The engine is a small block Chevy, for my dirt track Camaro. Last season I ran it out of oil (pan damage) and the machine shop had to line hone the block as part of the rebuild procedure.
Now, the timing chain (stock GM) seems a bit loose - not horribly bad, I'd probably use it on a street car, but not good enough for a race application. Picking up a new double roller set tomorrow, but when you line hone a block, does that move the crank closer to the cam or does it keep it the correct distance away?
I'm guessing that distance doesn't change or you'd run into all sorts of other problems, and the chain just stretched from abuse, but I thought I'd ask...
Also, how much of a nick is allowed on an intake valve stem before it's junk? Probably none, eh? Good thing I have VISA. ;)
Moves them closer by whatever amount they had to remove to get the bore back to spec. Usually they shave the caps and try to take as much as possible from the cap to keep the change as small as possible. The difference isn't usually enough to matter as ANY timing chain will stretch farther than the line hone difference anyway. Even a new chain in a new engine has some play. If your worried get a gear drive set-up unless the are outlawed.
For the price of a valve, toss it. Nasty things happen if a valve breaks when your on the final lap and sitting out front....
Thanks. It's not that bad of slop, but it kind of reminds me of what you'd see on a 100,000 mile engine for play, so a $30 replacement timing chain is going in.
I know... it's actually less of a $ thing than a time thing. This year everything car related is taking 3x as long, and with a 2 year old and a
5 month old, I'm getting 1/3 the car time, so things are going really really slow. There's only 2 races left for me and the motor is still torn apart, I need to change a balljoint, and replace my belts. I got a trailer this year, only took 10 hours to wire up the truck... it should have taken about 2 hours because the truck was mostly wired up - one missing fuse and one bad connection took most of the time to diagnose.
Hey, silly question - do I have to lap a new valve in or can I just swap it? The heads are like new, they only have about 300 race miles on them, so the seats are good, I just noticed one nick in one valve stem when we pulled one spring off to check, so now I'm going to check ALL the valves in both heads...
It's been a long time since I did any valve jobs in school...
Well the current school of thought is that if the seats are cut properly you don't need to lap. BUT I still would rather lap them to make sure I have a good seal. It's even relaxing to just hear the click of the lapping tool...
Line boring moves the crank closer to the cam, there's no other way to do it. Material has to be removed from both the block and the bearing caps (the "legs" where the caps bolt to the block are shortened and then the caps are bolted in place leaving the crank hole "oval" its then bored back round, removing material all around. The machinist should set up the machine to remove the MINIMUM amount of material from the block side, but some will be removed.
IF the timing chain is noticeably sloppy, then the block may be getting to the point it can't be used much longer (too much upward movement of the crank required to clean up the block). But another option would be a gear drive for the cam, which usually has adjustable tensioners.
When they rehone the crank journals or even rebore them, the center line of the crank stays put. The Main bearing journal size simply changes and you buy a bearing to fit it.
They don't 'move' the crank.
Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's - Gone to the rust pile... Canadian Off Road Trips Photos: Non members can still view! Jan/06
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fwiw, I'm going to call the machine shop today to ask them about this - I figure they'll either be able to set me up with the right size of timing set, or confirm that my timing chain is just a bit stretched.
In high school (way back when) I was told that if you put the chain flat on a workbench and the top drooped far enough to touch the bottom, it was worn. Now, this may only apply to traditional V8's up to the 70's I dunno, because we swapped the timing chain on the wife's Beretta V6 back in 99 and the new one was just as saggy as the old one. (We were swapping it for cheap insurance while I had the motor out to do a tranny swap.)
The motor has not been rebuilt before - it was a new GM Goodwrench 260HP longblock I bought in 2006 for my race car. I had a fancy oilpan on it with baffles and trap doors and stuff, but one of the baffles came off and left 4 spot weld size holes in the sump, and it's a dirt car, so no one can see you leaking... and when I lost oil pressure and shut the motor off, there was probably less than a quart in the pan. The bearings were literally melted and the crank and 6 rods were smoked. You can actually see the blue on them. We put new higher compression (legal for the class) pistons in there, and they line honed the block to clean up the slight warpage that was in there. Everything else was good, they didn't have to deck it or anything - the pistons are a standard bore size - there's not even 300 miles on the motor...
