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  1. #81
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    Quote Originally Posted by dian View Post
    50 thou off?
    Like I said, hard to believe. How do you take that first cut and not realize that you are a mile off center??

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    Guys I have a question, as a starting point how do you test the block is 'true' and everything lines up?

    Also, I know we are talking about the 0.001" clearances and tolerances but how is a machine shop able to keep tolerances to a high degree when boring or honing the block? How would they position the machine so that it's dead nuts in the center of the cylinder. Every slight misalignment/etc. would contribute significantly more than 0.001 - 0.002"?

    Thank you!

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    "Guys I have a question, as a starting point how do you test the block is 'true' and everything lines up?"

    Fact is, very few shops can. I'd say 1 in 10 have a 4 axis CNC machine which can locate the factory datum and measure from there. All other boring machines simply follow what's already there. There are some bolt on fixtures which help a little, but it's not remotely the same. The average machinist would be shocked to see how far off lifter bores are.
    BTW, I sent you a PM which should help in your quest for knowledge on engine machining.

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    lets take main/rod clearances on a ls7 as a simple example. looking up you see:

    production mains: 0.0008-0.0021
    service mains: 0.0008-0.0025
    production rods: 0.0009-0.0025
    service rods: 0.0009-0.0030

    so now what? its 1:3. do you go for the production average? 0.0014/0.0017? not sure. next you call thompson motorsports and the friendly tech tells you 0.0032/0.0021 for their road race engines. great. but wait, thats twice the clearence and rods are smaller now? you have to investigete, so you find this forum full of knowledgeable, friendly guys. you get told: 0.0035/0.0025 (guy is running straight sae 60 dyno oil in his 9000 rpm nito combo), 0.0012/0.0018 (guy has beeen playing with/reducing clearances for years and this has not blown up yet), 0.0017 for #5 and 0.0014 for the rest is the sweet spot (mains).

    so now you want to know. you go and buy a new ls7 and measure: plastigage shows weird values. were you supposed to measure cold or hot? is the crank dry or oiled? no idea.

    so you go and get a freshly certified id gage and measure 0.0020/0.0020. hm.

    next you take the aluminum block and heat it to 220°c. main journals grew by 0.003. o.k., you have been running 0.0025 on your iron small block for ages, so how does that translate? minus 0.0005 clearance? wait, no, the cap is syntered steel and the crank expands as well. so we have +3 for aluminum half, +1.5 for steel half, average is 2.25 minus 1.5 for the crank. thats 0.75 thou expansion. so you need 0.0175 for the mains. makes sense?

    in the end you decide to go with the production average of 14/17 thou. you have realized nothing opens up on the rods so they need a larger clearance, although they are smaller. so you have the crank machined and check it seven times with another certified instrument you have procured. its absolutely to your spec. so you order bearings. clearance is way off. oh, you ordered "under" bearings, you actually needed "over" (or is it the other way around?) so next set arrives, clerance is off by 0.001. what? oh, you remember that fed. mogul is looser than clevite. so next bearings and hurra, you have your clearance.

    after a short while running the engine you notice very high oil consumption. hm, what was the oil again you were supposed to use with those clearances? you go from 0w-30 to 15w-50. no good. what happened? you forgot you check lifter clearance. thats where your oil goes.

    next you realize the engine has a rew limit of 7200 rpm and the titanium rods you were so proud of might be deforming exessively at tdc of exhaust stroke because they are less stiff that steel. you should have gone for larger clearance after all. sleepless nights.

    now, on an engine without such a broad following the situation might be simpler: less info.

    question: would the ls7 with 0.0008 on the mains even turn over when cold?
    Last edited by dian; 05-10-2021 at 04:16 AM.

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    in most things that move together its better to be a bit loose than to tight and as far as locating most parts are held in a fixture / jig or piloted the machine takes off what you dial in to it but you have to understand the machine you have to know what its going to do and all that comes with time on that machine

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    I got back to doing some work on this and wanted to update the group...

    So the crank journals measure 1.714 - 1.715". Factory manual says they should be 1.716 - 1.717" so on paper it would appear mine are worn out ~ 0.002" and are out of spec, correct?

    We moved to the checking the inside diameter of the con-rods (big end) with the bearings installed and proper torquing in place. The factory manual says these should be 1.719 - 1.720", resulting in a tolerance/gap of 0.002 - 0.003".

