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V8 head machining question


HoSS
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I have a pair of early rover heads, which i'm preparing to swap onto my engine.

I plan to use the later composite gaskets & dont want to loose compression ratio.

what is the amount to remove if doing that?

How can i tell if if has not been done already?

cheers

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Some machining work is evident - there is a slightly rough 'sandpaper look' to the face. Really bad skims have zillions of curved lines, and an excellent one is dead smooth and was originally shiny. There is a measurement you can take from the gasket face to a point on the top (where the rocker gasket sits) usually.

These two pictures are of 4.6 P38 HSE heads that have been crack tested and then skimmed to the best standard. I would imagine it would be hard to tell if you didn't know already that they had been skimmed, so if yours are like this then you would have to measure them in order to determine if they had been done and how much has been removed from standard.

med_gallery_2_90_56366.jpg

med_gallery_2_90_239709.jpg

Les.

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If you machine a lot off, you will run into problems with fitting of the inlet manifold, however, you could also machine those faces as well. Also you will need to consider the lifter preload………..mucho off the heads will require rocker post shims, or better still, adjustable rockers ……………… checking the preload will be part of the next instalments on my engine rebuild thread.

On the heads there is plenty of ‘meat’ …………… but with reference to the above IIRC 55thou is the max…………………..

If you want to get right on the edge then also take into account the extra thickness of the composite into account. The thickness of the gaskets does vary from manufacturer to manufacturer, but here are some numbers (compressed),

Std tin 3.5 = 20 thou

Std tin 3.9 = 18 thou

RS 3.5 compo = 40 thou

RS 3.9 compo = 46 thou

I have measured some others and the numbers above stack up quite well ……………..

If you take off the gasket difference then you should be fine ............... Rover did it by reducing the combustion chamber size (28cc) on the later 4.0 & 4.6 heads

Its difficult to tell if they have already been skimmed, however, the trick of skimming for true is to only take off enough to 'true' the heads and on the V8's this is normally 5 thou or so.

In you position I would quite happily take off 30 thou ................

Be very careful that you get a machine shop that has done these before ................. due to odd angles and no right angles on the head shape, they are bar stewards to step up correctly ;) ................. expect to pay £30 to £50 a pair.

:)

Ian

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Is that a statement or a question?

What Nige is alluding to, is the fact that pre SD1 heads had smaller inlet valves .................. but I guess that there are not too many of these about now........

To make life easy I have used the common metric conversion sizes, but I have the decimal inch sizes to hand if you need them...

Pre SD1 inlet = 38mm exhaust = 33mm......................... these are parallel valves

SD1 and Post SD1 inlet = 40mm exhaust =34mm .................. these all have waisted stems (improves gas flow) and are the most popular

The largest valves you can fit onto the standard seats (re cut) is 41.4mm inlet & 35.5mm exhaust.

The largest valves that can be fitted into an RV8 head is 49mm inlet and 41mm exhaust (bowler wildcat heads .loads of ££££) ............... another popular upgrade size is 43mm inlet and 37mm exhaust

The standard SD1 & post SD1 valves are good to 300bhp (ish) ............ the bowler heads are good to 450bhp.............

Christ I'm sad .................... I really need to get out more............... :lol:

:)

Ian

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SD1 and Post SD1 inlet = 40mm exhaust =34mm .................. these all have waisted stems (improves gas flow) and are the most popular

Ian

Right well the new valves i have (bought in a caboodle with the heads) are 40/34mm waisted.

I'm not sure exactly the origin of the heads, as there is no serial/part number anywhere on them. The info on the v-8.org.uk site says there should be a part number in the rocker area, i only have :

612571

<BIRMAL>

10

cast into them.

I have new (early) guides also. So i'm going to machine the tops for the later type stem seals & bullet the ends on my lathe. (dimesions would be handy if anyone has that)

With ref to Mr BBC's thread - I'm still deciding wether to do any porting or not. Suppose i should since i have time, but not sure how much to take out / or if its worth it on an otherwise stock 3.5.

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Hoss,

Yes, porting will be good ………………… but also pay plenty of attention to the condition of the rockers / shafts ….

ALL high mileage RV8’s will have bucket loads of wear on the shafts and the rockers will be oval ……….. on the stripped engine that I am rebuilding , some of them have 25 thou of play, however, the hydraulic lifters hide this play as the lifter gently takes up the slack ………. A conventional engine would sound like a diesel bag of nails………

This had two effects ………… it reduces the valve lift and more importantly it retards the valve timing ………… which is very bad for low rpm torque.

I guess that you would have post SD1 heads ………….

With regard to the valve guides…………… the price difference between the old and later type is 49p ………… the later type cost a mere £1.20 each ………… is it worth your machine time ?

The new guides may also need to be reamed to the valve stem size ………..

:)

Ian

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Hoss,

Yes, porting will be good ………………… but also pay plenty of attention to the condition of the rockers / shafts ….

