Measuring Crank End Thrust | MG MGB Technical

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MG MGB Technical - Measuring Crank End Thrust

Hi All.

Got my engine back from the machine shop and started to put it together. Using a Haynes and the factory manual.

Only item I am unsure of is how to measure the crank end thrust. It appears that it can be done with either a dial indicator off the front or rear of the crank, but it also appears you can just use feeler gauges between the washer and the crank journal.

Should all the mains be on and torqued when checking the end thrust?

I would appreciate input on how to best measure the end thrust. A step by step on how you do it may be helpful to add to the archives for future engine builders.

I did purchase new standard thrust washers and in checking them against the ones that came out of the motor (67 GB motor, first rebuild, 100,000 miles), there is less than 0.001 thickness difference.

I noticed that with the middle main cap off, there is 0.003 clearance on the thrust washer towards the front of the motor and 0.005 on the back. This is after giving the crank a good couple taps with a leather mallet and measuring the gap between the washer and the crank journal. Would this be expected by

There also is a bit of difference between what the manual says is acceptable end float. Manual says.0.002 to 0.003, but in the archives I see different clearances by different engine rebuilders.

Bruce Cunha

Hi Bruce

I check thrust clearance with just the shells in and set clearance to 3-4 thou on the upper thrustwashers. I then fit the centre main cap and recheck the clearance. I use a dial gauge for convenience. The clearance is total not to front then to back? You imply you have 8 thou in total or is that 5 when you tap the crank towards the rear then 3 when you tap the crank to the front? If so the average would be 8/2 or 4 thou?

Peter

Peter Burgess Tuning

Peter

Thanks so much for the response. In my reading, no on indicated using an average. I used a feeler gauge on the lower bearing prior to taking the crank out when I took the motor apart as I did not have a dial gauge at that time. Gauge passed the washer at 0.003 on the front and 0.005 on the back. So I have an average of 4 thou.

I did purchase a new set of thrust bearings for this rebuild and in checking. I only find a difference of 0.001 between the old and the new, with the new being the thicker. From reading, it appears that there have been some issues with the thickness of the new bearings in the past.

Hoping to have it all set up to check the new parts today.

Bruce Cunha

I agree. We are looking at total clearance here. So fit the upper thrust washers and crank then tap the crank forwards and measure the clearance with a feeler gauge between the crank and the rear thrust washer. This should be .003". If you tap the crank rear wards this clearance should now transfer to the front thrust thrust race.

Iain MacKintosh

Bruce. The use of a feelers gauge is an indirect method of checking end float. It may be correct, but it may also give an incorrect reading depending on user experience and the exact shape of the parts being tested. The proper method of taking end float, whether a crankshaft, camshaft, or other part is to assemble the system and tap the part being measured fully rearwards. Then, install a dial gauge on the nose of the part being measured and zero the dial. Move the part being measured forwards until it comes to a full shot and read the end float off the dial gauge.

When checking the camshaft, remember that the thrust plate and the sprocket need to be installed before taking your reading. Without the sprocket, there is quite a bit of excessive end float of the cam. With the sprocket, which limits rearwards movement, the correct system end float may be observed.

Les

Les Bengtson

Les is absolutely correct but the reason for me suggesting the use of feeler gauges was because most owners may not have or have access to one. I had assumed that when the feeler gauges were being used that some care would be taken to ensure that the blade went directly into the clearance opening and did not foul on anything that was likely to distort the reading

Iain MacKintosh

Iain. The other problem with feelers gauges is that, to work properly, all of the parts must be absolutely straight and you must be able to ensure that the gauge goes into the part(s) fully. In theory, the parts should be absolutely straight and the exact same thickness on all of the sides. In practice, parts are not absolutely perfect and there may be some difference in the readings given with a dial gauge vice the readings taken from feelers gauges. Since Bruce mentioned that he had a dial gauge, I would suggest he use it and get the most accurate reading possible.

