Are you suggesting putting shim material between the bearing insert and the block to correct the error made by the machinist? That sounds like a horrible idea. Stand by for more opinions on this matter.
1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission
Not necessarily the crank grinder's fault... Deve's page gives me the impression that the block manufacturing(ie crank journal holes) wasn't that great to begin with.
My crank is ground .01" undersize & has 'matching' bearings
The manual i'm looking at says new bearing clearance should be .001-.003, used .001-.004
So I'm pretty close to spec... but the foil (a little less than 1 thou thick) makes me feel better by being on the tighter end of spec.
A better fix would be 0.002 undersize bearing inserts if you can find them. They used to be fairly common. [on edit] If you're already .010 undersize, you're out of luck. They never made 0.012 undersize. Have you miked the crank? Also, blocks can get warped a bit, possibly causing what you're seeing.
The tinfoil trick is a bit redneck.
Last edited by klhansen; 04/06/202012:33 AM.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
maybe i need an actual mic... But i did use digital calipers (nice mitutoyos) and got .010" under the minimum end of spec on all 4 journals. The spec has a .001" acceptable range.
There's an item known as a "taper shim" or sometimes called a "feather shim" that was used for correcting excessive bearing clearance errors. It's a piece of shim stock that's twice as thick in the middle as it is on both ends. For instance, if you want to correct for .002" too much clearance, the shim is .002" thick at the center, tapering to .001" thick on both ends. That prevents the bearing shell from being squeezed too tight at the sides, where the correction would be .004" with a shim the same thickness from end to end. It's a "shade tree" fix at best. It's also necessary to file the end of the bearing shell a little to prevent getting too much "crush" on the bearing shells as they're squeezed together when the bolts are torqued.
It's not uncommon to get a slightly wider clearance at the rear main bearing, especially if a rope oil seal is being used. I always check for clearance BEFORE installing a rope seal, and then I'm VERY careful not to sandwich a piece of seal fiber between the main cap and the block.
I print out some folks' technical advice, shred it and apply it to my garden! Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
I will go out on a limb and admit I am the one who gave Deve the tin foil trick to bring his bearing clearance to acceptable clearance.
Since I had little control over my machines work through the years, there were very few of the hundreds of engines I rebuilt or assembled that didn’t have at least one piece of tin foil in it.
It may sound like a redneck repair, but it works great, and if you are a sticker about bearing clearance, like me, you just have very few options.
There's a much better way to make small clearance corrections, but I absolutely refuse to post it on the open forum. Anyone who's interested can drop me a PM. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
A better fix would be 0.002 undersize bearing inserts if you can find them. They used to be fairly common. [on edit] If you're already .010 undersize, you're out of luck. They never made 0.012 undersize. Have you miked the crank? Also, blocks can get warped a bit, possibly causing what you're seeing.
The tinfoil trick is a bit redneck.
No red neck trick is using the page for the bearing catalog to shim the bearing. Seen worse........that worked for years.
No red neck trick is using the page for the bearing catalog to shim the bearing. Seen worse........that worked for years.
How about a piece of a coffee can? Does that qualify as redneck? I may or may not have done that. Fortunately there's no evidence left.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
Aha! Dave is the one with 235guy blog. I've run across that a few times... very handy info, thanks!
Jerry, you got me thinking some more & I measured my #4 bearing inside diameter with telescoping gauge & calipers... seems round as far as i can tell within ±.001 confidence I'll claim with the calipers. If it wasn't I'd definitely look for those taper shims.
Do you guys use sealant between the rear cap & block? The 'best seal' instructions for my rope say to use it, but that seems debatable.
I would say that if you have .004 clearance on the rear bearings, you would have no hope of getting a rope seal to work well. I have found on my 216s that the only way to get a good seal is to have the main bearing clearances down to a minimum.
Problem is you have “machine” the coffee can down to the proper thickness.
Sanding the bearing cap down on a belt sander might also be considered RN.........also seen it done in a pinch with success. Farmer that needs the tractor up and running in the morning might do what ever it takes.
OP did you rotate the crank and plastic gauge again ???? It might change the reading.........
60 years ago, I recall my Dad using a Prince Albert can (It's OK to use Google) when doing a shade tree lower end on our '49 Chevrolet 3100. It ran fine until he sold it and bough 55 ' ord a few years later. Of course Prince Albert cans are hard to come by these days.
Hmmmmm- - - -too bad General Motors didn't have a bunch of engineers back then who knew how to make engines out of aluminum foil instead of precision-machined cast iron! Think of all the money they could have saved!
Simply distorting the bearing at the bottom to make the Plastigauge look good is the equivalent of a doctor giving his patient a sugar pill and convincing him that the thing is going to cure his sickness. It's called a "placebo". It's a common practice when treating people with imaginary symptoms. I would be interested in seeing bearings that get the tinfoil treatment clamped up without a crankshaft in place, and accurate (like down to the .0001" range) taken all the way around. Yes, I've done that when building engines that were producing three times the factory rated horsepower, and the results are surprising at times. Also try measuring cylinder bores with a torque plate installed sometime! They are anything but round. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
Ott3R, are you 100% certain that your block is a 56? The reason that i ask is that the rear main cap was machined differently from 55 to 56 because 56 was the first year for a neoprene rear rear main seal. The 55 main caps are machined deeper to accommodate the rope style seal. The 56 and later blocks were specifically machined for the seal. The difference is why you cannot retro fit a 55 and earlier block with a 56 and later neoprene seal. So if you have a 56 block, you might consider using the neoprene seal. There was a reason that GM moved to that style seal as opposed to the older rope style seal.
hehe, an excuse for more pictures! Fairly certain it's a 56 block... has a 6 on the end of date code (attached) although it came out of a 59 burb. There was a rubber seal when i tore it apart. Hmm, I didn't know about the different machining back there starting in '56... I'm still hoping the rope will work well: when I installed it last night, the cap squashed the rope around the crank which still rotates, but has a lot of drag now (I have to grab the counterweights to turn it).