The first step is to "shave" the main bearing caps a prescribed amount, so that when they're re-attached to the block, the bearing retaining circles are no longer circular. Viewed end-on, they're all "football" shaped (sorta) since a thin slice has been removed from the center of the bearing circle.
After the caps are attached to the block and torqued to spec, the boring bar is set up so that (ideally) the "new" bearing circle is just exactly tangential to the old bearing circle at the very top of the circle (the block side). Of course that wold mean that the only material to be removed comes off the caps, and the crank wouldn't be moved at all.
But, you only line-bore for a reason, and that reason is frequently damage to the block casting and caps from a spun bearing. If that is the case, you HAVE to remove some material from the block in order to remove the damaged metal, and therefore the crank centerline does move up ever so slightly. The actual movement is on the order of a couple of thousandths of an inch, so its pretty negligible. It does affect the cam-to-crank spacing, and also moves the pistons up in their bores by cos(45)*the amount of material removed (for a 90-degree V-block engine), but again that's pretty trivial because numbers like that are generally less than the variability in the amount the head gasket compresses anyway.
I understand what you are saying, but when I rebuild engines I have to buy the correct bearing for the journal 'diameter'. Even at the factory they hone the bearings to different sizes in different blocks which takes material off 'both' the cap and block side of the hole. The center line isn't affected.
For instance on one Jeep engine, the main bearing diameters can be anywhere from 2.4901" up to 2.5001".
These stock bearings are even color coded and the block stamped for their 'factory' over and undersizes.
The OP only had his honed, not rebored as well.
Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's - Gone to the rust pile... Canadian Off Road Trips Photos: Non members can still view! Jan/06
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Use some Plastigage and determine what the limits are.Now, if my old walk behind trustry Briggs & Stratton engine lawn mower had/has a nicked valve, I would push it till it croaks and then go buy a new Briggs & Stratton engine lawn mower for about a hundred bucks.I have no idea though of what condition (my condition is in) the old valves in my 1978 Dodge van are in.sometimes, when I crank it up, they make a racket, sometimes not, quiet as a church mouse.That's just the nature of those old Dodge engines though. cuhulin
Mike there is a BIG difference in grinding the crank and grinding on the block. The crank is ground on a machine using the original center of the crankshaft. The new bearings are physically thicker INSIDE the shell to make up that difference.
However when you line hone a block you CANNOT use the original bore that the factory machined for the crankshaft. Either damage or wear has made that bore out of round or tapered or just FUBAR. It is now too large for the factory bearing shells to fit in. Nobody makes a set of bearings that have a larger diameter to fit that hole. You need to make the hole smaller AND round to bring it back to the factory size.
My Jeep engine's bearing specs clearly have the 'different' journal size (diameter) measurements I gave above and the bearing insert measurement for the different crank sizes.
To quote your link even:
If an engine overheats or loses oil pressure, one or more bearings on the crankshaft or camshaft may seize and spin. The resulting damage to the bearing bore must then be repaired by either machining the hole to accept a standard sized bearing or an oversized bearing.
And:
Many engines can handle a few thousandths variation in the position of the crankshaft centerline but others cannot because of changes it causes in other critical dimensions
End Quote
I think if they end up having to move the crank more than a hair, the engine is trash.
For different crank journal sizes YES. If the BLOCKS bearing bores are oversized your SOL. The oversized bearings you get for Jeeps are ALL the same external size. They are different on the inside only. You mic. up the different journals and use undersized bearings to give you the correct clearances. For instance a SBC uses a block with the mains bored to 2.6406-2.6415. The bearings are made so they fit a journal diameter of 2.450. Now if you grind the crank to .010 under. The bearings STILL have to fit that
2.64 hole, but they will be 2.440 on the inside. You can sometimes find bearings in .001/.002 undersizes as well. They are still made to fit in that 2.64 bore.
Same thing with your Jeep bearings. The undersizes are to fit worn crankshaft journals. The bore in the block still has to be the correct size or the bearings will be loose and spin.
True. However the meaning is that you correct the block back to the 2.64 size so you can install new bearings that fit the crank. If you use a new crank you use standard bearings, if you cut the crank you use an oversized bearing.