Les

Les Bengtson

Thanks all. Picked up a nice magnetic dial indicator at our local flea market. Got it set up and ran the thrust for both the old and the new thrust washers. Measured 0045 to 0049 on the old washers and 0022 to 0026 on the new ones. My manual says 002 to 003 is desirable, so it looks like I am right there with the new washers.

Crank turns over very smoothly. So on to the piston rings.

In other engines I have done in my youth, we would measure the ring gap by fittig a piston without rings on it in a cylinder. We then put a ring in the cylinder using the piston to square it up and then measuring the gap. Anything different for the B?

Bruce Cunha

If you use two of the new thrusts one side and two of the old on the other you may get 3-4 thou which does seem nice on the engines we have built over the last 30 years. If you have the Grants rings be careful as you can fit the oil ring pack incorrectly and burn oil forever.

Peter

Peter Burgess Tuning

Thanks Pete. I will try that. Only takes a few minutes to swap them out.

I have heard that for the rings. I will triple check that.

Bruce Cunha

Bruce-
The secret to measuring crankshaft endplay (endfloat) lies in remembering that steel is actually more elastic than rubber. The elasticity of steel is such that if you drop a rubber ball and a steel ball bearing of identical diameters from an identical height, then the steel ball bearing will bounce higher every time! Consequently, it is always best to gently push the crankshaft as far as it will go rather than to tap it and risk it bouncing either backward or forward. Remember, in this case we are dealing in thousandths of an inch! If you gently push (or lever) it as far as it will go, your measurements will then be much more accurate. These measurements should also be done dry, i.e., without oil, and with the center crankshaft main bearing cap loose on its machine bolts / studs, so that the crankshaft lines up the two sets of crankshaft thrust washers. Only then should you tighten down the center crankshaft main bearing cap and check the endplay (endfloat) of the crankshaft with a dial gauge on the nose of the crankshaft. While too little play is worse than too much, it should be noted that whenever endthrust is applied to the crankshaft (such as when applying the clutch), the lateral acceleration of the crankshaft increases with its lateral movement. Thus, the greater the endplay (endfloat) of the crankshaft, the greater the impact loading against both the crankshaft thrust washers and bearings will be, and, as a consequence, the greater and more rapid the wear will be. The endplay (endfloat) of the crankshaft should always be the same whenever the crankshaft is moved as far as it will go (this is what machinists call “repeatability”). Note that some, if not all, of the new thrust washers currently available are not flat. Instead, they have a bit of a "Belleville" shape. When measured with a micrometer you will only have a thickness measurement. When set on a good flat surface and measured with a depth micrometer (not a digital caliper) you will have the dimension that controls the endplay (endfloat).

Stephen Strange

Bruce, how are you progressing with the engine rebuild

Cheers

Gary

79mgb

gary hansen

The rebuild is moving ahead, but a bit slower than I would like. The delay is due to my building an apartment for my father, that is taking up pretty much all my time.

I did get the pistons and rings installed. Checked the ring gap and all were very close to specification. Also plastigauged the rod bearings and they are all > 0.0015 and < 0.002 so that looks exactly where they should be. Unfortunatly Moss and VB were backordered on the rod bolts, so I have not done the final torquing on the bolts yet. It took a while to get them. Came in this week, so hoping to have them torqued down and install the cam this weekend.

The rebuild Kit I purchased has a upgraded cam and an adjustable timing gear. I am thinking that the adjustable timing gear will require some questions on my part.

Any articles on these out there that will give me more information on them? I do have instructions, but would like to read up a bit on setting these up.

Bruce Cunha

Hi Bruce, pay close attention to the end float and mark up the the vernier against your old gear to give you a starting point.
Peter

Peter Burgess Tuning

Thanks Peter.

The thrust plate has minor wear marks on it. I have read that a light polishing with fine sand paper on a glass plate is an acceptable fix as long as the end thrust is still within specs. Your take on that?

Bruce Cunha

Sounds a good idea to me. we usually find we have to take 12thou off the vernier gear to get the clearance correct, probably depends on make though.

Peter

Peter Burgess Tuning

This thread was discussed between 12/11/2014 and 11/12/2014

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