Pete Phillips Posted October 16, 2016 Share Posted October 16, 2016 Approx. 5,300 of these 1938 Special Sport coupes were made, model 46-S. I have TWO of them in my shop right now: Both are black, both have the optional dual side-mounts. What are the odds of that...??? The first one lives in north central Texas and is beautifully restored, runs well, but loses its oil pressure after a few minutes when the engine warms up. We have taken the engine apart and diagnosed it as badly scored and worn bearings--being repaired now. The second car lives in central Texas, about 380 miles from here. Its engine problem is more of a mystery, and I would like to see what others' opinions are. Owner says he bought it about five years ago; it ran fine. They had it on a tour this past spring. Second day of the tour, he goes out first thing in the morning, starts it up, but it had no power during acceleration. He says it took a mile or more just to get it up to 35 m.p.h. He took it home, checked the compression, and it was low in all cylinders, varying from 36 p.s.i. to 67 p.s.i., when it should be closer to 110 according to shop manual. He removed the cylinder head, had a valve job done on it by a professional mechanic, reinstalled the head, no improvement in compression (both wet and dry). Next, he took the timing cover off and found that the timing chain was off its correct setting by a few teeth. He reinstalled it according to shop manual specs, and re-tested the compression. No improvement. He says the car has always run fine prior to this, and had no other symptoms. He thinks the engine was rebuilt prior to his ownership, when the whole car was restored. We suspected a worn camshaft, and removed it, but it has very little wear, consistent with a fairly recent engine rebuild. The timing chain is nice and tight, again consistent with a fairly recent engine rebuilt. I guess we will have to remove the engine from the car and take the crankshaft and pistons out next. I'm thinking the only other thing that can cause poor compression is worn piston rings and cylinder walls. Here are some photos of both cars. They are beautiful, twin sisters. Pete Phillips, BCA #7338 Leonard, TX Link to comment Share on other sites More sharing options...
LAS VEGAS DAVE Posted October 16, 2016 Share Posted October 16, 2016 If you squirt oil in the cylinders and the compression does not come up then installing new pistons and rings will not bring it up either. Could thee cam gear have slipped on the camshaft somehow. It sounds to me like its not drawing in enough air to compress. My guess is somehow the cam is not timed correctly to the crank. A degree wheel would verify the correct cam timing. If the car was fine during the first day of the tour it didn't wear out the pistons and rings at night while it was sitting. Link to comment Share on other sites More sharing options...
nzcarnerd Posted October 16, 2016 Share Posted October 16, 2016 (edited) On the stock 6.15:1 compression ratio cylinder pressure would be about 90 psi (14.7 x 6.15). To achieve 110 psi compression ratio would have to be nearer 7.5. Edited October 17, 2016 by nzcarnerd (see edit history) Link to comment Share on other sites More sharing options...
Daves1940Buick56S Posted October 16, 2016 Share Posted October 16, 2016 Don't forget the distributor. First, if the cam gear slipped a couple of links the ignition timing will be off as well. Also, look at the centrifugal advance, maybe a spring slipped or broke. I would get a timing light on it tout de suite. Obvs won't cause the compression issue but might explain the sudden loss of performance. Link to comment Share on other sites More sharing options...
Mark Shaw Posted October 17, 2016 Share Posted October 17, 2016 (edited) I agree with Dave "If the car was fine during the first day of the tour it didn't wear out the pistons and rings at night while it was sitting." Do the easy stuff first... Check it out with a timing light from the low to high RPM range to make sure the spark advance is working. Edited October 17, 2016 by Mark Shaw (see edit history) Link to comment Share on other sites More sharing options...
HuntzNSam Posted October 17, 2016 Share Posted October 17, 2016 An engine with a restricted exhaust will also lose compression. If the compressed gasses cannot exit the cylinder, fresh intake gasses cannot get in. I remember one customer, a skilled restorer, chasing a similar problem. After much time, money, and frustration he found the exhaust just packed solid with mouse nest. Link to comment Share on other sites More sharing options...
LAS VEGAS DAVE Posted October 17, 2016 Share Posted October 17, 2016 48 minutes ago, HuntzNSam said: An engine with a restricted exhaust will also lose compression. If the compressed gasses cannot exit the cylinder, fresh intake gasses cannot get in. I remember one customer, a skilled restorer, chasing a similar problem. After much time, money, and frustration he found the exhaust just packed solid with mouse nest. A restricted exhaust would affect the compression reading. Could mice cause that in one night. Another troubling thing is that there is a difference of 30 pounds amongst the cylinders. I would do a Leak Down Test to get a better understanding of what is happening if the cam is timed properly. The thing that keeps coming to my mind is that I am taking for granted that the car was running good the day before, maybe that is incorrect. Link to comment Share on other sites More sharing options...
DonMicheletti Posted October 17, 2016 Share Posted October 17, 2016 Fascinating problem. I'll be interested in the actual solution. My similar experience with sudden low compression was that my compression gage went haywire! 2 Link to comment Share on other sites More sharing options...
Thriller Posted October 17, 2016 Share Posted October 17, 2016 I'm far from an expert, so my question is just that. Could the timing chain being off by "a few teeth" be enough for some piston / valve interference to have occurred, damaging valves in most or all cylinders? Link to comment Share on other sites More sharing options...