Also true. GM specs allows a centerline difference of +/-.004 from the factory. However you still try to minimize the loss. On a couple Chrysler engines the allowance is only .001.
Not really, on the Jeep straight 6 they allow a difference of +/- .006
fwiw, and this is probably why they had to line hone my block, is when I ran out of oil, the crank bent .060. I now have a new crank, still cast due to $$$, but I don't know if a forged one would have done any better without any lubrication.
Racing is a good way to accumulate a large collection of blown up parts. Blocks with air conditioning holes, tires that consist of one sidewall, wheels that look like pretzels, hubless axles, 4 piece camshafts...
Rod and main bearings for normal auto engine overhaul purposes are sold as either "standard" or "undersized" (not "oversized"). The term refers to the inside diameter of the bearing or the outside diameter of the crank journal. They are sized to the crank pins, and in some OEM situations, a factor of both the crank pin *and* main or rod journal, but that is not appropriate to this discussion.
The OP is talking about a normal engine rebuild situation using either a standard sized new crank or a crank turned to some undersize. We don't know what size as it hasn't been disclosed, but it doesn't matter anyway. The block main journals were trashed and needed to be line bored/honed to get them back in alignment and back to the correct diameter.
This is how the Chevy engine rebuild went for the OP....... The diameter of his main journals is supposed to be... well, lets say 2.5" for a point of reference. The OP brings in his messed up block to the machine shop. The machinst IDs the block and looks up the correct journal size in his reference book. He starts by taking some metal off the mating surfaces of the block and the journal caps, then he runs the bar through the (assembled) caps/holes until the diameter is once again exactly 2.5". This moves the crank centerline a hair closer to the cam centerline; there is no avoiding this assuming that some of the block side metal was damaged from spun bearings. He can't do this operation properly without cleaning up the damaged surface on the block side of the journals. Whether the operation is being done to correct a misaligned crank or to clean galled metal it *will* bring the crank closer.
Now that he has his block back he needs to put a crank in it. If it is a new crank and machined accurately, standard bearings will do. They will always be 2.5"-ish outside diameter and about 0.0015" larger than the crank mains on the inside. If the crank has been turned undersize, then the correct fitting undersized bearings will still be
2.5"-ish outside, and about 0.0015" larger than the undersized journals on the inside.
You mentioned your Jeep straight 6 having different types of main bearings. Sure, that is the norm. The thrust bearing will be different, and sometimes the rear main and/or the front main will be different depending on engine. I have yet to see an engine that has more than three different types of main bearings, but there's no reason that all of them couldn't be different. Unfortunately, if they have different diameters, line boring becomes more difficult. The same concepts as above are still true though.
I mentined some bearing sizing methods that take the block and the crank into consideration. On these types, including the 4.0 IIRC (rebuilt one at a Chryco dealer in the '90s) you take stamped numbers on the crank and block and reference them to a chart to find the correct insert. This does allow you to fit an insert that is sized both externally and internally (sort of) to a FACTORY UNTOUCHED crank and block. Once you machine either part then that is all thrown out the window and you simply use standard or undersized bearings either from the dealer or aftermarket. Since the OP is taking about a (re)machined engine this type of measurement doesn't apply.
If you are worried that the block mains and the new bearings aren't a perfect fit together, you can always verify the installed bearing size with a snap gauge or plastigage. If that measurement isn't the correct mate to it's respective crank pin, then either the bearing is made wrong or the machining is inaccurate.
Toyota MDT in MO
PS I think that Jeep also used color-coding on the back sides of the bearing inserts to designate size. IIRC they even allowed two different color-coded insert halves to be installed in one journal if the measurements called for it. As stated above, this system was for factory finished blocks and cranks only.
I'm the OP, and I don't know if the replacement crank was turned or not. I didn't ask, and don't really care. They just mentioned that they had to line hone it to bring the bottom end back to square. They didn't tell me if it was .001 or .01 or .1 off - I didn't ask - I trust them to machine it square for the race car, and that's all I need to worry about. :)
FWIW, I got my replacement timing set tonight, so I'll be doing that tomorrow.
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