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Plastigauging for checking crankshaft clearences

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Hap Waldrop

Hap Waldrop

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I thought I would start a new thread on this subject rather than hijacking one of the existing threads. I know Jack is getting ready to do this, so I thought now would be a good time to go over this. Plastigauge is a decent way for the average DIY engine builder to check clearences, but if you don't have alot of experience with this product, much can go wrong to askew your readings.

Checking the mains is easier because the crankshaft has much less chance of moving, the rods are much harder to check because while torqueing and lossening the rod bolts it's real easy to move the crankshaft and if the the crankshaft moves then your palstigauge reading are useless. So the big thing with palstigauge is too make real sure that nothing moves during the process and great care is take when removing rod and main caps, doing it a couple of times is a good way to double check yourself. If the crankshaft moves while you're plastigauging, then it will smear and this will make you think you have less clearence than you really do have.

Jack if you didn't have your rod's big ends resized, then by all means atleast have a machine shop check them.

FWIW, I build all my street engines to .0015" on rods and mains.
 
Hap,
is that .0015 per side or total?



mark
 
Boy am I glad Hap is back.

See my answer on another thread Hap.

Except that my engine guy was so jump on the problem and echoed what you all have been saying you almost got an engine to check.

Will be at Anderson track first weekend in Nov. and was just going to come by.
 
.0015" will be what you see on the palstigauge or what you will looking for. Mark think of it this way, the diameter of crankshaft journal is for example .0015 than the housing bore of the rods or mains loaded with bearings, so that dimension is constant at 360 degrees.

You guys doing clearence can use this simple formula, if you have access to a bearing book, on paper you can do your math and spec to the machinist the clearence you want. Remember if extra clearence is desired (for racing for example) get it on the crankshaft journals, never work outside of the sepc of a rod or main housing bore this is what holds the bearing and should always be within factory specs.

Housing bore diameter - bearing shell thickness x 2 - crankshaft journal diameter = clearence.


Jack you coming up for a Legneds race, yes please look me up. Thanks again for the lead on the legends car buyer in Alabama, unfortunately I could never get the legends car owner to call me back, he's a busy guy. Anyway he called me me last weekend and told me to help sell it for him again, he was ready to get it out of his garage (too many toys) Anyway I put it on Ebay, reserve is $4K, so if any of your legends buddies need a cheap car, tell them to check out the Ebay ad.
 
That seems like an excellent price. It just may go fast.

Yep will be in Anderson for the Legend races. Had in mind to biring my engine to you at that time but my guy down here knew all the answers with out the questions and told me exactly what I needed to do as did you all. So I just gave him the whole job.

Check mains and journals and make em right.

I expect that somehow I got the wrong size mains in her as I get no noise from the engine and would I think if rods were loose.

Anyway, he said he could get on it before the end of the week and would call as soon as he checked it out.
 
Yes too much clearence will for sure result in lower oil pressure, so you may be on to something. The factory spec range on rods and mains clearence can easily get people into trouble, as most just take thier stuff to the machine shop and the machinist if not told a specfic spec will just end up in the spec range somewhere, but on our little cars, that could be as much as .002", which works great in a race car engine at 7000-8000 rpm, but not real good in a street car where such clearences can easily net low oil pressure, that's the main reason I blueprint every motor I build whether it's for the street or racing. When I take a block in for line boring or send a crank off to be ground undersized I spec the exact number in the spec range I want the job to end up at, so in the end I can control the amount of clearance I have. This is something that often times gets overlooked by DIYers, in many way we put too much trust in out machinist to make such decisions for us.
 
You in Anderson Ben? If so PM me an address and a phone number so I can chat if time permits.
 
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