The smaller Fiat engines are unique in that the center and rear cam bearings are NOT presized, so that means that they most be bored AFTER they are installed. Some time, in an emergency situation, I made a very simple boring bar that allowed these bearings to be bored, as not one of the 5 machine shops in-and-around Raleigh NC was set up to do this work. At the time I need only to do the center bearing, so I made a bushing to fit in the front bearing and the rear bearing to hold the 1" diameter boring bar. However, I needed a better solution so that I could do the rear one as well. Since I was re-engineering the whole set-up, I thought I would also make the bar capable of line-boring main bearing bores at the same time.
One of the largest concerns is doing line boring is repeatability. As the recommended cam bearing clearance is 0.001 inch, very close tolerances must be maintained.
I started out by making two high tolerance bushes that would fit in the front and rear camshaft journals.
- Bearing Bushings.jpg (59.41 KiB) Viewed 76 times
The front bushing was a standard bearing with an insert pressed in and then bored to tight sliding fit for the 1" alignment bar. The rear bushing was machined for 2" bar stock. These two bushings are inserted in the block and the alignment bar inserted.
- Front bushing in block.jpg (47.71 KiB) Viewed 76 times
The above photo shows the bushing inserted in the cam bearing journal and the alignment bar inserted. In the rear of the block there would be a similar bushing.
Once the alignment bar is in place with the close tolerance bushings, two support plates are attached to the front and back of the block. The 6mm bolts that would normally hold the front timing cover and the rear crankshaft seal hold these two plates in place. The bolt holes in the plate are countersunk and purposely made slightly oversize. Each support plate has two large 2.250" holes bored in it ( one for cam bearing boring and the other for main journal boring). The rear side of the laarge holes, up against the block, are relieved about 1/4 inch to allow a place for material when the cam bearings are bored.
- Plate on block.jpg (53.11 KiB) Viewed 77 times
You will notice that the alignment bar is supported in a larger diameter bushing on the support plate, which is held in place by two allen head bolts. The sequence is that after putting the alignment bar in the block with the two bushings in the cam journals, the support plates are attached to each end of he block loosely. The larger 2.250" diameter bushings are then slid over the alignment bar, at end of the engine block, and pushed home and secured with the two allen head bolts. After this the support plates are tightened securely to the block. The support plate bushings are then temporarily removed from the plates, and once out the alignment bar is removed, the bearing journal bushings are removed from the block. These bushings are stored for future use as they are what the alignment bar precisely in the center of all three cam journals.
For convenience I made three different boring bars, however it could have been done with just one. The three bars have boring bits adjusted to do the middle, cam bearing bore, the rear cam bearing bore and the main bearing journal bore. This is done so that once set these do not have to be disturbed and it makes the job of line boring cam journals fairly quick and painless. The appropriate boring bar is inserted in the block, and then the support plate bushings slid on each end of the boring bar and secured to the respective support plates. Each of the support plate bushings has a small oil hole to allow oil to be applied to the boring bar during the boring operation. Since the support plates, and their bushings were lined up from the alignment bar located in the exact center of the bearing bores, all that is left to do is the boring itself.
- Plate on block.jpg (53.11 KiB) Viewed 77 times
If there was a need to bore the main journal bores the you would use the same procedure. Insert bearing bore bushings in the front and rear journals, torque the caps, and place the alignment bar through them. Attach the support plates loosely to the block. Slide the support plate bushings on the alignment shaft and then push them home in the support plate holes and secure with the two allen head screws. Now secure the support plates firmly to each end of the block. Remove the main caps and remove the journal bushings. Take the three main caps and, using a cap grinder or a surface grinder, remove 0.002-0.003 inch from the mating surface of the cap. Now reinstall the caps to the block and torque to specification. You are now ready to line bore the main bearing journals. I have not shown the boring bar in the main journals, but the idea should be fairly clear.
Cam bearings can actually be done with a hand drill (if you are VERY careful) as the element to turn the boring bar. Advancing the bar steadily takes a bit of practice. I do it by using a right angle head on my Bridgeport Mill. I set the head up on one end of the mill table and the attach the end of the boring bar with a double u-joint (so precise alignment is not necessary) to a collet in the right angle head. Once this is set then the head stays stationary, and using the X-axis handle (or the automatic feed on REAL slow) to move the table, and hence also the block, while the bar stays stationary. I have been able to maintain tolerance of less than 0.0005 inch with this arrangement. This is quite good enough for either bearing clearance and/or shaft alignment.
I welcome anyone wanting to use this design, to go ahead and use the ideas presented here without restriction. The same principles could be used to line bore cam bearings and crankshaft journals in almost any engine, so long as you have the correct bushings and support plates.
