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spark plugs

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hemm999 Avatar
hemm999 Ian Hemmingway
Manchester, lancs, UK   GBR
Hi all,
will be receiving my morris minor 1958 on monday,
will be re-building the engine with new rings gaskets ect.
i want to do a compression test before i re-build it, what
thread size is the spark plugs 14mm or 18mm.
Thanks Ian.

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bufferzone Avatar
bufferzone Gord Smith-Ritchie
Surfside Colony, California, USA   USA
14

hemm999 Avatar
hemm999 Ian Hemmingway
Manchester, lancs, UK   GBR
Thanks Ian.

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66jalopy Avatar
66jalopy Silver Member Phillip Jolliffe
Lake City, FL, USA   USA
I never did figure out how come spark plugs were measured in mm threads from way back when.

0123 Mike D
Biddulp, Staffs, UK   GBR
Blame Bonapart Phil he
invented the wretched things sad smiley

MMs not spark plugs.

A Belgian invented plugs,
he'd have been stuck
with the.wretched things too sad smiley

Probably why we are getting out smiling smiley

14mm actually refers to the size
of the "Port" (hole) in your head.smiling smiley



Edited 2 time(s). Last edit at 2017-06-23 07:11 AM by 0123.

66jalopy Avatar
66jalopy Silver Member Phillip Jolliffe
Lake City, FL, USA   USA
Shocking revelation

0123 Mike D
Biddulp, Staffs, UK   GBR
mowogmaca Avatar
mowogmaca Silver Member Grahame Mc (Maca)
Bullaburra, NSW, Australia   AUS
Hi Ian,

I see that you intend doing a compression check ...
Prompts me to ask - what compression reading would you be looking for on a MM with good standard head and good cylinders?

I can't find that spec in my Workshop Book.

Thanks
Grahame

0123 Mike D
Biddulp, Staffs, UK   GBR
You can't find a figure in the
Manual because there is not an
answer to your question Grahame sad smiley

With a working motor.

If you have less than say 10psi between
the highest and lowest readings, that's good.smiling smiley

Someone will dispute that idea,
which is also goodsmiling smiley

Between say 10 and 20psi between highest and lowest is ok.

More than say 20psi between highes and lowest bad.

I compression test a different way from most.
I swing the handle a couple or three times only.

I regard anything more 80% of 15 times the comression ratio as good..
So, with a compression ratio of 19 to 1, 120psi will make me very happy.smileys with beer

emjay Jim English
Marietta, PA, USA   USA
For Fred in the shed, what value does compression values actually tell you that you can't tell behind the wheel? Let's say the differences are more than 20 psi or below some spec value, are you going to say "better tear it down and rebuild"? Or you going to just drive until you can't get up the daily hill you drive? Sure it will tell you which cylinder is failing, but you will probably see that when you take the head off. I can see the value at time of purchase or for a professional to evaluate an engine, but for the DIY if it runs fairly smoothly, soldier on.

Mike, do you think 10 to 20 psi difference is the criteria for all engines? Those numbers may be rather significant on a low compression engine compared to a typical high compression like your 19:1.



Edited 1 time(s). Last edit at 2017-06-25 10:38 AM by emjay.

0123 Mike D
Biddulp, Staffs, UK   GBR
Hey come on Phil, get real thumbs up

Grahame asked "What compression reading would you be looking for on a MM with good standard head and good cylinders? "

I answered Grahame's question.
That's it and all about it.

mowogmaca Avatar
mowogmaca Silver Member Grahame Mc (Maca)
Bullaburra, NSW, Australia   AUS
Thanks Mike and Phil for your comments. I have a reading of 150psi +/- 3 on my 948 on +20th bores. The head is shot with valve guides very worn and oil fuming out the rocker cover breather. Reco head on the way.

I'll retest when the new head is pn. Presently it struggles in 3rd down to 40k ip a hill I have done easily in 3rd.

We'll see!
Thanks Grahame

hemm999 Avatar
hemm999 Ian Hemmingway
Manchester, lancs, UK   GBR
Did a compression test.

pot 1 = 135 pot 2 = 145 pot 3 = 130 pot 4 = 110 Dry

pot 1 = 100 pot 2 = 115 pot 3 = 115 pot 4 = 125 Wet

are these ok.

shoebone Stuart F
tempe, arizona, USA   USA
Mr Hemmingway,
The pressures should of been higher when wet !! Overall, your pressures are on the low side so you should expect to have some work to do sometime.

rusty bucket Avatar
rusty bucket Cam H
Baltimore, Maryland, USA   USA
So I did a compression test of the 1960 four door I bought
Cylinders 1-3 have110 psi and number four has70 ☹️

It misses a bit and is out run by a crippled dog going up modest hills

Did like to swap in a 1275 and 5 speed so I can have a fighting chance, beside I'm deaf from all the people behind me leaning on their horns..

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