Jerry, you make some good points. I will have to think on it, and decide where to strike my better vs good enough vs laziness vs optimal decision. Placebos are known to work every time 30% of the time right?
I have a 55 2nd production engine that I completely rebuilt in 1973 -- crank ground, cylinders bored, cam bearings etc. It has pinned upper main bearing shells like the earlier blocks. I installed a rope seal and rolled it flat with a hammer handle. As soon as I installed the seal the crank no longer turned freely. It was scary tight but I proceeded with the main cap shims per the book and set the mains clearance per the book. The engine ran leak free for many years but eventually developed a leak that I couldn't find. I pulled the flywheel to check the cam plug when I had it apart for a clutch. That wasn't it. It would leak intermittently. Eventually, I changed the rear main seal for a neoprene one. It has leaked all the time since and eventually got worse and soaked the clutch. I put a full set of bearings in it (now with 150,000 or so miles) and used tapered shims on all the bearings to adjust for crank wear of about 0.002 (still round). With the old bearings, the rear main plastigauged at 0.0045, which is why I checked all the bearings. I felt that this was enough clearance to flood the rear seal and might be the source of my leak so I bought a set of bearings. The rod bearings were out to about 0.004 and the number-two, upper-shell came out in two pieces. The rods never knocked, go figure.
Anyhow, I shimmed the mains per the factory procedure but I set the rear main tight at 0.0015, feeling that less oil would flood the seal. That appears to have worked. It still leaks but not very much and it has pulled many times without complaint at full throttle for 15 minutes at a time over one of our many mountain passes. It's beginning to burn a little oil so its due for an overhaul soon. But I can't complain after nearly 200,000 miles.
1951 3800 1-ton "Earning its keep from the get-go" In the DITY Gallery 1962 261 (w/cam, Fenton headers, 2 carbs, MSD ign.), SM420 & Brown-Lipe 6231A 3spd aux. trans, stock axles & brakes. Owned since 1971.
I don’t know if you know this or not, but the new rope seals are not like the old rope seals. The old rope seals were a asbestos based material, while the new rope seals are a Fiberglas graphite material and do not work anywhere near as well as the old rope seals. Failures are common. They just do not hold up. The neoprene seals were an improvement for the higher pressure later 56 and up 235 motors. So why are you going backwards here? Just asking of course. Your block can use the neoprene seal yet you want to use a less sturdy and less reliable new manufactured rope seal.
What does "pressure" have to do with how well a crankshaft seal works? If there's any pressure at all in the crankcase where the seal rides, there's something dramatically wrong with the engine. Rear seals, either rope or lip type, leak because the crankshaft is bouncing up and down on excessive clearance, and the seal can't keep up with the movement of the shaft. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
I don’t know if you know this or not, but the new rope seals are not like the old rope seals.
A former engine development engineer told me exactly the same thing. He said to toss the rope seals that came in the new gasket kit, and find a pair of old rope seals that contain... (gasp).. asbestos. I took his advice, and have not had any problems.
The neoprene seal you all are talking about, is it one piece or two piece?
Phil Moderator, The Engine Shop, Interiors and Project Journals
1952 Chevrolet 3100, Three on the Tree, 4:11 torque tube Updated to: ‘59 235 w/hydraulic lifters, 12v w/alternator, HEI, PCV and Power front Disc Brakes Project Journals Stovebolt Gallery Forum
It's a 2 piece seal that fits into the same groove as the rope seal did. If you rotate the seal slightly so the ends of the two pieces don't align with the parting line between the block and the main bearing cap, they seal very well. 1/4" is plenty of offset. A much better fix on the Chevy engines is to machine the back of the block and the main cap for a Chicago Rawhide #38647 full circle lip seal or the equivalent. It solves the rear main bearing seal leak problem for good as long as the main bearing doesn't develop excessive clearance. Since GMC inline sixes have a flywheel flange bigger than the seal surface, the full circle seal can't be used on those engines. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
My seal selection wasn't thought out very much... the rope just seems like a cool novelty to me! Graphtite rope, i think it's teflon rope impregnated with graphite. Drag is significant on mine ~10ft-lb at the moment.
A new rope seal usually makes a crankshaft very difficult to turn. I've seen an improperly installed seal actually lock up an engine where it couldn't be turned with a wrench on the crankshaft snout. 30 or 40 ft-lbs of torque to get the crank to start turning isn't uncommon with a new rope seal. It "wears-in" pretty quickly, however. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
No, you cannot retro fit a neoprene two piece seal on a 55 and earlier 235. The rear main cap is machined for a rope seal, so it’s machined a bit deeper. The neoprene seal will seat itself deeper into the cap and just leak like a sieve if you do. The 56 blocks were the first to use the neoprene seal and are machined specifically for that seal. Best gasket did not have the neoprene seal for a while but I understand they have them available.
The rope seals were notorious for leaking. It’s why our beloved motors mark their territory. But gm changed over to the two piece neoprene because they worked better at keeping oil from blowing past the rope seal. You have a 56 block so you should use the better seal. Just my opinion.
