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PlastiGauge help please

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ichthos

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I read the link about PlastiGauge that Dr.Entropy posted. I understand the math and explanation of how it works, but I still have a few questions. Is the purpose of this to check the clearance after the crank and new bearings are put in? Is this a normal practice that should be carried out when rebuilding an engine? Basically the PlastiGauge gives a clearance between the main and rod bearings and crank shaft. What is the clearance between the bearings and crank supposed to be, or where do I find them? Any help is greatly appreciated.
Kevin
 
Your exactly right in what it measures. :thumbsup:

Recommended clearances should be in your BOOKS/MANUALS.
 
Yes, the Plastigauge is one tool to check the clearances after the engine has been machined, but prior to run-in. I like to assembly the bottom end and torque everything down. Insure that everything turns OK. Then take one cap off at a time, insert the Pastigauge, then reinstall and torque. DO NOT TURN THE CRANK! Then remove and compare the width of the squeezed guage with the card with the instructions. Write it all down and continue on the rest of the rods and mains. I have seen clearances posted for .0015 to .0025" for both mains and rods, but I think that is too wide of a spec. I would recommend that the clearances be kept as close as possible to .0015"
You can't do your engine now as with the worn bearings will give a big clearance.
The other tool used is a micrometer to measure the crank journals. This finds out if the crank is worn. Your bearing should list on the back what size they are. The factory size will either say nothing or STD., If they are undersize, they will be printed 10, 20, etc. A resizing of the rod journals, i.e. making sure they are the right size and round, only makes sense.
Scott in CA
 
When you say resizing the rod journals, you are talking about the four areas the piston connects to on the crankshaft, correct? When you take in a crankshaft to a machine shop to be checked/polished/ground, do you normally have to specify that both the rod and main bearing surfaces are to be checked? Do you normally have to tell them what specs you want both to be? And when you say 10, 20, etc, you are referring to thousandths, correct?
Kevin
 
Kevin, the above sounds like you have it. The machine shop will have to have at your crank again and they will be able to order the correct rod and main bearings. Assemble as Scott said and check with plastigage. Since your pistons and rods are assembled I would'nt worry about having the machine shop resize the rods unless your plastigage check shows something wrong. A good machine shop is your friend and they will be willing to help you out. Course a bad one may try to sell you more than you need but this bunch is here to advise!

Kurt.
 
When I rebuilt my 1275 engine (now +40), I went through the same learning curve Kevin. My lips were definitely moving as I read articles such as "how to rebuilt your engine" and ""Plastigage- your new best friend"
grin.gif

I made some mistakes, but happy with the end results.

One thing that was always emphasized, no matter what phase of the rebuilt, keep things clean, and use engine lube when needed (pistons, cams).
 
Not sure if you misunderstood or I misunderstood you. Anyway, you don't tell the shop what specs you want on the crank. They check it and tell you what it needs to be ground to. Also, I would expect any shop to check everything on a crank but it can't hurt to say what you want done.
 
The main reason to do this is to ensure that the crank and bearing are indeed the sizes specified. You can find occasionally mismarked bearings or cranks very slightly undersized enough to go out of spec.

When using plastigage everything needs to be dry, no oil or assembly lube as these can cause readings on the tight side. And I personally like to do on set of readings, then move the crank 90 degrees and do another set as I had a crank that somehow was slightly oval when redone.
 
Mike,
I never thought of redoing it after a 90 degree rotation. (that could be taken out of context).
Definitely something to keep in mind when/if I rebuild an engine in the future.
 
So... if you have a machine shop oval your crank... how do you get compensated? Obviously, you don't ask them to do it again.
 
you should NEVER ASS-UME that a crank grinder or an engine assembler got everything right.

There is a big difference in an engine assembler and an engine builder. The latter will run the motor on the dyno and give you a sheet with all the numbers, while the assembler will work to thousandthS and not tenths (.0001).

Some times one has to know thier limitations in what they are doing. Kinda like doing brain surgery after reading the "How To" book.

As to crankshafts........a competent crank grinder will look up the journal dimensions in the book and grind using those numbers. Unless you want "a little on the high side" ....for example....... when doing A motor rods you might want a little more big end to crank cheek clearance to eliminate pinching at higher rpms. But then you should be asking for specific numbers and should know the drill as in the paragraph above. THEN , when you get the crank you take your micrometer and measure the journals. Side to side (no taper) and at 90* (roundness). PLASTIGAGE will only get you in the BALLPARK. Reading the thickness is plus or minus half a thou. You just can't get closer. All this points out that there are front runners and backmarkers in life and in assembling (or building) a motor. That is not to say that there is a broad spectrum of assembly and what is expected. Sometimes a budget street motor is all that will be expected and God love the homebuilder with his Haynes manual.

To answer Trevors question........CHECK EVERYTHING. Get your parts back and measure everything. Borrow tools if you have to. If you have an oval crank you should not have to pay and you have a 25 pound piece of scrap metal unless ANOTHER COMPETENT crank grinder can take another .010" and make it right. Before you have someone do machine work you shoud see what kind of work the shop does. Ask the RIGHT QUESTIONS so you get the RIGHT answers. In other words ask folks that are heads up to parts. Have fun.
 