    However, when measuring mine I am getting a value of 1.717", which also appears out of spec (range 1.719 - 1.720"). But the tolerance/gap for mine is within spec (since journals are 1.714 - 1.715" see first line above).

    So I'm scratching my head and wondering whether if the gap is in spec, do the factory numbers in the manual matter as much? Or could it have been variability on this engine at the factory (recall it's a 1960s design after all). Note the engine has <10,000 miles before the rebuild. The bearings are Vandervell.

    Thanks

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    Assemble the rods with the bearings to the crank using "Plastigage"and see if the clearance shown is in spec. Engine bearings from some mfgs are available in .001 and .002 undersize. Measure the wall thickness of the bearing at the center of each half to see if undersize . That would explain your readings on the small side. The 2 most important things on a rod journal are roundness and taper. In any engine I ever worked on ,and there has been more than a few, .0005 was the max taper or out of round before regrind. And any performance apps neither were allowed. Refresh me what engine are we working on?? I may have bearing specs. Was not uncommon to polish journals .001-2 undersize for high RPM use. That is if you had adequate oil pressure and volume .

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    not so sure about plastigage. do you do it? dry, oiled, lightly oiled? at what temperature? how many measurements to get reliable value?
    Last edited by dian; 06-09-2021 at 03:48 AM.

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    Hell, just put put a small block chevy in it. You can build them in your backyard with a little grass and dirt thrown in for good measure and they will still run for a long time!

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    Quote Originally Posted by JohnEvans View Post
    Assemble the rods with the bearings to the crank using "Plastigage"and see if the clearance shown is in spec. Engine bearings from some mfgs are available in .001 and .002 undersize. Measure the wall thickness of the bearing at the center of each half to see if undersize . That would explain your readings on the small side. The 2 most important things on a rod journal are roundness and taper. In any engine I ever worked on ,and there has been more than a few, .0005 was the max taper or out of round before regrind. And any performance apps neither were allowed. Refresh me what engine are we working on?? I may have bearing specs. Was not uncommon to polish journals .001-2 undersize for high RPM use. That is if you had adequate oil pressure and volume .
    With pastigage the clearances are in spec. Rod journal roundness and taper will be checked again but they seemed matched the first time (round and no taper).

    What's a typical out-of-round guideline for the con-rod big end? I had one shop recommend they resize the big end if needed but that could throw off the gap/tolerance between journals and rods/bearings?

    Thanks!

  11. #91
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    Quote Originally Posted by dian View Post
    not so sure about plastigage. do you do it? dry, oiled, lightly oiled? at what temperature? how many measurements to get reliable value?
    Plastigage at room temperature, locally oiled is what was tried and it was in spec.

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    i never used it, but i keep hearing, its not reliable.

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    Quote Originally Posted by dian View Post
    i never used it, but i keep hearing, its not reliable.
    Measuring with plastigage is akin to measuring with those throw away 6" Chinese calipers. You can get a rough idea but use real tools if it's important.

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    Plastigage was introduced as a way to feild measure engine bearing clearance at the time of rebuild when most mechanics do not have the measuring tools or the experiance to measure the parts with any positive repetitive results. If it is fresh and not been sitting around it is fairly reliable and has been proven for years. Is it the best , no. is it absolute, no but for what it is it works.
    I routinely give my guys and myself a journal. The same tools, mic,bore gauge ect, and we measure and compare measurements. 2 of us are the same to the tenth, 1 is different not much but different.. so the question is who is more accurate? I know you should be able to measure or your not a machinist, but every body has different feel for the tool.
    This is why there are tolerances on the running parts. If your going to run 0w-20 oil better not be at the loose side.


    Sent from my iPhone using Tapatalk

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    Thanks guys - yes this is a way to check at the install time with the plastigauge. Going back to the beginning of the thread I was describing the issues with rebuilding this engine and the opinions I have gotten from the local shops.

    In this case as we started to look at the crank itself the puzzle of whether the factory engine manual could be wrong presented itself. It makes the difference between accepting the slightly smaller (~0.002") journal diameters, and correspondingly smaller (~0.002") big-end bearing diameters to the point of the tolerance being in spec (which is the least amount of changes/work/risk to the engine rebuild) or grinding down the crank to the next under size and using the undersize bearings to end up with the same gap but factory manual ranges for the diameters.

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    With apologies, I gotta ask. Are you using the same micrometer for both measurements? .002 off on both I'd question your measuring device.

