Alignment of cylinders and crank bearings

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mr-mechanical

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Hi all

I plan to start making a straight 4 cylinder engine in the near future. My first one.

Like lots of you probably do, I spend lots of time thinking about the setup for each part

Recently I've been thinking about the setup(s) required for machining the block and how to properly align the cylinder centres with the crack center and how best to achieve alignment of each crank bearing. I.e. preferred method / setup.?

Can anyone point my to some posts / photos that would help / show me?

Thanks in advance
 

Asm109

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Machine a flat surface on block. Could be head mounting surface or the surface the barrels bolt to if they are separate.
Machine an edge along the side of that surface.
Use those two surfaces to set up and line bore the bearings.
Use those two surfaces to setup and bore the cylinders or the barrel locating holes.
 

gbritnell

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Rather than trying to 'line bore' main bearing bores on small engines I started making the openings as milled slots.
The bearing are then made as a tight fit in the slots. This way the accuracy only needs to be in making the bearing inserts and not trying to line bore a small diameter hole.
 

gld

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Mr,
I am currently building George Britnell's 4 cylinder overhead valve engine. I started out on the bottom of the block locating the Centerline of the engine. I then drilled and tapped the holes for the main bearing cap bolts. Then I made a bar of aluminum the size of the main bearing caps drilled and bolted it in place on the bottom of the block. I stood the block on end in the Mill Vise located the crankshaft Centerline and drill the hole full length of the block. I wanted a 3/8 finished hole so I drilled 11/32 hole and reamed it with a 3/8 reamer. Then I laid the block down hollowed out the bottom end. When that operation was finished the main bearing caps were left standing ready for the inserts.
I know I'm going to get heavily ridiculed for this technique but I have a very free turning crankshaft.

George check your e-mail
 

gbritnell

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Hi Gary,
I didn't receive any emails from you, yet.
gbritnell
 

peterl95124

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I use a ball-end-mill at 1/2-diameter depth to cut the case,
ditto for the caps. works like a charm.
Peter.
 

MachineTom

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Most engines (full scale) have the crankshaft C/L offset to the cylinder C/L. This to reduce the angle of the connecting rod on the power stroke. This will increase engine torque a bit. combined with offset in the piston wrist pin, makes the piston skirt bear a bit heavier to one side, thereby reducing the sound of piston slap.

Although running time on most model engines is measured in minutes per year, not hours.
 

Steamchick

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Not seen it mentioned, but 2 factors are most important. 1 is the perpendicular it of bores to crank alignment. Less than 1 degree error is most important. Nothing less than "spot-on" is the desire, so set-up is critical. If you can set-up from a precision bar along the main bearings, set in a pair of v-blocks, or similar, and checked with DTI before progressing, then bored. If you use a datum face, or flange face, you must be absolutely certain it is parallel to the crank centre-line.
Second point. Bores, and crank bearings, must be made with a rotating tool, which traverses a stationary workpiece. Knowing the rule, I tried to "Cheat" once, and put the workpiece in the lathe (on surface plate) and bored with a normal projecting boring bar from the tool-post. WRONG! I simply copied the error of mis-alignment between the bed and headstock of the lathe, and could feel the taper between piston and bore when assembled. Careful honing and a larger piston rectified the engine, but everything since has been from a rotating tool traversing the bore - as I was taught 50 years ago!
Please learn from my mistakes so you get it right first time and every time.
Many "professionals" will laugh at my stupidity, but maybe someone will learn from my lesson.
Enjoy!
 

markn1061

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I use a ball-end-mill at 1/2-diameter depth to cut the case,
ditto for the caps. works like a charm.
Peter.
I was going to do the same too. Did you use an oversized reamer with the caps bolted In place to clean up the holes?
Mark
 

peterl95124

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I was going to do the same too. Did you use an oversized reamer with the caps bolted In place to clean up the holes?
Mark
i put split bronze bearings in there, so there's no reason to clean up the case and caps, and besides if you try to the reamer will catch on the ever so slight discontinuity between the case and caps and you won't be able to turn it.

I will add that the case will come out perfectly aligned, but the same can't always be said of the crankshaft itself especially if it is long (like for the V-12 I made recently). the solution I found was to put garnet lapping compound on the bearings and turn the crankshaft in place while slowly tightening the cap bolts. After a couple rounds of that I ultra-sonically cleaned everything, oiled it all up and reassembled it, and it turned over like silk !!!

Peter.
 

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