Isn't it true that the A-series crank can be ground only 0.010, i.e., only once, unless it is rehardened? I seem to remember that from my distant past. Rehardening is possible but probably not economical. Even if I'm wrong about that specific item, there are a lot of little things like this that you won't know about unless you check it out in a good shop manual. I don't know if the machine shop will have this info or will just assume it's OK to regrind as much as needed. Certainly, you can get bearings up to 0.040.

Kevin, some of the questions you ask make me suspect that you don't have a manual. A manual is really essential. Doing an engine rebuild without a good manual has the same safety profile as wing walking. You can get a Haynes manual for $15, and reprint editions of the original shop manual are available.
 
A series cranks, especially 1275 cranks are factory hardened, and that hardeneing is good to about .030", so .010", or .020" journal do not concern me very much, once you get down to .030", then you getting to the point the factory hardeneing is about gone, and you need to think about rehardening.

AS for rod and main clearence on one of these engines, I use .0015" on rods and main for my street engine and .002" on my race engines. my engines are bluepint built, thats a real fancy term for just saying, I meaure everything, determine what needs machining, then when machining is done I dictate the size, for example I might say to my crank grinder "the rod journals need to 1.6154" ", because that is the number I need once I resize the rods to get me to .0015". I have alot a precision measuring tools to do all this with, but lets just use a rod clearence for an example, I make sure the size on my rod journal on the crank is right, my crank grinder, same one as Mike uses, (I turned him on to them) they're outstanding, they will hit my number +/- .0001", then I resize the rods in house on my Sunnen hone and use my Sunnen AG300 rod gauge to make they are to the exact size +/- .0001" I want, I use the bearing shell thicknesses printed in a bearing book to see what the bearing thickness should be, use a ball mic to actually measure those bearing shells, then I use this simple formula,

Housing bore - bearing shell thickness x2 - journal size = clearence.

I still use plastigauge sometimes to double check, and it's OK, it leaves lots of room for human error, like rotating the crank even a little bit while doing this , or tapping a cap on to hard, so plastigauging can fool you some of the time.

I'm often asked, "whast the big deal, so I have extra .0005" more clearence that what you stated, whats the crime if I go with say, .002" vs .0015" ", well it actually pretty important a extra .0005" too much clearence could compromise oil pressure 10-20 psi, and oil pressure is why I go to all this trouble.
 
Hap said:
I'm often asked, "what's the big deal, so I have extra .0005" more clearence that what you stated, whats the crime if I go with say, .002" vs .0015" ", <span style="font-style: italic">well it actually pretty important an extra .0005" too much clearence could compromise oil pressure 10-20 psi</span>, and oil pressure is why I go to all this trouble.
Click to expand...
(Italics added for emphasis)

Amen! That's what "occasional" engine builders need to understand, Hap. Good stuff. :thumbsup:

I 'clearance' the engine builds with the math and use PlastiGauge as confirmation I'm in the ballpark. The stuff is not infallable to be sure but with reasonable attention to detail it can be a reassuring last check. There's NO substitute for good instrumentation and a bit of math, tho.

Heck, I don't trust those fancy-scmancy digital mic's! Learn to read a vernier scale and develop a proper "touch/feel" with micrometers and you can't go wrong... err... well, you have a much better chance of holding tolerances. And a good machinist is worth his weight in gold, IMO. I'm passable with lathe work but when it comes to crank journals and such, I whimp out and hand it over to the REAL guys. :wink:

Reminds me: Our trusted machine shop is seeing the retirement of too many of the "old guys". I may need to talk to ya about a Lotus crank and some rods I intend to use for the Elan soon. :smile:
 
Whoo Doc. About time you started thinking about getting that Lotus on the road.
 
Just brought all my toolboxes back to th' hovel from the shop on Sunday, Jack.

I am render'd whole again!! :smile:
 
Hap Waldrop said:
a extra .0005" too much clearence could compromise oil pressure 10-20 psi, and oil pressure is why I go to all this trouble.
Click to expand...

Yikes! I must have vacuum! :eeek:
 
Doc, I don't use digital mics either, I think guys using digital mics are probably work with bigger tolerences than would be with crank clearence. Just using a mic is a developed skill, I starting working with my assistence last week on mic measuring and reading, I did so a 1275 crank I had already measured and not revealed those finding to him, he consistly measured too big. He paid very close attention to my advice to always check a mic again the standard to see it how the mic feels on the standard set at 0, and look to replicate that feel with that mic on what he was measuring, and I think he was being overly concerned aboht that mic's feel on the standard, most others when I taught them this almost always measured too small/tight. I told him he still needed more time with mics, but it was good start. I don't think most folks realized how 'touchy-feely" mic mesurng is when you're dealing with tenths of a thousands, like you do on crank journal measurments, it take time, and practice to get it right.
 
Those digital things are just flinky bits o' "bling" IMO. May be good for measurin' golf ball diameters but I wouldn't trust 'em for accuracy on a crankshaft... not one I'm responsible for, anyway. :wink:

Hap said:
I don't think most folks realized how 'touchy-feely" mic mesurng is when you're dealing with tenths of a thousands, like you do on crank journal measurments, it take time, and practice to get it right.
Click to expand...

Yup. A skill/art largely unknown and disappearing, I fear.
 
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