# crank clearance



## Topkat (Sep 9, 2010)

Hi awesome folks 
The book says 'crank clearance .0002 to .002
Going to finally do it right and plastigage everything :lurk:
does pg come in different sizes, how mant packs would I need to do all the mains and cranks?
thanks as always!
(on recent trip discovered the joys of Jamesons Irish Whiskey)
rock on all!
and thanks as always
Tk


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## geeteeohguy (Feb 2, 2008)

Rods .0005-.0026, and mains .0002 to .0017, in my old MOTOR manual. Packs come in different calibrations, may need two. Cheap. Tried Bulleit bourbon on a vacation last week and wasn't impressed......have a bottle of Baker's and it's my favorite so far.....


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## geeteeohguy (Feb 2, 2008)

Forgot to say, the mains will actually be 'ok' as loose as .003". but .002" or a tad less is better.


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## Norms68 (Jul 17, 2013)

The pg you're looking for is green and you could do it all with one strip of it but it's not the most accurate.

I got myself a set of outside micrometers and a dial bore gauge to really know the exact measurements.

-Norm


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## geeteeohguy (Feb 2, 2008)

Norm, of course is correct. Actual micrometers and calipers are superior to plastigauge, but plastigauge will work fine for this application. It will definitely be 'in the ballpark'.


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## PontiacJim (Dec 29, 2012)

Here is a tip you may want to look into. I don't know the history of your build, but, it is very important to get the correct "crush" on the bearings. You need some tools to do this, or have a machine shop do this for you and you can sleep at night.

Torque the main caps down, without the bearings. Measure the inside of each bore. My info states that the bore is measured at the up and down 12 and 6 o'clock position. The bore for the 400CI 3" mains should measure 3.1880 to 3.1885. 3.25" mains should measure 3.3480 to 3.3485. If you do not get the correct bearing crush on the main bearings, then you will not get a true readings from your bearings. Best way, as mentioned in an earlier reply, is to then insert your bearings, torque down the main caps, and measure the bearings while in place at the 12 ans 6 o'clock position. Use a micrometer on each main journal and measure these, writing them down. Subtract the main crank diameter from the inside bearing diameter to get the true clearances. You may have some variances and you can switch bearings around to get the best placement of your bearings. This can be done with the rods as well.

If your crank has been cut, make sure your bearings have a large enough chamfer on the edge to crank radius or you can destroy your bearings.

Factory crankshaft end play is listed as between .0030 to .0090. If not in this range, then you may need a different thrust bearing.

Check the clearance between 2 rods with a feeler gage. My info states that the factory manual is .006" to .011". However, the book I have for building a street Pontiac states .014" Clearances here are important as too much will cause oil pressure loss and overload the cyl walls and overload the oil scraper ring on the piston.

My Pontiac build up book states main bearing clearance as: Street- .0005 to .002, Ram Air/Super Duty- .0015 to .0024, Drags - .0025 to .0032.

Rod clearance as listed as: Street- .0005 to .001, RA/SD- .0013 to .0033, Drags- .0020 to .0035

My Chilton Pontiac factory engine specs, 1968 to 1973 GTO/Lemans/Tempest calls for: 
Main bearing clearance @ .0002 to .0017
Rod bearing clearance @ .0005 to .0026
1968/1969Rod side clearance .006 to .011
1970 to 1973 rod side clearance .012 to .017
1969 RA IV mains 1,2,3,4 @ .0012 to .0028 #5 main .0007 to .0022
rods @ .0015 to .0031
1970 RA IV mains 1,2,3,4 @ .0007 to .0023 #5 main .0007 to .0022
rods @ .0015 to .0031
Personally, I have never done this on any engine I built as I have simply "assumed" my machinist knew what he was doing when he turned down the crank and told me what bearing to order. I have spot checked with the plastigage, but never gone all out. My last 400CI had oil pressure problems and it eventually dropped to "0" and don't know if my "assuming" was part of that problem. You can bet this time around I will take the time to personally check each and every tolerance this time around. Engines today cost way too much to guess or "assume" and I can't just go to my local junkyard and buy a good running 400CI for $150.00 anymore! HahaHa

Hope this helps.


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## tjta (Jul 24, 2020)

My engine builder finished rebuilding/assembling my 400 short block. I asked him what are the main and rod bearing clearances. He mentioned 3 thou on the mains and 2.5 thou on the rod bearings. The bearings are considered a high performance bearing not stock type. I believe I mentioned the specs and part number in a previous conversation with Jim, but can't seem to find that thread now. 

