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MG MGA - Getting to know your engine

After tearing apart my “no history” 1600 engine, I was pleased to see STD stamped on the pistons and virtually no ridge at the tops of the cylinders. I figured I had an almost new powerplant. I carried on with the tear down to put my mind at ease, just to see what I had.
I took the block in for a hot tank and I found out that the cylinders were actually .030 over…. and after some head scratching, it seems that I have 1622 pistons in a “1600” block. I’ve since ordered new pistons (the old ones were a little scrapped) and the new ones will be 9:1 compression 1622 pistons (to match the rods). I am also planning on putting on an MGB aluminum head.

Will this all work? 1600 block +1622 (9:1) pistons + MGB head (making eyebrows for valve clearance)? Any problems ?

Any advise is appreciated.

-Cheers
Bruce
B Suelzle

That works.

The 1500/1600 engines used the "15" head with 38cc chamber volume and dished pistons.

The MGA 1600-MK-II used the "16" head with 43cc chamber volume and larger valves, and flat top pistons to make the compression ratio come out right.

Early MGB 1800 engine also used the larger valves and 43cc chamber volume, combined with slightly dished pistons for similar compression ratio.

The key is matching piston shape (dish or not) to chamber volume in the head. Flat top pistons with small chamber could be too high compression for a street engine. Dished pistons with large chamber would be very low compression at a loss of some power (and fuel economy). The second case is easier to fix by shaving the head. Alloy head can tolerate a little higher CR. Fast cams commonly like a little higher CR. Check the cam maker's spec sheet for best recommendation on compression ratio.

If you shave the head (or use the shallow chamber 1972-1974 "L" head) or install a high lift cam, then you may need clearance eyebrows in the block. Clearance from block to valve head at full should be 0.080-inch minimum.
Barney Gaylord

Bruce, I've just finished my 1600 engine and had it bored out to 1622+30 thou. Used 1622 pistons and early MGB con rods, which were used in the early 1800 Bs. I used the original cyinder head but fitted MGB valves and hardened valves seats. I also used the later MGB timing cover with the oil seal and timing marks at the top. You should add this to stop the notorious oil lead at the timing chain cover. I know of one person with the cross flow head on his 1622 and he loves it. I don't see any problems with your proposal.

Good Luck

Andy
Andy Preston

Glad to hear I'm going in the right direction.

Barney am I hearing you correct? The 1622 pistons (VB)I've purchased will be flat top (9:1) and will work with any MGB head 43cc, (but with a 72-74 head I need to cut eyebrows)?
It's hard to know what your purchasing,when looking at a comp screen, the MGB heads seem to fall into either early or late with very little information up front.

Andy I thought about x-flow head, but it's to costly by the time you convert everything over...still it is tempting.

Thanks
Bruce
B Suelzle

Bruce,

There is a website which shows the different MGB cylinder heads.

http://www.flowspeed.com/cylinder-ident.htm



Mick
M F Anderson

Thanks Mick,
That helps alot!

-Cheers
Bruce
B Suelzle

Even with the aluminum head you may still want to have your machine shop port and polish it to acheive maximun flow efficiency. The new ones out of the box apparently are not much better flow-wise than the cast iron ones.
Mark J Michalak

Bruce, -- Yes, 1622 pistons should be flat top, and the 43cc chamber is as original for the 1622 setup. However, the "L" head (used '72-'74 in North America, possibly later in Europe) has one size larger intake valves (good or bad depending on you point of view).

The "L" head is also slightly thinner originally with smaller volume combustion chamber, like it was shaved down to 38cc or thereabout. This puts the valves closer to top of block, so these engines (and all later 1800's) came originally from factory with eyebrows in the block. If you install a high lift cam the eyebrows may need to be ground deeper.

Also if you use the 38cc "L" head with flat top pistons you may have substantially higher compression ratio than you had in mind. This might give you problems for street use with a standard cam and pump grade fuel. If you install a higher performance cam, then the higher compression ratio might be just what the cam maker is looking for.

You will need to measure or carefully calculate the combustion chamber volume in the head (38cc for the "L" head before shaving), volume of the aperture in the head gasket, and volume in the block above the piston at TDC. Add these together for total combustion chamber volume (CV). Add cylinder displacement and CV. Divide that by CV to get mathematical compression ratio.

Check the spec sheet for the cam you intend to use to see if your CR is a reasonable match for the cam.
Barney Gaylord

Thanks Barney
Lots of good things to consider.

Mark I will very likely get the new head ported and polished, if only to get it checked for precision.

-Cheers
Bruce
B Suelzle

This thread was discussed between 18/11/2008 and 19/11/2008

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This thread is from the archive. The Live MG MGA BBS is active now.