The MGA With An Attitude
LINE BORING, Crankshaft Bearings, Rear Seal - CS-107

At 07:31 AM 3/27/05 -0700, Wyatt James wrote:
"I'm trying to line bore a MGA 1600 block. What is the diameter of the mains without bearings? Also, What is the diameter of the rod's big end without bearings?...."

Check the preceding article for Crankshaft Bearing Dimensions.

Just out of curiosity, why are you line boring the block? People sometimes do that if the bearing caps were missing and replaced. If nothing else was wrong, you should be able to measure the original block bore size by measuring laterally across the edges of the block bore (very near the split line of the block journal bore). The block itself is very stable and will generally maintain the proper bore for the main journals indefinitely. I have had one block journal be damaged when a crankshaft was broken just ahead of the rear main bearing. It wasn't really the bearing cradle being damaged, but it made a mess out of the mechanical rear seal bore when the broken stub of the crank and the flywheel were wobbling about at high speed.

The bearing cradles are bored with the bearing caps in assembly. Most of the time the split line will not be exactly on the center of the bore, so bearing caps always have to be mated with the original pairing. If the caps are switched to a different location (or different engine block) the bore size changes. If the bearing caps were lost, and replacement caps were procured, then it has to be line bored to restore original bore size. You can assemble the caps and measure the bores first to disclose the size of the largest bore in order to determine how much it must be reduced. Typical procedure is to shave 0.010" of all bearing caps to reduce the bore. Then reassemble the caps to the block and rebore the cradles in line 0.005" higher than original to restore original bore size. Be aware that this will screw up the gear mesh for half speed shaft drive gears on the Twin Cam engine, but it does not bother the cam drive on the pushrod engines which have a tensioner on the timing chain. Any small change in cam timing will be negligable.

When you line bore the block you need to pay particular attention to the bore size and clearance for the mechanical rear seal, which is not necessarily the same diameter as the block journal. At some point in production the clearance for the rear seal was opened up slightly larger, and the tolerances were tightened up some. End result is that the minimum bore would be larger but the maximum end or bore size would be about the same as before. This may have been done to prevent mechanical contact in the seal when the main bearing might become slightly worn.

There was a case a few years ago where a bloke in Australia had a 1600 block machined narrower (font to back) at the center main bearing cradle in order to install a 1622 crankshaft in the 1600 block, This is commonly done with a race engine to use the stronger crankshaft when the 1622 engine block is not allowed in the 1600cc engine class. In this case, when the job was finished the newly rebuilt engine had a very stiff rotation of the crankshaft. The owner was somewhat impatient and ran the engine without determining the problem. The engine ran for about 20 minutes and seized up. Upon disassembly and autopsy the problem was revealed to be too little clearance (or some interference) in the mechanical rear seal.

At the time I recall seeing all of the dimensions and clearance numbers for the rear seal area published somewhere for the early 1500/1600 setup, and the later 1500/1600 setup, and the 1622 setup. But a couple of times more recently I have not been able to find it again. This is one of the items on my wish list, to find these dimensions and post them on my web site.

The rear seal dimensions are somewhat tricky. The target for the rear seal would be to provide minimal clearance, but to leave enough clearance to allow for a few thou of wear in the bearings without having mechanical contact in the seal area when the crankshaft wobbles a bit in the worm rear bearing.

One additional point also comes to mind. Sometimes when building a race engine the clearance in the crankshaft bearings may be opened up a bit to allow more oil flow for better cooling of the bearings. This is done by grinding the crankshaft bearing journals slightly undersize. This results in lower oil pressure at low engine speed. To compensate for this a high volume oil pump is used to provide more oil flow at low speed to maintain the minimum required oil pressure. An oil cooler must also be used to dissipate the extra heat generated by pumping the increased oil flow past the pressure relief valve at high speed. Having done all this, the crankshaft then has more space to wobble around a bit more within the main bearings (especially at low speed with minimal oil pressure). It is then necessary to open up the clearance in the mechanical rear seal accordingly to avoid mechanical contact there in worst conditions of tolerance buildup. To avoid changing dimensions on the engine block, this change is best done by reducing the seal diameter on the crankshaft to the same reduction as the main journal diameter.

The rear seal dimensions might be calculated to allow proper running clearance (following the spec's for clearance in the main bearings).

In case you have a leaky rear seal, and you may think the cost of line boring is a bit expensive, you might consider installing a custom built rubber rear seal. The rubber rear seal may also be a suitable solution if you don't want to completely disassemble the engine for line boring.

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