1939_Buick Posted October 17, 2016 Share Posted October 17, 2016 4 hours ago, Pete Phillips said: Approx. 5,300 of these 1938 Special Sport coupes were made, model 46-S. I have TWO of them in my shop right now: Both are black, both have the optional dual side-mounts. What are the odds of that...??? <snip> Here are some photos of both cars. They are beautiful, twin sisters. They look good How far apart are the chassis/frame & engine serial numbers? Link to comment Share on other sites More sharing options...
jenz38 Posted October 17, 2016 Share Posted October 17, 2016 (edited) I agree with HuntzNSam... Maybe the old exhaust rust /whool is loosen inside and block the small bow above the rear axle ? I would loosen the exhaust flange to bypass the at the Manifold and give'em a trail . Edited October 17, 2016 by jenz38 (see edit history) 1 Link to comment Share on other sites More sharing options...
Pete Phillips Posted October 17, 2016 Author Share Posted October 17, 2016 I unhooked the exhaust at the manifold, blew compressed air back through the exhaust, and I get a good air flow out the tailpipe, so I dont' think it is an exhaust restriction. Just did a (dry) compression test with the camshaft and timing chain removed from the engine, so all valves are closed. This should give a pure reading of cylinder compression. Here are the results: 80-50-60-70-75-35-40-60. Doesn't this sound like bad or broken rings? Pete Phillips 1 Link to comment Share on other sites More sharing options...
LAS VEGAS DAVE Posted October 17, 2016 Share Posted October 17, 2016 Pete, you can not get a meaningful reading with the valves closed because there is no way for each piston to suck in air. If the cylinder can not suck in air there is no air to compress. The intake valve allows for air to be drawn into the cylinder and yours is not poking. Also the choke on the carb must be open and the throttle should also be wide open. You CAN do an accurate leak down test with all the valves being closed and that will tell you the TRUE condition of the rings and valves. A leak down teas requires a compressor and a special leak down tester. Link to comment Share on other sites More sharing options...
Daves1940Buick56S Posted October 17, 2016 Share Posted October 17, 2016 Does kinda look like some broken rings, possibly, esp. on 6 and 7. But I agree with Dave, you'll get better results with the valves operational (assuming the valve timing is correct). However, since they're all closed now, I agree on a leak down test. Harbor Freight carries a pretty good test kit for this. And you really can hear the hissing as the air escapes past the broken rings! Ask me how I know... Cheers, Dave Link to comment Share on other sites More sharing options...
Pete Phillips Posted October 17, 2016 Author Share Posted October 17, 2016 Pulled three pistons so far; found broken rings on each one. One ring was broken in 12 different places! Pete Link to comment Share on other sites More sharing options...
Larry Schramm Posted October 17, 2016 Share Posted October 17, 2016 11 minutes ago, Pete Phillips said: Pulled three pistons so far; found broken rings on each one. One ring was broken in 12 different places! Pete Top ring in 12 pieces? Link to comment Share on other sites More sharing options...
LAS VEGAS DAVE Posted October 17, 2016 Share Posted October 17, 2016 (edited) Now you know you have major problems. Best scenario would be if cylinders are not scored and you could just re ring it. More likely is it needs to be bored next oversize and rebuilt. Edited October 17, 2016 by LAS VEGAS DAVE (see edit history) Link to comment Share on other sites More sharing options...
Pete Phillips Posted October 17, 2016 Author Share Posted October 17, 2016 (edited) The second ring from the top was broken in at least 12 places. Have now pulled all of the pistons and found broken rings on all of them. Also found main bearing shims that were installed backwards and got partly chewed up. Amazingly, the cylinder walls are not scored. Edited October 17, 2016 by Pete Phillips new information (see edit history) Link to comment Share on other sites More sharing options...
Ben Bruce aka First Born Posted October 18, 2016 Share Posted October 18, 2016 And it ran good the day before?? Strange! Ben Link to comment Share on other sites More sharing options...
LAS VEGAS DAVE Posted October 18, 2016 Share Posted October 18, 2016 1 hour ago, First Born said: And it ran good the day before?? Strange! Ben IT MUST NOT HAVE RUN GOOD THE DAY BEFORE Link to comment Share on other sites More sharing options...
DonMicheletti Posted October 18, 2016 Share Posted October 18, 2016 With that many broken rings on a "new" engine, it sounds as if the end gap on the rings was to tight - maybe not checked at all. With the screwed up shims, it sounds like a really sloppy engine assembly. I had one "restored" engine that had no shims at all (rods and crank). Needless to say, the engine would not turn. At that point the owner / mechanic gave up his restoration. Link to comment Share on other sites More sharing options...
LAS VEGAS DAVE Posted October 18, 2016 Share Posted October 18, 2016 If that was a rebuilt engine it was a waste of time and money. There are still reputable shops that will rebuild these old engines correctly, it would be wise to have one of them do it. Link to comment Share on other sites More sharing options...
Pete Phillips Posted October 18, 2016 Author Share Posted October 18, 2016 "There are still reputable shops that will rebuild these old engines correctly, it would be wise to have one of them do it." Dave, I prefer to think that my shop can rebuild this straight eight correctly, having done about 7 or 8 of them in recent years, none of which have come back for problems. That's why this car is here, sir. Some other shop didn't do it correctly. It's going to be rebuilt correctly now.--both of these '38 engines are. Pete Phillips 1 Link to comment Share on other sites More sharing options...
LAS VEGAS DAVE Posted October 18, 2016 Share Posted October 18, 2016 I am not doubting you can do it Pete. Whoever did it the last time was a con artist. Link to comment Share on other sites More sharing options...
critterpainter Posted October 20, 2016 Share Posted October 20, 2016 Pete: I sent the owner of that car to you. I knew you were up to a good project. Bill Link to comment Share on other sites More sharing options...
Pete Phillips Posted December 28, 2016 Author Share Posted December 28, 2016 I finally got both engine blocks back from the machine shop, and looked at the engine numbers. One is 43518498. The other is 43491662, so they are 26,836 units apart. Pete Phillips, BCA #7338 Link to comment Share on other sites More sharing options...
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