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Crankcase mating surface tolerances

T

tomf

Regular Contributor
"OK, another one in the endles

"OK, another one in the endless series of 15hp rebuild questions...

How close is the fit between the crankcase cover and cylinder block supposed to be? If I place the crank cover on the block, without any bolts, and shine a light at the seam, I can see a very thin stripe of light when peering into the crankcase. Is this normal?

The reason I'm asking is that the mating surface on the crank cover looks like somebody used some pretty coarse sandpaper to remove a gasket, so I'm wondering whether this might affect the fit. History & running condition of engine is unknown.

I don't have a measurement for the gap, but even where it appears to be largest, my thinnest feeler gauge (0.05mm/0.002") isn't even close to fitting in. So I'm guessing it could be something in the ballpark of 0.02 mm or 0.001"."
 
"There is no gasket here. Put

"There is no gasket here. Put all the bolts in and tighten them down, and that gap will disappear.

Jeff"
 
"Woops, you're right Jeff

"Woops, you're right Jeff
blush.gif
,really must start wearing my glasses,and stop jumping to conclusions."
 
when building engines at work

when building engines at work (aircraft) we use silk thread on one case half held down by permatex gasket compound. Just one strand along the inside edge of the bolts or one strand down the length going around each bolt. The thread smashes and takes up any gap. Yes my real job is in aviation.
 
"Its all the same, wrench bend

"Its all the same, wrench bending 101. I just have an o-sheite factor when I mess up."
 
"That is a new one for me, but

"That is a new one for me, but I like it a lot. I will add that to my bag of tricks."
 
"Thanks everybody. Hand-tighte

"Thanks everybody. Hand-tightened the bolts to about half the required torque and no sealant, and the gap went away almost completely, so looks like I'll be fine.

Just wanted to make sure since I've spent the last 5 weeks tearing down and cleaning this thing & didn't want to end up in a dead end on the long climb up Mt. Reassembly...
happy.gif


Cool trick with the thread, JB, have to try that. Certainly can't hurt.

Tom (total amateur marine mechanic & aviation wannabe)"
 
It should be a simple matter t

It should be a simple matter to lap down the parts.Use a surface you know is flat like a drill press table or a milling machine table take a piece of 440 grit sandpaper& tape it to the table. Spray some wd40 on it . Put the part on & move it in a circular motion it will "clean up in a few seconds.No more daylite
 
You can not do that with the c

You can not do that with the crankcase halves; it would change the way the main bearings and crankcase halves fit together.
 
"Check. But in other applicati

"Check. But in other applications, such as fitting a "car engine style" cylinder head where bearing fit does not come into the equation, giving it a light sanding on a flat block would be standard operating procedure, right?"
 
"It is a good way to clean up

"It is a good way to clean up a flat surface, but I do not like to use it for truing cylinder heads. If you have a head that is warped you really do not get a perfect surface this way."
 
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