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MG MGB Technical - Head Skim Advice on 18V Head please

Hi,
1978 8:1 Compression Ratio...engine has 44K, I added Delta 270 Cam and Roller Rockers, (Harland Sharp), have probably lowered my effective CR further doing this. Car runs twin SUs/eurospec dizzy/peco...runs well, but lacks punch...reading everywhere aboput skimming the head to up the Comp Ratio...head has never been skimmed and is in good shape, Uncracked)...would like to get to about 9.4:1.....will skimming 70thou be about right to effect this?
P J KELLY

70 thou is 10.0 cc. If you are 8.0 cr now, that is a chamber of 56.2 cc. Reduce it by 10.0 to 46.2 will raise the cr to 9.7
If you skim 70 off, you will need eyebrows in the block for valve clearance. For 9.4 cr you need to skim 58 thou, also probably need the pockets. Assuming you know for sure the existing cr and you are going to use the same compressed gasket thickness.
Art Pearse

You may want to use reduced dish pistons such as the ones on the 8.8:1 18V engines and then less of a skim.
David Witham

am trying not to disturb the bottom end....too good to recondition....yet
P J KELLY

Hey PJ, beware of too much head skimming. The motor in th ecar I purchased was running 220 pounds of compression. About 80 to 100 thousands skimmed. Can't keep a head gasket on the darn thing. Check the "abandoned project" thread. Regards, Tom
Tom

Thanks Tom,
my car runs 125psi across all 4 cylinders, late 18V LOOOOW compression motor....never skimmed....so from what I'm reading online and in the Burgess book about 70 thou should get me back over 9:1....looking for feedback from folks who have skimmed heads to increase CR and what their results were.
P J KELLY

Tom...read your thread....answered a lot of my questions....I'll probably take 70 thou off mine...my block was never bored so I have room to make the move up in compression. Regarding your motor, I would think you'll be best off with a new head or a thick gasket....how many miles on your head?
P J KELLY

Hey PJ, "0" miles on the head. the previous owner went for uprated cam, valves & valve springs. Like I said, can't keep a head gasket in place. Tom
tOM

PJ - do the math first (or double check mine). It is not rocket science!
Art Pearse

Hi PJ

70 thou is as near to 7ccs as you can get.

Peter
peter burgess

OK. I was figuring using the bore size, so 70 off the deck is 10 cc. The head has smaller open area.
Art Pearse

After research and reading a very successful midwest MGB marvel a Mr. Hap something, says he takes 70k off any head, some which have 5-10 already gone. He feels much eyebrow cuttung is unnessary! I just changed my first head gasket blow in 36 years with these cars on the 77 and just took off a 65k cut ( to demonstrate my reserve) shoved a couple of hunks of clay in 1-4 tightened up things, ran through a couple rotations plugs out. No real show of clay squish. The valves were impressed in the clay, but not that close to the deck ( and then there is what another 35-40k for squish? I'd like to hear how yours comes out. Cheers Vic
vem myers

Hi Vem

The problem occurs if you don't measure the valve face to the head face related to valve lift. A clearance of 35 thou of a crushed gasket isn't enough for safety of valve bounce. I paid for a new piston many years ago which was my fault as I didn't know the problem ( the customer fitted the head himself and I accepted the responsibility).We have also seen the results of contact on many MGA and MGB blocks. Did you lift the valves that little extra to see when they touch? Of interest the position of the ex valves seems to vary, I don't know if it is the position in the head, bore positioning or both.

An average standard B will have the valve faces approx 9mm or 360 thou from the head face. This plus the gasket will give a clearance of say 135 thou with the ex cutout. which means your 70 thou off is safe. The problem occurs with higher lift cams and if there is no cutout. You have approx 65 thou clear, take the valve lift to 420 or so and things could be getting tight.

The earlier heads have 40 thou deeper chambers to start with so 70 thou off an early head is like 30 thou off a later head.

Peter
peter burgess

Peter...am I right that my 1978 18V engine already has block cut-outs from the factory? I DO have a 270 Cam, (Delta) and also Harland Sharp Roller Rockers fitted which I know alter the acceleration and valve lift. My reason for considering the head skim is that I've lowered my effective CR even further from the 8:1 dished pistons I already have. I know I should REALLY re-bore the block and put in .060 Over Pistons which would up the CR by increasing swept volume. But seems a shame to pull a good low mile motor if I can get away with decreasing chamber volume.....what do you think?
P J KELLY

PJ=
Read Tom's "abandoned project" thread. All the info you need is there or linked in my posts. Yes your engine has cutouts, but it may not be enough; good detail re valve cutouts on mgbmga too.
Maybe you and Tom should "put your heads together" (sorry), figure it all out, and trade heads. You're practically neighbors.

And to Vem - "thousandths" IS NOT abbreviated using "k"!
And "No real show" or "not that close" IS NOT engine building terminology, especially if the intent is to educate those who have little experience in the field.

FRM
FR Millmore

Hi PJ

The best idea is to fit the head after milling and then get valves to full lift (ex) and lever the valve down to make sure you have 80 thou spare or so.

Are you on double springs to take full lift?

Hi FRM

I think Vem is just having a 'play' he is deadly serious how he works.

Peter
peter burgess

Thanks for everyone's replies....am going to do the 70thou skim and will post back in a few weeks.....!
P J KELLY

here's a pic

P J KELLY

Nice looking car P J. Are your rockers 1.5/1 or 1.6/1? Are they the newer ones with the bushings pinned in place?