My oil pump is 60 psi. I had read that these clearances are still acceptable but may cause possible oil pressure at idle?

My question is what is a safe oil pressure at idle after it has fully warmed up. If my psi is lower then the recommended idle psi, will this cause any issues? 

The engine will on the dyno for a couple of hours before it goes back into the car. 

Thanks


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## Old Man Taylor (May 9, 2011)

You should have no problem. With 10 psi at idle, pay attention. At 5 psi at idle start getting worried, but it's still probably OK.


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## tjta (Jul 24, 2020)

If that is the case.....just recalled and should have posted it as well, before the same engine got rebuilt, I remember that my gauge was hovering around the 30psi mark warmed up at idle. The bearings that came out of the engine were quite worn and looking stock like.

The pump was a 60psi and is being reused for the rebuild since it was only 2 years old and very little miles on it.


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## Old Man Taylor (May 9, 2011)

Just make sure the pump is clean.


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## tjta (Jul 24, 2020)

Sorry my fault.....should have been more clear in my question.

I would like to know if I will have the same oil pressure ( ie around 30psi during idle after a warm up or sitting at a traffic light ) with these clearance numbers. OR if I have lower then 30 psi during the same idle scenario, will there be issues.
Thanks


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## Old Man Taylor (May 9, 2011)

It will be somewhat lower, but it should not be an issue.


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## PontiacJim (Dec 29, 2012)

It also depends on the weight of the oil being used. With those clearances, which are on the wide end scale, I would use a 15W-40 oil - which was also the required oil for the Ram Air IV engine. If they were tighter clearances, then I would use a 10W-30.

Don't use a synthetic oil.

With the heavier oil, you want to be sure you have installed an aftermarket oil pump shaft to replace the factory one as it will take more pressure to pump the heavier oil and you don't want the oil pump shaft to break at the "ears" that slides over the oil pump shaft.

You may not see 30 pounds at hot idle with the larger clearances. What you do want is 50-60 PSI at 2,600 RPM's which is how Pontiac rates their oil pumps. Your idle speed can also affect your oil pressure and increasing idle RPM's can bring up oil pressure if you think it is a bit low. The Pontiac Super Duty 421 race engines had to maintain 1,000 RPM's to keep oil pressure up as well as the use of an oil additive due to the large rod/crank clearances.

Use a good filter that has minimum flow loss. Many recommend the Napa Gold/Wix oil filters. I use these on my Hyundai with no issues.


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## tjta (Jul 24, 2020)

I was thinking and also have Joe Gibbs 10w 40 with the added Zinc. Will that suffice? Would I still need to change the oil pump shaft using the 10w40?

The oil pump was changed to a 60 psi about 2 years ago by the previous engine owner. But don’t know if that also meant a change in the oil pump shaft cause I don’t have any paper work on that.

Thanks


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## Old Man Taylor (May 9, 2011)

Unless the engine builder ground the tangs off of the oil pump drive rod, you will not be able to take it out without taking the engine apart.


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## tjta (Jul 24, 2020)

The oil pump shaft is located off the oil pump which is accessible by removing the oil pan I’m assuming?

10w40 with the added zinc will be heavy enough to keep up the oil pressure during idle?


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## PontiacJim (Dec 29, 2012)

tjta said:


> The oil pump shaft is located off the oil pump which is accessible by removing the oil pan I’m assuming?
> 
> 10w40 with the added zinc will be heavy enough to keep up the oil pressure during idle?


Joe Gibb oil is good. Brad Penn is the other recommended oil with the added zinc.

If the oil pump shaft is factory, it has to small "pinched ears" on the side of the shaft that prevents it from being pulled up and through the distributor hole once the distrubutor is removed. Often, the aftermarket shafts don't have these so the advantage is they can be dropped from the top if needed.

Your engine builder may be able to answer what shaft is in the block.

Oil pump shaft at left is factory, note the "pinched ears" about 1" down from the top. That end fits to the oil pump. The shaft on the right is aftermarket and has no "ears" where you see the ring.


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## tjta (Jul 24, 2020)

Thanks Jim and “ Old Man Taylor “. I will inquire about this shaft and see what he has to say about it and the 10w40 oil weight.


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## Old Man Taylor (May 9, 2011)

tjta said:


> The oil pump shaft is located off the oil pump which is accessible by removing the oil pan I’m assuming?
> 
> 10w40 with the added zinc will be heavy enough to keep up the oil pressure during idle?


Yes, the oil pump is accessible by removing the pan. But the first step to remove the pan is to remove the engine. 10W40 will be fine.


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