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MG MGB Technical - Dished Pistons - Re-Using

Is it true that dished LC pistons can be retained if modifications, ie skimming the head or block, or both, are carried out, as opposed to throwing out these original pistons, and fitting HC pistons.

Somewhere I read this creates the same, or similar effect of converting to HC pistons.
Jack

Sounds as though you were drinking too much liquor, and not enough branch water with it because you seem to have foxed our fellow MG'ers out there as well as yourself.

Seems a silly idea.
Bob

Seems a pretty expensive way for saving a few $$$$$. skimming the head and 'decking' the block are irreversible and the next time your car may blow a head gasket you may need to replace the head.
M Landskroner

Jack. Doug Jackson has the information to figure compression ratio on his website, something to the effect of www.mgbmga.com . As M notes, you can only skim a little from the block. More can be taken from the head, but you can then run into problems with the valves hitting the side of the cylinder wall (depending on size of valves and which engine you are starting from). The "low compression" pistons are 8:1 while the "high compression" pistons are about 8.8:1, not a great deal of difference. The reuse of old pistons can be done, but is not a great idea. There are two reasons for this. First, the pistons can show some cracking and wear in the grooves where the rings fit. Thus, the old pistons would need to be cleaned, measured and crank inspected. Secondly, if you are using the pistons currently in the bores, the bores are probably worn sufficiently that the pistons are no longer a proper fit. The engine has to be disassembled and the bores measured for wear. Thus, while it is possible to modify the block and cylinder head to raise the compression ratio when using the LC pistons, it is not a particularly effective modification from either the engine life or the cost effectiveness viewpoints. Les
Les Bengtson

the pre '75 heads had a large combustion chamber and used pistons with a shallow dish. Using the shallow dish early pistons with the late -post 1974- heads will increase compression. Pistons are way cheaper than milling a block & head and as said above are not irreversible like the milling process. A firm in Australia sells flat top pistons (four ring versions only). You can also buy flat top forged pistons (strong & expensive).
Barry
Barry Parkinson

Thanks Les and Barry.

The engine is a low mileage '79, and the bores have been inspected with a slight nominal ridging, at the top.

Unfortunately, one or two rings were stuck and snapped during an attempt to unstick and clean.

Would like to re-use them, but Les's comments strike home, although when I read Barry's comments I wonder if it is possible.



Jack

With a low mileage engine you should be able to install new rings and reuse the old pistons assuming the piston wear is within limits and there no evidence of damage. Check that cylinder diameter wear is within acceptable limits. Remove the cylinder ridge and home the cylinder walls to break the glaze. Skim the head enough to assure a good seal and check the flatness of the block deck. Skim the block deck if needed to assure a good seal. I would also replace the big end bearings. For a normal street driven car you'll probably mot notice any difference between HC and LC pistons. What I have described is known as an overhaul rather than a rebuild and it's a common practice in automobile repair. The overhaual isn't as good as a rebuild but it will cost less than half the cost of a rebuild. As for the LC pistons I believe they were used beginning in model year 72 for USA cars. FWIW, Clifton
Clifton Gordon

Jack. My daughter has a 77 with header and a Weber DGV. Her engine seems to perform as well as my 68GT which is stock, but has the HC pistons. She has no trouble keeping up with me when accelerating or when travelling at highway speeds.

As Clifton notes, on a low milage engine, if that can really be documented, it may be possible to use the old pistons. However, the fact that the rings broke would be, to me, an indication that there may be other damage. Why did the rings break and did they damage the pistons in the process? When we rebuilt the engine in my daughter's car it too showed minimal ridge at the top. However, when the bore was measured, it had sufficient wear to require a rebore. The camshafts for the RB cars are also a problem. They have been referred to as "soft". This seems to refer to either the metal used in making them or the heat treatment process. In either case, I have seen sigificant wear on several I have removed, more than would allow the cam to be reused effectively. (Yes, the engine would run, but the cylinders would be out of balance due to valves not opening as far as they should.) Yes, new pistons are expensive, about $200 for a set with rings. However, that is quite a bit less than the cost of a rebuildable block if a piston should let go. Les
Les Bengtson