You should be OK on a street car with the Harland Sharps.

Paul K

thanks...they are 1.6/1 and are the newer ones with pinned bushes....got them on ebay very reasonably....they were easy to fit and are really well made. My tappets sound 'normal' now!
P J KELLY

I had to switch to Titans on the race motor. The bushings on my Harland Sharps were starting to crack, but that engine runs best between 6 and 8K RPM. Very hard on parts, especially at 12.5/1 compression.



Paul K

Peter, thx for the kudo and spot on. I was trying to show that this Hap Walthrop, Midwest, avid B racer for 22 years, took latitude with big brush strokes in his layback speak, and just does not feel there is ANYTHING to worry about.
How many you all had strange encounter with the valve/piston , or with blocks lacking eyebrows ( eyebrow creation nicely demonstrated with common tools, and complete procedure pix on Barney Gaylord's MGA website) .
also Peter, I have the P285 and chilled& drilled lifters ( followers).If it is a factor for failure at 50-70 K skim, then there must be a few stories to tell from some of this group. On the otherhand, lacking ahandful of stories, maybe we got us another urban legend ( such as the touted lope and high idle of the 285- 8 years,, with Peter's newbill HRPiper 285 and FRBV flowed head, Doug Jacksons 1.55:1 rock and rollers and the car idles just fine , and I can sense a rythym at idle at 650-700, but no lopping


FR...so very swell to hear your incessant knitpicking again. I thought you were gone away! Vic
vem myers

Hi Vem

There is a person who races MGAs in the UK who reckons I can't be bothered to skim race heads to the right ccs and just plonk on any old head off the shelf and away we go! Do you think if we did it properly we might win more races and championships? Kind of the reverse of your fellow who skims off 70 thou come what may? Funnily enough I cannot recall ever selling the MGA man a head or working with him.

Peter
peter burgess

Peter- Life is a gas, and whatever floats your boat,is what floats your boat.


vem myers

I'm interested in the discussion of rocker ratios between Paul and P J Kelly. I have two sets of rockers on arms that have come off what I know to be B heads but most probably 1500 rather than 1800.

Are these likely to be different ratios from my B engine, or are the different ratios an aftermarket item? What is the ratio of 1800 head rocker arms? Is there any way to distinguish between the 1.5/1 and 1.6/1 alternatives exampled in the discussion eg markings or measurements?

Would appreciate any enlightenment as I would not want to confound my engine by fitting the wrong ratio nor 'assist' a fellow enthusiast by donating a 'wrong' rocker for a B 1800 head.

Regards
Roger

Roger T

Roger,

My question to PJ was regarding the Harland Sharp roller rockers, an aftermarket part. They are stamped on the rockers what the ratio is. Titan's likewise, they are 1.625/1.

The standard B rocker arm ratio is 1.4/1. Didn't mean to confuse you, as we were mostly discussing performance-oriented aftermarket parts and modifications.

Cheers,
Racer Paul
Paul K

Paul,
Thank you, I learn as I go along. Any confusion was of my own making. I do feel better about possibly passing a spare rocker or two among our local Club members.
Regards
Roger
Roger T

Roger T...sorry for the late response...been out of town....my Harland Sharps are in fact 1.5:1 rocker ratio....not 1.625:1 that many after-market items are. They fit on the std rocker shaft and do not require any mods to anything else....they gave me a seat of the pants 5hp and definitely help the mid-range....BUT....any increase in rocker ratio increases valve lift somewhat....and reduces Compression ratio also....hence my original questions about skimming off 70 thou....my engine needs the CR restoring so it can make real use of the cam/rocker/twin SU HS4/Eurospec dizzy I have on the car. The way it is bnow I am probably somewhere well under 7:1 effective CR given my 270 cam/roller rockers and 16cc dished std 18V pistons in my 1978 NAS MGB.
Am dropping the head off to the machine shop today!
P J KELLY

I somehow never seem to get the math right when I try to do it, but if you want to know your effective compression ratio, can't that be back calculated from the values of a compression test?

In a similar way, you can calculate your theoretical compression from the static compression ratio.

It seems that I somehow get balled up in how to treat the difference between gauge pressure and absolute pressure.

So, does someone have the proper formula handy?

Charley
C R Huff

effective CR ain't so simple because the factors that influence the simple difference between swept volume compared to TDC include cam duration, overlap and even cam timing...the Burgess book explains this very well.
P J KELLY

Charley, I suppose you could, but it is complicated. You would need to know the suction pressure right at the intake valve, and the air temperature there. Real compression is accompanied by a temperature increase which is related to the compression ratio. The geometric compression ratio assumes constant temperature. There are too many assumptions to make.
Art Pearse

Just to throw a spanner in the works, if you are working out CR theoretical, don't forget the area above the rings and below the piston crown.

Dynamic pressure is relative only, eg 120psi - bad 180 - good 220 - high!

Peter
peter burgess

Well then, I guess that explains why I never got the right results.

Charley
C R Huff

Result; took 65 thou off the head....seat of pants additional 10hp. The car is definitely more throttle responsive. No issues with exhaust valve/block contact and remember I have a Delta 270 AND roller rockers with a 1.5:1 ratio.
Definitely worth doing to any low compression motor....cost me $75.00 plus $30.00 for a head gasket....now I just need a better pair of SUs....mine are getting hard to tune.
P J KELLY

This thread was discussed between 25/06/2009 and 20/07/2009

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