skim - how much?

[Peetee]

Some time ago my modified 1275 was rolling roaded and the compression was shown to be a bit low. Not suprising really as there was a fair bit taken out of the combustion chambers thanks to Mr Vizards excellent tuition.
What sort of skim would be recommended to bring a compression of 150 back up to a respectable level?

[Mogwai]

http://www.infomininl.org/readarticle.php?article_id=17 just type in 1275 & the base figures will come up

This wont tell you how much to skim off but useful for determining the compression ratio & the combustion chamber volume
I think I had about 10 thou of mine which took it down about 1cc or there abouts

[bmcecosse]

I would say 40 thou - but are you sure the low compression is due to ground out chambers, and not just wear and tear? to get an answer for sure - you would need to measure the chamber volumes - perspex and burette!

[Peetee]

I would say 40 thou - but are you sure the low compression is due to ground out chambers, and not just wear and tear?

Pretty sure. it's a good engine, doesn't use oil, no undue noises, no crankcase pressure issues and the head was clean when it was removed. I suppose i should have done a compression check when i started just to check especially as it's an 'unknown' A+. I'm not aware of a low compression version of the A+ units. The pistons have a small amount of dish and look pretty much like this:

[Matt]

Flat top A+ pistons are rocking horse and AFAIK they never came from the factory.

And there were low compression 1275 a+ as fitted to marina/ital commercials.

When you are taking the head off see if there are any markings on the pistons that could be used to identify them. (BTW I have a single low compression piston on a rod sitting on my transit dashboard if you wish to compare!)

[Peetee]

Number on the piston is 21961+ 508

[d_harris]

(BTW I have a single low compression piston on a rod sitting on my transit dashboard if you wish to compare!)

But not use

[Peetee]

Hello Matt?

[Mogwai]

Number on the piston is 21961+ 508

Has it had a rebore? as its got +508 on the piston which is metric for 20 thou

[Peetee]

Quite probable. It is a Gold Seal unit.
I haven't got Vizard to hand. Does that make it a 1293? And, can't quite get my head round this at the mo (too late ), won't the overbore have some bearing on the compression and the optimum skim? Would be great to have a ballpark answer. it's booked into the engineering shop on Wednesday.

[bmcecosse]

+20 makes it 1310cc. Yes it has a tiny effect on the CR - make that 39 thou if you like..........
My estimate at 40 thou is just that - it will raise the Cr about 1 more than you have at present - whatever that is. The only way to do this accurately is to use a burette and piece of perspex, and find out the true chamber volumes. This would of course be done anyway when the metal was being ground out of the chambers - to ensure they are all exactly the same and make for a well balanced engine...........

[Peetee]

So how would ml equate to thou? If the chamber were say, 28ml instead of 26ml.

[bmcecosse]

The chambers on these heads were 21.4cc . Which is why - if used as they come, on 1098 engines - they give a nice big increase in CR without skimming. If used on 1275 engines - if metal is removed from the chamber then skimming will be required. My 'guess' is 40 thou - but I have no idea how much metal you have removed to start with! Hence my guess of 40 thou - but could be 80 thou is required!!

[Peetee]

so 40 though would bring it roughly back to standard Cr of 8.8. I'm looking to get nearer 10:1. What would that look like on a compression tester? 190? More? Im thinking I'll have a go at 60 thou and compression test when built then take it from there.
I've read that 120 thou off is possible from that head but I need to check if anything has been lost from the 69.9mm head depth already. The 150 compression reading should give a clue as to what I've taken off. That's what the rolling road operator measured across all cylinders less than 200 miles ago and he was quite happy with the general health of the engine.

[bmcecosse]

You will really 'push' it if you take 120 thou. I once skimmed 100 thou from a head - and the deck was then so thin it could be deformed with finger pressure........... Your original CR was surely rather more than 8.8 to 1 ? I very much doubt skimming will make all that much difference to the readings on a compression gauge - yes it will help - if you get 170 you will be doing well. 190/200 is getting into competition engine range! All heads start at 2.750" thickness.

[Peetee]

I took 8.8 from the Haynes Marina manual.

[bmcecosse]

Oh - that seems lowish. Maybe has quite heavily dished pistons? Well - if you feel brave - go for 80 thou off the head! But just check it hasn't already been skimmed first - by checking it is 2.750" thick.

[don58van]

Have a look at this video by John Twist. He has done quite a few interesting tutorials on YouTube.

http://www.youtube.com/watch?v=OGuOMItsCXk

Cheers
Don from Oz

[bmcecosse]

His vids are good - if there is ever a dull night on the TV ( ) these fill the gap!