Oil leak rebuilt 348

#1
Hi guys,
Had a 1958 engine rebuilt by a machine shop in CA. that has a great reputation. The 348 runs like a top but is leaking oil on the garage floor.
My mechanic has not had it up on the lift yet and I have tightened the oil pan and valve cover gaskets. I’m unable to find the source of the leak and am soliciting suggestions please. This is my first 348 and I am hearing that the rear main is probably the source. Has anyone else had this issue and if so, what was your fix.
many thanks!
Jim
 

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Tim

Well Known Member
Supporting Member 4
#2
When I put my seal in there are 2 black tabs that go under the rear main cap(oil pump). If they aren’t in right or no silicone sealant added to each part then it will leak. Just one possibility
 

Tim

Well Known Member
Supporting Member 4
#5
We use dye and a black light at work to find leaks. Dump a vile of dye in the oil run it till it’s hot and put the black light to it. Find it every time.
Had to do it with a coolant leak on a 2011 avalanche I had. Could always smell coolant on the pass side after it ran for a bit. Never on the ground or wet anywhere. Put the dye in ran it for a day and found a pinhole in the engine block. GM replaced the whole motor N/C.
 
#6
Thanks Gents!
the oil on the floor is about the size (Footprint) of a loaf of bread or if you will significant. I’m in line to get the vehicle up on my mechanics lift and will advise. He is telling me that the 348 is problematic with oil leaks reference the rear seal. The rebuild to include securing another head (the old was cracked) was $5600 and that in my mind should have delivered a worry free engine. Thanks for your suggestions and please keep them coming if you think of anything else.
Cheers!
Jim
 

boxerdog

Well Known Member
Supporting Member 5
#7
I have seen several rope-seal rebuilds require 3-4 attempts before one finally "took". Not just 348s either. I think some engines (Ford?) had a finely knurled sealing surface which helps. My machinist mumbles something about asbestos vs graphite I think, and looks for NOS seal material. I got lucky on mine, just used a current Best brand, no leaks.

A run stand always helps.
 
#9
Road draft tube. I will have my guy take a look when it’s my turn on his lift. His speed shop is high demand so it will probably be a couple of weeks since he is a one man show.
Thanks!
Jim
 

Rickys61

Well Known Member
Supporting Member 3
#10
Road draft tube. I will have my guy take a look when it’s my turn on his lift. His speed shop is high demand so it will probably be a couple of weeks since he is a one man show.
Thanks!
Jim
Also take a look at the bellhousing bolts... the one behind the oil filter housing goes all the way through. It can leak out there, and if there’s too long of a bolt in it, the bolt can damage the filter housing.. causing a leak there...
 

chuckl

Well Known Member
#11
I have a .030 over 61 348 truck block, 333 heads, big valves, 0950 cam, Tri-power and M-21. It was balanced and has no vibrations. This motor seems to have all the right "secret" combinations. It gets about 15 MPG, runs like hell and has more torque than I imagined. The problem: It has leaked oil from day one. The oil is not coming from the valve covers, intake seal, draft tube, filter area. The back of the block is dry. It has a 2-piece rear seal. I have photos of the correct installation of the rear seal and the rubber tabs under the rear cap. The oil is dripping out of the flywheel lower cover and must be coming ( I think) through the rear seal.

The amount of oil is about a teaspoon 1 hour after shut down. Don't know if it is leaking during driving. It is annoying, but manageable with floor pans. The only thing I can think of is that I did not apply any sealant to the rear cap tabs. Is there any 348/409 out there that does not leak? :read
 

409newby

Well Known Member
Supporting Member 10
#13
Years ago I had a buddy offer to give me a 302 Ford just rebuilt but leaked like a sieve he had sealer oozing out of every part that bolted together trying to stop the leak I should have just took it but showed him his machinist left out the soft plug behind the cam! He had it in an old Jeep wagonier so it was no fun to pull or install he had it in n out 3 times trying to fix the leak :crazy
 

Toms63SSQB

Well Known Member
Supporting Member 7
#15
Most, but not all 348/409s leak a little or a lot at the rear main, all 4 of mine did, certainly there are some that don't, I'm just not aware. Mine now leaks just a drop or two after cruising, "almost" nothing.
 

chuckl

Well Known Member
#16
Rear main seals on old cranks seem to have potential issues.
The crank was polished and mic'ed standard. I put standard main bearings in and everything mic'ed standard or better. Is there anything I am missing? All oil galleries are threaded and plugged. The cam plug is not leaking. Is it just the nature of the beast and will I be no better off with a new seal? Does anyone make a different style of rear seal? Since there is no direct oil pressure behind the main bearing or on the seal, the drain-back gallery should handle any oil coming through the bearing. There is no noticeable blowby--the oil filler tube and draft tube show no vapor or oil, only a rare drip from the draft tube. And lastly, the oil pan/pan gasket is not leaking. :mad4 :mad4
 

427John

Well Known Member
#19
Rope seals are tough to get leak free and aren't nearly as flexible and forgiving as rubber,the least little out of roundness at the seal area of the crank can contribute to leakage.Boxers machinist is right on the money the new replacements for the old asbestos rope is even less flexible and less forgiving and more prone to leakage.
 

61BUBBLE348

Well Known Member
Supporting Member 9
#20
I have built 2 engines with rope seals, no W's though, one was a Flathead Ford the other was a GM Holden 6 (1958), when I did the Holden a wise old Mechanic told me "son soak those rope seals in oil for a few days before you put them in and roll them into place with a piece of broom handle", I was about 17 at the time, neither of these 2 engines leaked from the rear main seal. Whether this is right or wrong, but it worked for me at the time.
 
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