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MG MGA - Engine rebuild
|my winter project is to rebuild the bottom end of my 1622 engine and I'm starting to think about the things I can do and those I can't. At the moment I'm undecided about whether one of my pistons (or others) can be refurbished or need to be replaced. In the pic the piston on the left is similar to the other two with smooth sides but has 'burn' marks by the gudgeon pins. The piston on the right however has scouring on its side as shown that you can feel with the hand. Can these be buffed out or should it be discarded? Is there a definitive guide to engine rebuilding that anyone has seen?|
|J H Cole|
|Should have added pics
|J H Cole|
|Surely there must be some scouring on the bore too? If it needs to be re-bored then you will need new pistons.|
With reference to engine rebuilding (and tuning) I have a couple of engines to do and have been planning to get this:
Does anyone have this book? Any comments? Maybe I should ask on the MGB forum?
|Neil, yes there is scouring but very feint and not that you can feel, more a patination.|
|J H Cole|
|Moss sells an MGB rebuild video (in VHS? and CD formats) by "Dr. Doolin". I don't know who he is, or what his credentials are, but I purchased this video a few years ago. It came with a comprehensive set of notes, most of which applied to the MGA engine as well. I never watched the video as I had a good set of notes from attending a University Motors/John Twist rebuild seminar, but I did refer to Doolin's notes along the way. And don't ignore the shop manual, although there is an error there about the direction the crank thrust washers are to face upon installation; the shop manual has it backwards. |
|JH, look for a wear ridge near the top of the bore where the rings end their travel. If there is an appreciable "step" then you will need a re-bore and, as Neil says, new pistons anyway. If your bores are not badly worn or scored, then you can clean them up with some fine emery paper to remove any glazing and re-ring the pistons. That scored piston might be re-claimable with a good clean and some easing of any scoring, but it does look, from the photo, that something has happened there, difficult to tell from a picture and without having it in your hand. Old pistons should be thoroughly cleaned to remove all the varnish and carbon, especially in the ring grooves. The pistons don't form the seal, they are merely carriers for the rings, but they should be smooth and free of scratches or bumps where the alloy has "picked up". A nail file is quite a good tool for easing high spots on a piston, don't use emery on them because it can become embedded in the alloy.|
|George, my issue 5 copy of page A12 shows them the right way round.|
|I seem to remember that the text wording was incorrect in how it described inserting the thrust washers. I'll check it when I get home.|
|George, see what you mean the text does seem to be ambiguous, "replacing the thrust washers in their correct positions at the centre main bearing with the oil grooves towards the bearing." main bearing or thrust bearing surface?!|
|The oil grooves must go to the face where there is relative motion, so the crankshaft side. that's where the oil is required!|
Re the piston scratches - that is an unusual place to have wear. Most wear would be at 90 deg to the gudgeon pin, this is where the sidethrust is. This is also the area where you can check the piston to bore clearance and compare it to the book.
The other test is a compression test. If that is OK, likely you do not need a rebore or pistons. Also what is the oil consumption and blow-by?
|Yes Art, I know that. The picture shows the correct orientation of the faces of the thrust bearings, but the text that goes with it leaves a lot to be desired! When I pulled my engine to pieces, I found one pair of thrust bearings the right way round the other pair the wrong way round. The previous engine builder must have been as undecided as the text!|
|The BMC company released a Service Memorandum advising that the Workshop Manuals had an error concerning the fitting of crankshaft thrust washers. Image attached.|
|M F Anderson|
|I found the Dr Doolin video to be very helpful when I rebuilt my engine a couple of years ago.|
|Art, unfortunately I don't have figures for when the engine was running so I'm just inspecting the components. The scouring on the piston is strange because there's hardly anything to feel on the bore but there is some markings. If I do go for a rebore are the flat top hepolite pistons still available? Thanks for all the comments.|
|J H Cole|
|Checked my notes from the rebuild seminar, the thrust washer slots should face the crank not the block, as Art said.|
|JH, clean the carbon from the unworn part of your cylinder right at the top, remove one of your piston rings and fit it in the bore so that it is absolutely flush with the block surface and measure the ring gap with a feeler gauge and note. Now push the ring a couple of inches down the bore with the skirt of the piston and re-measure the ring gap without disturbing its position. Post the two readings on here and somebody will tell you whether or not you need a re-bore.|
|Thanks LS. I will try and get this done over the week end. One thought though -what about ovality of the bore, does this come int it? Should I take readings at 90 degrees?|
|J H Cole|
|Bore ovality by itself does not matter. What matters is the clearance to the piston and how much the piston can rock during the strokes. The clearance that matters is measured at right angles to the gudgeon pin. Most modern pistons are in fact made slightly oval, being tightest where I mentioned.|
|The ring gap should tell you enough about bore wear no matter where you arrange for the gap to be, but you could try turning it to see if you get different readings. Generally though, if you can catch your finger nail on the wear ridge about 1/4" down the bore, it is usually an indicator that a re-bore is due.|
|The piston on the left is normal used, carbon build up where it doesn't touch the wall. The marks on other one are probably a result of rust in the bore from the engine sitting at some point. If there is no scoring on the thrust faces = the clean parts of the LH piston, then it should be OK. The marking on the cylinder wall is likely minor rust pitting, not a problem unless it is deep. |
One point is that if the pits in the wall are in a ring form, they can result in a "catcher" for the piston rings; that calls for rebore. Another is that I have taken apart a number of engines that were "stuck", freed up, and run with broken rings - that eats the ring lands. If you reuse these pistons, check ring side groove clearances carefully.
As far as the step at the top goes, it can be removed if within limits, followed by honing. In fact a light honing will show up how bad the wall rust is. Since you asked, I would suggest that you have the parts examined by somebody who knows what they are doing (automotive machine shop); too much work and money to do it twice!
|That black looking piston is still a worry. I had a problem which necessitated a second rebore and rebuild.|
The second time I discovered that the con rods were still mixed up from before I owned the car. Note - the con rods are not all the same. Nos 1 & 3 have a different Pt No to Nos 2 & 4. There is a big end offset to make room for the main bearings.
The offset is very small but if in the wrong bore it puts a sideways strain on the conrod and forces the gudgeon pin and piston to one side and gives a very, very tight engine! Could that be your problem? Not saying it is, but its a thought...
When all is refitted, with the engine upside down, just to make sure, look to see that you can see equal amounts of gudgeon pins glinting up at you either side of the small ends.
Hope this helps.
|Thanks Pete, My con rods are stamped 1-4 is it simplistic to assume they fit in this order? I'm in Eastleigh, where are you approx in Hant?|
|J H Cole|
|The WSM shows the offsets in the engine sectional diagram|
This thread was discussed between 10/09/2010 and 19/09/2010
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