HI new here hoping to get some input. I purchased a 1961 buick 364 nailhead I plan on installing it into an old nash coupe. I did a quick compression check on only a few cylinders as I did not have much time. Two of the cylinders were at 90 one at 130 from what I remember. I have not got time to go back out and check on it yet but wanted to get some ideas on how to proceed. Could the tests have been wrong? it was a rebuilt motor with no more then 15k according to the previous owner no papers though. It looks nice not sure where to start another compression check leak down test?? I really have no idea.
It sounds like it may have had a Dupont overhaul. Even 130 psi is on the low side for this engine. 90 psi is bordering on a healthy 5.5:1 or 6:1 compression ratio era engine (like my Ford Model A). Was this compression test done cold? I would assume that number would come up a little if the engine were warm. Be sure you have the choke and carb wide open when cranking. A healthy engine should have about no more than a ~10% variation between the highest and lowest compression pressure reading. I'd perform a full compression test on all cylinders and then a leak down test to determine the cause. If you cannot do the leakdown test you can get an idea of the condition of the rings by squirting oil in the cylinders and rechecking the compression. If it is substantially higher then your rings are not sealing. If the engine has been sitting long it could be that the rings are stuck. If the engine were rebuilt maybe it is suffering from improper break-in and some of the cylinder walls are glazed over. Why did the previous owner sell this engine? Properly rebuilding a nailhead isn't cheap. For comparison, my bone stock (internally) 1964 401 with ~105,000 miles has compression ranging from 145 to 165 psi. This test was done with a warm engine.
Thank you for your reply. The compression test was done cold as you suspected. Could testing it cold throw the readings off that much? According to the previous owner the car was slow and he was doing an LS1 engine swap. He purchased the car 3 years prior and the work was done by the previous owner. The car looked super clean and the motor looks super clean and has tons of new parts. The car was a 61 lesabre with dynaflow trans so I would expect it to be slow. Looks like I will need to set it in the car or find a way to run it before doing a full compression test then do the leakdown test.
I don't know which compression ratio you have, but my 1959 manual says that a Dynaflow engine is 10.5 to 1. At a normal cranking speed (160 rpm), the compression should be 185, with a variation between cylinders of less then 15 lbs. It has been my experience that engines that have not been run in a long time may have much lower compression. the real test would be to run it for a bit, flog it a little, then check the compression again.
