Thread: Engine re-assembly techniques

I beleive that you should always check bearing clearances, either with Plastigage or micrometers, the clearance between bearing surface and crank journal is critical to oil pressure. Too much clearance (over spec.) will result in lower oil pressure and premature wear. Crank machining can be a hit and miss affair, I once had a Cummins (14lt. truck engine crank) come back from a reputable Machinist shop with a 10 thou. step in the middle of one of the main journals. Not picked up by the shop, but noticed by me when miking the journals befoe installation. Cost $5000 for a new crank, back in the mid 80's. With cranks the bearing journal fillets are critical and should be checked against specs. after machining.
I always gap every ring in it's respective bore before fitting to pistons (likewise piston clearance). I once had a set of rings to fit to a Mercedes truck engine (6cyl.), the ring packs were supposed to be ready to install, came in a pack of 6 so they couldn't be mixed up. 2 of the compression rings, for different cylinders were actually overlapping by over 20 thou. when they should have had a gap when measured in the bore.
I also like to deburr excess sand casting marks in engine blocks and I have a range of wire and fibre bottle brushes for scrubbing out oil galleries, I always replace every welch plug, water and oil.
Any extra time to make sure everything fits/specs is time well spent, Regards Frank.

you raise some interesting points... I often wondered how much is myth and how much is based on reputable "physics".

FWIW Re. the use of plasti gauge for example, my "understanding" is that even when the 'numbers' are stamped on the bearing box, so to speak, there are manufacturing tolerances, which however slight can add up to work against you, so you need to assess the correct clearances. Provided the bolts are the proper bolts, and they are torqued well within their tensile range, there should be little/no concern about do up /undo a couple of times.

I also got to thinking about how things are done in 'new car land'. There'd be no time to use plasti gauge on an automated assembly line for example... Perhaps it's because in this case they're dealing with rigidly enforced precision specifications and CNC machines which can effectively replicate a perfect copy... whereas with a DIY rebuild, everything is measure,measure, measure and...measure again!!!

Using locktite raises other issues: if it begins to cure before final torque, then the expected relationship between specified torque vs clamping force is compromised as some of the torque is used to overcome the Locktite and ... So, stick with oiled threads unless OEM instructions specify otherwise. my 2 bobs worth

you put oil on the bits that are supposed to move and not on bits that arent....

This means no putting oil under the back of bearing shells, I used to believe that you oiled both sides of the shell but was then shown the difference between the clearances (using plastigauge) of shells that went in dry and shells that went in lubed on the back. That was the day that I shifted my concept to lube the crank, not the bearings.

if you're torquing a nut or bolt it needs some kind of lube unless the manual says otherwise (dissimilar metal threads, some locking bolts/nuts)

I lube the bores and piston prior to ring assembly, I use a rag drowned in a thick oil (40/70) for the bores and a very thin oil on the piston and rings

I like to use plastigauge as it confirms the calculated clearances, The first time I came across a cap with a crack in it it had measured up fine but plastigauging came up with a clearance about 3 times what it should have been.

I prefer to use the Feeler gauge method of setting crank and cam thrust clearances over the DTI method.

Thanks for the response guys this is exactly what discussion is good for showing everyone's thoughts

I personally can see the advantage of using plastiguage as I have also had problems with "reputable machinists grinding a crank way to small" luckily for me they are a cheap and easy crank to find unlike a diesel one such as the one tank had...however for some of us isuzu owners we need to be carefull as our cranks are nitrided or "tufftride" which prevents grinding so measuring is the only thing that can be done, if found to be standard size I can't find anything that can be changed
as for bolt strength I cannot comment to much as I do not know enough about the topic however I do doubt it would weaken the bolt like mentioned but something just tells me its not that good for the threads etc etc

Good points about oiling vs loctite

Simply put, measure everything that can be. If you've gone to all the effort to tear it down to whatever level (rings / pistons and rings / pistons, rings, bearings / etc) then does it REALLY take that much longer to measure things? I found something interesting with my V8 build. Atmospheric conditions made substantial difference to the measured ring end gap clearances due to expansion. Final check and end gap adjustment had to be done all in one night to get them the same. My experience with machinists is mixed, my experience with new components even more so. Measure everything. The new "ready to go bearings" supplied for my build were not ready to go without machining the centre crank journal for correct bump clearance. Little oversights like that can be show stoppers. Bump done with feelers, all other bearing clearances done with plastigage.

Being clean is another thing. It takes a heap of work to clean a block after being worked on, and freshly bored cylinders need very careful cleaning to remove any abrasive debris that will chop up pistons and rings. I scrubbed mine with normal A4 paper pads soaked in shellite. This liberates any grit from the bore and binds it to the paper. When they come out clean, they're clean. Any gray and you've still got work to do. Cleaning my block took hours, but I was happy knowing it was done fully. It's important to oil the bores with engine oil straight away to prevent corrosion, and I just use small pads of clean cloth and engine oil - only enough to seal them, not drench them. Rings also got the same clean and oil treatment.

Originally Posted by Tank

I beleive that you should always check bearing clearances, either with Plastigage or micrometers, the clearance between bearing surface and crank journal is critical to oil pressure. Too much clearance (over spec.) will result in lower oil pressure and premature wear. Crank machining can be a hit and miss affair, I once had a Cummins (14lt. truck engine crank) come back from a reputable [snip]
Regards Frank.

I agree with everything Frank has said as that's how I was taught too, and I always do a trial build,then pull down and rebuild for real. (or did, it's been a long time since I've assembled a complete engine.)

I'd never used plastiguage until I did the big ends on the Tdi a few years back, always micced bearings in the past.
When building race engines, whether right or wrong, we just felt it was more accurate. Interesting little anecdote by Dave re the cracked cap, so it shows that the Plastiguage has its place and was a huge help that time.

As Dave said, lube on cap bolts to get the correct tension.
When done to the correct tension (stretch) they'll never come undone.

Having read ARP's little story on lubes and fasteners I'd be using their thread lube for cap and head bolts these days. Supposedly much more consistent tension is achievable than engine oils and moly added home made solutions.


And clean clean clean, scrub and re-clean everything.
You can't be too clean.

Especially for a novice builder (me) Plastigauge is the bee's knees. Used it for every possible clearance when I re-ringed/bearinged my Passat engine late last century. Was so much fun matching each bearing to it's journal. Same with the Big Ends. That and being super-clean.

Engine did another 200K, still with no excess oil consumption when it was sold.

I also find what blknight said about an unforeseen crack is picked up when using plastiguage an interesting point..

I have never used plastiguage I've always done as rick said and the old bloke that shows me how he preps his race engines also micces everything too but all these points are very interesting...