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MG MGB Technical - 1971 Pistons

G'day all.
Could anyone please advise how to determine if I have High or Low
compression pistons?
Also if the top piston face should be level with the block face?
Thank you.


231

231
High comp pistons -- 8cc dish
Low comp pistons -- 16cc
The piston tops don't quite get to the top they vary a bit std probably 1/2 to 1mm down
If your car was an Aust. delivered car it will most likley be high comp. What year model have you got there
willy
William Revit

G'day Willy
I have a 1971 Oz built Roadster.
I require the block face machined and and I'm concerned about the Compression Ratio and the piston/block face.
Thank you for the info.
Dave


231

Dave
If you have to have the block machined, it's just going to have to be. Usually your engine machinist would guide you there
Normally it would be a small amount that has to come off to correct that blown headgasket burn and won't effect things TOO huch but if it's bigger than that a replacement block or fitting cylinder sleeves are options Your machinist should be able to advise the best way to go
How big is your problem area and between which cylinders
willy

Where abouts in WA are you - There are plenty of good MG fellas over your way
William Revit

G'day Willy.
I think I only require a very small amount off the block, but the head also requires a skim and I'm concerned I may be pushing the Compression Ratio a little.
Also require new pistons, and I'm not sure if I need to order High or Low Compression ones.
I live in Kalamunda up in the hills.
Dave



231

231-
Since yours is a 1971 model, your engine should be an 18GK version with a 12H 2389 head casting that has a 43cc combustion chamber. The Original Equipment piston crown to deck depth is .040". Research by Peter Burgess has established that the optimum squish is achieved at a depth of .012". That would give you an increase of compression ratio of about .5:1. The piston should never be flush with the top of the bore as due to the 002” to .003” (.0508mm to .0762mm) "stretch" of the connecting rods at high engine speeds the pistons will hit the bottom of the cylinder head.
Stephen Strange

Dave
You're not far out of Perth there so good fuel shouldn't be a problem
Your choice of pistons will come down to which fuel you are/intend to run it on
Because of having good fuel available to you there, to stick to the high comp' pistons which with the machining you mention should still keep comp. under 9.5:1 and that would run fine on 95 premium
However if it's a std. fun car and you don't need optimum performance I'd be tempted to go for the low comp. pistons and you should end up at about 8.5:1 on them which is a tiddle lower than the 8.8 std. high comp ratio spec but would save worrying about engine pinging etc

One thing to watch
Your engine I imagine should have circlips holding the gudgeon pins in
With low comp. pistons their are two types
With or without circlip grooves - the later versions had press fit pins and no circlip grooves - so care will need to be taken when ordering to get pistons with circlip grooves if you go the low comp way

There are some good guys down at The Sportscar centre at 55 Poole St Welshpool which isn't far from you
Might be worth calling in and showing them what you're up to
hope this helps
cheers willy
William Revit

Thanks Guys.
All your advice appreciated .
Dave
231

Dave as Willy said you will need to be careful when ordering pistons to get the correct gudgeon pin type but I would stick with hi-comp as the small amount on a head and block clean up will only lift the compression a few points. One of the main ingredients to make a B go is compression. (it takes about.o60" for one full unit) and I feel you maty be disappointed with lower compression. Denis
Denis4

This thread was discussed between 10/12/2014 and 14/12/2014

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