all things being equal a hi compression engine will outperform a low compression engine any day.(power & mileage) Measure the taper in the cylinder. I rebuilt a chevy 350 v8 years ago and threw in stock size pistons and new rings. It worked real well as long as I didn't go over about 3000 rpm or about 65 mph. If I started driving 70 - 75 mph (3500 or so) the oil mileage dropped down to about 400 miles per quart. A high reving engine as the MG is ( by necessity because of the 3.9 rear end ratio) needs a nice straight cylinder with no wear taper.
Barry
Barry Parkinson

The pistons were all gunged up because the car had done no mileage over the preceeding ten years, bar an occassional rev up.

The pistons themselves are stained, is it varnish, but otherwise appear as good as new.

I was also intending to put a better cam in, is it a piper 270, (just observed one or two recent threads on similar topics), so that would hopefully get around Les's observations.

I want a bit more power, and hoped these dished pistons would be re-useable, provididing modifications were made elsewhere, if possible.
Jack

The original pistons should be Hepolite, a plus point, and an engine reconditioner told me that by skimming the head, the compression could be raised to the same level as if you fitted new High Compression pistons.

Sounds as though you can maintain originality, continue to use quality pistons, and maybe save yourself a few bucks by skimming the head.
Phil

Jack- I've just completed a rebuild as you've proposed. The original intent was a Han's OZ Super Charger, but I gave up after going over cost numbers and potential problems: I had, however purchased the new, dished pistons for the 8,0:1 comp ratio needed for the SC. I mocked up the shortblock, and was surprised to learn the distance from top piston - deck was 33k. Burgess is emphatic about 10-12 thou for proper squish and flame prop, so I decked the block 20k( $100) and took another 10k off the heads ( $40) for a balanced cc of 38 per chamber. I now have a calculated 9.75: 1 CR using the CI head, and 60 over dished pistons. I installed the Piper 270 which "degreed in" exactly after scrupulous double and triple blind checks with stock key by nothing more than "lining up the dots ( blind checks are achieved by promising yourself not to look at the degree wheel until the dial indicator says "TDC"), new Doug Jackson Rockwell hardness rated followers, ARP head studs, Auto Zone dizzy speced to 64 curve ($64 complete), pertronix, Doug Jackson roller rockers ( 1.55:1), respindled/bushed HS4, K&N, and Peco.
I'm just running her in now, but must tell you the performance is a smile: quick as a scalded cat.
vem myers

Vem. I would, most strongly, suggest you check the advance curve of the AutoZone distributor. I have pulled them apart and the advance curve was not per factory specification. The last one I checked ran a full 16 deg (crankshaft) more than was correct for the early cars even though it was purchased as being a "68 Specification" dizzy. This can result in detonation. Les
Les Bengtson

Vem, that sounds almost identical to my rebuild. Except I just installed a new rocker shaft, not roller rockers. [can'tafforditjustyet] And my cam is a local regrind up from the stock '73 engine to about halfway to a 1st stage.
I did have the block 'decked' down to the top. or close to the top of the 8.8 pistons. Then all was balanced, rods lightened, flywheel lightened to 15 pounds. [stock is 22 lbs.] It's not as light as the aluminum unit, but I can afford it now. Home built port and polish. seems to work ok. Still could get some more performance if I could lose some weight. Not me, the car. [okicouldtoo]
Now a crossflow aluminum head, aluminum flywheel, two Weber's, or F.I., Electramotive distributorless ignition. $$$$$$$$$$$$$$$$$$$$$$
Safety Fast
Dwight
Dwight McCullough

Vem, am I correct in assuming your new dished pistons were +60th, seems to read that way.

If so, I cannot use my original pistons, and shave the head & block as you did, if I wish to follow your speedy spec.

Is that correct.
Jack

This thread was discussed between 06/05/2003 and 19/05/2003

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