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    Quote Originally Posted by strokersix View Post
    With apologies, I gotta ask. Are you using the same micrometer for both measurements? .002 off on both I'd question your measuring device.
    Same micrometer, and measured at the same time -- we can triple check the numbers again to be sure but I feel pretty confident that the crank journals and the rod inner bearing measurement are within tolerance relative to each other but off (each of them) by 0.002" from the factory manual ranges.

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    Measuring journals or any round part is a art all on it's own. Gauge blocks are easy as there is no feel and this is what most zero their mics on.
    Round parts, round master and zero. Amazes me how many make this simple gauging mistake.

    Plastigauge, a big fan of when first doing engines.
    Not so much now. A new bearing goes in and is crushed on it's ends. This can throw the piece of plastic off by a bit or a lot as this happens.
    Second it has been crushed I do no want to take it apart. No crush on second bolt together.
    Indicating mics tend to work better than normal ones.
    Some race car people like a tad more clearance than the factory spec so a whole new issue.

    I'm sure there are many viewpoints and this is just one.
    Bob

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    I am new to this thread and reading through it, but I will chime in as I have worked for a long time as an automotive machinist. If this is a "rare" car, or a car that has < lots of replacement parts available, TAKE THE ENGINE TO A REPUTABLE MACHINE SHOP. I read just one post of the thread owner and there are several red flags. Here are a few things to keep in mind:
    -The factory "specs" are just that...factory. They vary largely depending on production date, use, oil viscosity, engineered use, and many MANY other variables. The specs are just guidelines, an experienced machine shop will be able to guide you on which side to run the specs…loose or tight.
    -Measuring any kind of bearing journal should be measured to the 0.0001" place, and measured at the left/center/right and at 3-6-9-12 o'clock positions. Automotive bearings are not the same as industry bearings, they are subject to impulse load at specific clock positions and hard torsional variations throughout the rotation. Therefore the bearings and journals do not wear in circular patterns. Often in an elliptical shape.
    -The fillets on the crank need to be checked for wear as some older European engines has issues with oiling and rod shifting, in where the rod would wear to the left or right causing premature damage to the crank.
    -Thrust bearing clearance plays a huge factor however they are often overlooked. There are many transmissions out there that were destroyed as the thrust bearing clearance in the engine was enough to toss the transmission into failure orbit.
    -Bearing clearances are EVERYTHING when it comes to a short block, for example standard SBC you and be +/- "bananna" and it will still run. Maserati +/- 0.00025 and you can still have problems. Miata +/- 0.00015 and throw the engine out.
    -Modern oil is a far cry from the oil that was available at the time this engine was designed, therefore that must be taken into consideration. The additives that were available then are no longer available, leading to the need for additional additives to be placed in the oil for operation.
    -Surface finish for head gasket adhesion is another big item. If you own a Roloc holder and Scotcbrite discs keep them for bodywork. If you Scotchbrite the gasket surface on an engine or cylinder head you are in for trouble. There are plenty of cars running around burning vast quantities of coolant because the person who built the engine hit the deck surface witch Scothbrite discs and provided 1000 different valleys for the fluids and gases to travel back and forth. There are a few engines that (although the build manual will never say it) benefit from spraying the head gasket with aluminized spray paint. Don’t believe me, try taking apart an old Continnental flat head and getting the head to reseal.
    -Surface finish and cross hatch on cylinder walls, another big one. Put too fine of a finish and the rings will never seat, too rough of a finish and the rings will never last. Some rings require a “plateau honed finish” in order to seat properly.
    -Camshaft…well that is an entire page worth of information.

    Automotive machining is way different than traditional machine shop practices. Exponentially increase your chance of failure the more exotic your application. Dodge slant six…you practically don’t even need oil (they call it the leaning tower of power) for a reason. 5.9 12valve Cummins… you can fill the engine with the hopes and dreams of a long past childhood and it is still almost a million mile engine. Old Volkswagen diesel will run on a good Burbon or fine Scotch and just keep going. Older Cosworth engine….good luck finding the necessary parts, or the engine out of a Studebaker Golden Hawk good luck in general. Take the engine to someone that knows what they are doing, and has the reputation to back it up. Long post in one sentence… Minimal experience gets you a minimal quality product.

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  23. #100
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    One last item I will add to the previous post... if you are doing work that is near impossible to find replacement parts. Try the website below Egge, they are the go-to guys for hard to find or old and obsolete parts.

    Egge.com


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