulling the engine through a few cycles.
-The old cam cap nuts were discarded and the new ones have been torqued per WSM specs during every reassembly.
-This is not the first shim-and-bucket valvetrain in which I've adjusted clearances but this is the first time I've done it with a Lotus Twin Cam.
These seemingly illogical failures to get the clearances where they should be is becoming aggravating. Am I nuts or is there something I'm missing here?
Before taking it all apart I checked the clearances and found all 4 intakes to be at .004" and only one exhaust was correct (exact figures are on a scrap of paper down in the garage). I measured the shims, did the math, ordered some new shims and carried on with the other work.
After re-assembling everything I found the clearances way off. Fair enough, since everything had been disturbed and I wasn't the last person to have it apart so I can't account for anything in the 'as found' condition. I took the measurements, took the valvetrain apart again, re-measured the shims (they were all what I thought they were), ordered a few more.
When that order of shims arrived I verified their sizes, installed them, re-assembled the valvetrain again, checked the clearances and found many off again, but to the extreme in the opposite direction. Some that had been very tight were now very loose. Pull it apart again and do some math, swap some shims around between various positions and my growing collection, put it back together and one more is in tolerance, but the very loose ones are now very tight again. The excursion of these changes have been much greater than the change in the sizes of the shims. Bigger shim to reduce clearance, smaller shim to enlarge it, right? And it's only five of the eight that are being troublesome like this, the ones on the exhaust side being the worst.
Additional details:
-When I reassembled, I used the same type head gasket as before but converted from head bolts to studs since DBE was out of the bolts.
-Head torque has not been disturbed since the head was re-installed.
-Every shim has been verified (some more than once) with a Mitutoyo digital micrometer than has been calibrated by an A2LA-accredited vendor within the last 60 days (borrowed that from the tools I use in my job ).
-Everything was well-oiled during reassembly.
-I verified during each reassembly that the camshafts had not been swapped and that their bearings were fully seated.
-Upon each reassembly I verified that the timing was all still lining-up after pulling the engine through a few cycles.
-The old cam cap nuts were discarded and the new ones have been torqued per WSM specs during every reassembly.
-This is not the first shim-and-bucket valvetrain in which I've adjusted clearances but this is the first time I've done it with a Lotus Twin Cam.
These seemingly illogical failures to get the clearances where they should be is becoming aggravating. Am I nuts or is there something I'm missing here?