ALL high mileage RV8’s will have bucket loads of wear on the shafts and the rockers will be oval ……….. on the stripped engine that I am rebuilding , some of them have 25 thou of play, however, the hydraulic lifters hide this play as the lifter gently takes up the slack ………. A conventional engine would sound like a diesel bag of nails………

This had two effects ………… it reduces the valve lift and more importantly it retards the valve timing ………… which is very bad for low rpm torque.

I guess that you would have post SD1 heads ………….

With regard to the valve guides…………… the price difference between the old and later type is 49p ………… the later type cost a mere £1.20 each ………… is it worth your machine time ?

The new guides may also need to be reamed to the valve stem size ………..

:)

Ian

Yeah, since i am trying to build these heads up complete before i remove my old ones, I will try to put new rockers & shafts on (when i find a good deal)

Are the early/later valve stems a different diameter?

I already have a new set of (early) 40/34mm waisted valves & guides - i didnt really want to buy the valves agian , but would buy the later guides if they are that cheap.

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Is that a statement or a question?

Sorry HOSS - Statement !

Typed quick pre a meeting...

The EARLY heads (P5b P6B etc) up to early SDi era inc MG V8 RR etc around 1976/7 is poor,

smaller valves and port shaping 3 angle 36cc chamber and 38 inlet 33 exh valves. and pre SDi used dual spings

With later efi type headfs - + SDi Vitesse V8 post 1977ish still 36cc inlet is upped to 40 and exh 34mm, these improve rpm and flow, vitesse heads have a stem mod - this increses flow more, these have single springs, and although dual springs have advatanges you can't fit to these headss without machining work. Later 3.9 heads have a machined lip on the back of the valve - this aids restriction of gas not backflowing EFI have a tweak re injectors and their are minor head differences between 3.5 3.9 etc and some model years

I may have misread what you deem as early :lol:

Hope the above helps

Nige

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Yes, the stems are the same size ..................

Older guides ......... Real Steel CX2300 ........... 81p each

Later type for hooded seals ....... Real Steel DW200 .............. £1.20 each

You can also get modified guides that have 0.1 inch machined off for high lift cams .................... £1.50 each

I guess non of it is bank breaking stuff ................

:)

Ian

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Yes, the stems are the same size ..................

I guess non of it is bank breaking stuff ................

:)

Ian

Only prob is i am out in switzerland, so i can't just pop round to local main stealer.

I'll prob machine them, the i can bullet the ends too - how hard is the guide material? (never having tried before)

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Only prob is i am out in switzerland, so i can't just pop round to local main stealer.

I'll prob machine them, the i can bullet the ends too - how hard is the guide material? (never having tried before)

sort of high on the "Kin Hell" scale. :(

Why not use paypal, begging PMs / requests on the forum, get what you want posted out ?

Nige

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sort of high on the "Kin Hell" scale.

Why not use paypal, begging PMs / requests on the forum, get what you want posted out ?

Nige

ok, sod it. i will order with craddocks. EPC is a bit confusing...

"24G from xxx to WA152500 - ERC3648 no longer available"

"24G to WA152501 - LGJ100900" only used on 1 motor!?

or

"24G (no change point given) - 603554"

any ideas which is the correct later guide?

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sort of high on the "Kin Hell" scale. :(

Why not use paypal, begging PMs / requests on the forum, get what you want posted out ?

Nige

Actually no, after a test cut - they seem to be easily machinable.

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wots the engine number that the heads are off ?

I'll then have a lookie in my books and bits

Nige :unsure:

thats the prob, these were an ebay purchase.

where other heads have a LR part number i only have:

612571

<BIRMAL>

10

and "19 1N" stamped on a flat on the exhaust side.

Back to the 'how much to take off the heads" thing...

Its clear i need to do the 20 thou (ish) to use the comp gaskets, but i'm tempted to go a bit further, as i run LPG & that is supposed to benefit from an increased CR.

I dont know the calculation that derives CR from amount machined off. Or more importantly how far i can go before problems occur.

p.s i already have a set of ARP studs lined up. I guess adjustable pushrods or rockers might be an idea if budget allows- the last time i adjusted preload was a pita with the wire up the lifter to gauge it. & even then its a compromise on the average value.

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Pressing out the old and then pressing in the new can be a bit of a challenge........ I bloody hate replacing guides !

let us know how you get on with this - I was told that with aluminium alloy heads the matal 'smears' as you press the guide out so instead the guides are bored to fit an insert.

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let us know how you get on with this - I was told that with aluminium alloy heads the matal 'smears' as you press the guide out so instead the guides are bored to fit an insert.

Well i bought the heads as an 'unfinished project' (so the guides were already out) & a bag each of new valves, guides & springs.

I'll use the 'kitchen engineeing' method & run to the press with oven gloves on ;)

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