Random question..

[TexasNomad]

When your building the bottom end of a engine and you use the plastic crush gauge to test the tolerances between the crank and bearings, what do you do if it fails the test? get smaller bearings take the crank back and have it machined down? get drunk and cry?

 

[89frankenford]

haha well you could do either. if it fails the test you can get the grooves ground down and get thicker bearings or get a new crank that is slightly bigger(the bearings would be cheaper though)


you can get drunk and cry though too lol

 

[TexasNomad]

haha well you could do either. if it fails the test you can get the grooves ground down and get thicker bearings or get a new crank that is slightly bigger(the bearings would be cheaper though)


you can get drunk and cry though too lol

Hehe Well every thing i've read of watched about engine building shows the building doing the crush test but nothing has what your supposed to do if the test fails.
So the crush test is really to see if the crank is big enough?

 

[89frankenford]

yes basically. providing you have new bearings in a motor and are using the old crank it will tell you if your crank is big enough or even too big

 

[TexasNomad]

Ah I see. Thanks Travis

 

[89frankenford]

no problem at all




and thats Words of Wisdom from Travis lol

 

[Fellro]

Did you have the old crank checked? If not, it needs to be to find out what size it is. If it is undersize, you will have to get it machined to the proper size, and get the proper set of bearings to match.

On a side note, in some ways I am surprised to hear anyone checking that, few do it seems these days. I had a tough time getting my hands on some for a simple maintenance check on a diesel motor for my dad. The kid at O'Reilly's didn't have a clue what I was talking about, I had to explain it all to him...

 

[blacksnapon]

So the crush test is really to see if the crank is big enough?

The test with plastiguage is to measure the clearance between the bearing surface an the crank journals. The amount of clearance determines your oil pressure (among other things). Less clearance-more oil pressure. More clearance-lower oil pressure. Theres a tradeoff though. Less clearance, the more difficult for oil to circulate (may use a lighter oil), theres a better chance to have increased friction, lower fuel mileage, more heat as well.

 

[Fellro]

Basically, all plastigauge does is check oil clearance. As long as you know the oil clearance spec, it is useful. If you don't know the spec, it doesn't help as much. AS stated, more clearance, less pressure. Less clearance, more pressure, at first at least, and possibly more drag, depending on oil used and design tolerances.

 

[TexasNomad]

The test with plastiguage is to measure the clearance between the bearing surface an the crank journals. The amount of clearance determines your oil pressure (among other things). Less clearance-more oil pressure. More clearance-lower oil pressure. Theres a tradeoff though. Less clearance, the more difficult for oil to circulate (may use a lighter oil), theres a better chance to have increased friction, lower fuel mileage, more heat as well.

Really! So the tighter things are the better lubrication you will have in your engine.
Or is it a much more delicate balance then that?

 

[SuperCab]

That's the jist of it...

 

[blacksnapon]

Really! So the tighter things are the better lubrication you will have in your engine.
Or is it a much more delicate balance then that?

Its a balancing act for sure.

 

[surewhynot]

It is a definite balancing act. If you look in the service manuals, they will give you an allowable service range for clearances. Too tight is just as bad as too loose.

 

[Dirky]

Using new bearings on an original used crankshaft can create too much bearing to journal clearance. The crank will have worn over its life span, and when rebuilding a motor, you must check the clearance between the two components, and ENSURE the clearance is correct.

Sometimes the crank is in good enough shape that the machine shop will only need to polish the journals. Other times the crank journals will have to be turned (ground) to the best available under size depending on journal wear, which means that over sized bearings must be used. So if your crank was ground 0.010", you would need 0.010" over sized bearings.

Just curious, what was the reading on your plasti-gauge test?

 

[TexasNomad]

OH! I haven't done any readings yet Derek, just a random questions
Thanks for your reply's guys

 

[Dirky]

Ooo I didn't read the question properly first. My bad.

Let's hope none of use run into this problem!

 

[gsxr1238]

Hey guys,I know that this is an old thread but my 2 cents on this.What you are looking for is called plastigage.It comes in different diameters for testing different clearances when crushed.You need to have a pretty good idea of your intended clearances to buy the correct stuff.Your machine shop probably uses it,if not they should be.They also probably sell it,if not most auto part suppliers do.Good to have around to check your stuff as you build.Bearing to journal oil clearances are what you are looking at,and these are published measurements.Really handy if you are using undersize cranks,oversize main bores,etc.Hope this helps.

 

[LEB Kris]

Ill add...

you want .001" of clearance per 1" of journal diameter. A stock 460 has 3" main and rod journals so optimum clearance is .003. The tighter the clearance the more oil pressure you will have but less oil flow because its harder for the oil to move through tighter clearances. Plastigauge is really the most inaccurate way to measure clearances but its better than not checking at all. A dial bore gauge is the best but if youre not building engines rather regularly its tough spending a pile of money on the correct equipment. Never take journal diameter at face value. Just because you had .010 taken off dont assume .010 bearings will be a perfect fit.

I had a great deal of trouble specing out my 521. Forged steel stroker cranks are 3" main and 2.200" Chevy rod journals so my clearances should be .003 and .0022 respectively. When I bolted down the main caps with stock bolts with 30w oil and the correct torque specs I came up with .003 and a perfecly free crank. When I bolted down the mains using a main girdle and ARP bolts with moly lube and the correct torque specs I came up with .0025 or .0005 too tight. When I bolted down my H-beam rods using ARP2000 rod bolts with moly lube and the correct torque specs I ended up with .0025 or .0003 too loose. I fought that thing for weeks without too much luck before finally calling Adney Brown (3rd place in Engine Masters last year). VERRRY long story short I left the clearances as is for a 50/50 race motor that will see some nitrous.

Cold with 10w40 Ive got over 80psi of oil pressure at idle and hot Ive got about 55-60psi. Half of that is clearance related and the other half is running a Kaase oil pump. Anyone building a motor making over 500hp should seriously consider buying a pump from John Kaase (www.jonkaaseracingengines.com) Very beefy design eliminates the housing breaking at the stem that is common with Melling HV pumps at higher horsepower levels. The only downfall is its so big you have to cut one window out the girdle to clear the pump.