Kevinrpayne Posted June 15 Report Share Posted June 15 When I bought my GT6 I didn’t realise that I actually had a LC engine most probably from a 2000 saloon or other. some years ago I decided to return the performance by having the head skimmed and worked out capacities and the amount to be taken off but this is now lost in the mists of time reading the article by Adam Egeland-Jensen in the April Courier mentioned that you should shim the rocker mounts ro compensate for the head skim……this I never did and will struggle as I’ve lost the calculatios on what should have been taken off. Now this is a real long shot, has anyone else done the same as me and has an indication as of how much might have been taken off? I’ve absolutely no idea if it was 5 thou or fifty ! Anything will be better than nothing and nothing is what I have right now ! responses and guidance appreciated please thanks Kevin Link to comment Share on other sites More sharing options...
johny Posted June 15 Report Share Posted June 15 I think if youve been able to adjust the tappet clearance correctly all these years then you havent got a problem Kevin. When so much metal is removed that there is no longer sufficient gap left between pushrod and tappet you either have to shim the rockers up or reduce the length of the pushrods... Link to comment Share on other sites More sharing options...
JohnD Posted June 15 Report Share Posted June 15 Kevin, I skim my heads to achieve a CR of 10.5:1 - see my article on Sideways: https://sideways-technologies.co.uk/forums/index.php?/topic/7551-how-to-raise-the-compression-ratio-safely-and-effectively/#comment-99739 But recently I realised that I never adjusted the rocker shaft mounts to match! The objective is to ensure that the rocker movement above and below the shaft centre is equal. This may require new push rods. See the Revington web site: https://www.revingtontr.com/tr3/information-sheets/is0066-rocker-geometry John Link to comment Share on other sites More sharing options...
micklewis Posted June 15 Report Share Posted June 15 if skimming the head , either shim the rocker pedestal or reprofile the push rods . if not the operating radius is incorrect and can result in broken rocker arms . Link to comment Share on other sites More sharing options...
Kevinrpayne Posted June 16 Author Report Share Posted June 16 Thanks. The pushrods are down to the very minimum when adjusting and only just within the adjustment range so whilst I don’t need to do anything, it could be a better setup if I shimmed the pedestals a bit, the challenge is how much. kevin Link to comment Share on other sites More sharing options...
johny Posted June 16 Report Share Posted June 16 John mentions the angle of movement of the rockers but I cant see that changes with a head skim because the rocker doesnt change position relative to the valve. However if you start jacking up the rocker pedestals then yes that angle will change so I would say if you must raise them do it by the minimum amount necessary to get the tappets within safe adjustment range. Then as he says if the shims needed are excessive new shorter pushrods would be a better option... Link to comment Share on other sites More sharing options...
Iain T Posted June 16 Report Share Posted June 16 (edited) Not setting up the rocker arm arc relative to the centre of shaft also makes a small but measurable difference to the valve lift. My pedestals have shims but the arc still isn't correct so I'm thinking of removing the shims and getting shorter pushrods. I get easily bored and love to tinker! Iain Edited June 16 by Iain T Link to comment Share on other sites More sharing options...
JohnD Posted June 16 Report Share Posted June 16 3 hours ago, johny said: John mentions the angle of movement of the rockers but I cant see that changes with a head skim because the rocker doesnt change position relative to the valve. However if you start jacking up the rocker pedestals then yes that angle will change so I would say if you must raise them do it by the minimum amount necessary to get the tappets within safe adjustment range. Then as he says if the shims needed are excessive new shorter pushrods would be a better option... johnny, If you skim the head, then the rocker shaft is lower relative to the camshaft. With same length push rods, the range of movement of the rockers will be further downwards, more below the horizontal. A shim under the pedestal will raise the shaft again. This pic doesn't show what I mean but may help thinking. Link to comment Share on other sites More sharing options...
johny Posted June 16 Report Share Posted June 16 But surely you undo the tappet to compensate for the higher position of the pushrod (if theres sufficient adjustment of course) and the rocker hasnt moved position relative to the valve? As I see it adjusting the tappet is exactly the same as shortening the pushrod... Link to comment Share on other sites More sharing options...
Iain T Posted June 16 Report Share Posted June 16 (edited) 39 minutes ago, johny said: (if theres sufficient adjustment That's the issue on skimming the head say 40 thou there is insufficient adjustment to even have any tappet clearance (there wasn't on my rocker assembly) so shims are the only quick option. The way to reduce the load required on the system is to use the mid line rocker option and to get the rocker swipe on the valve stem as central as possible to minimise valve guide wear. I used a black marker on the stem top and cycled the engine to see the contact area. However if your valve seats aren't even you will get different heights. My valve stem heights before I had it sorted were like a scenic railway.... Iain Edited June 16 by Iain T Link to comment Share on other sites More sharing options...
johny Posted June 16 Report Share Posted June 16 Dont forget though that this is a head skim from LC to HC so could still be in range of tappet adjustment (done ages ago and presumably had correct tappet adjustment since) or maybe a shorter pushrod was used on the HC engines and Kevin could get these... Link to comment Share on other sites More sharing options...
Iain T Posted June 16 Report Share Posted June 16 42 minutes ago, johny said: Dont forget though that this is a head skim from LC to HC so could still be in range of tappet adjustment (done ages ago and presumably had correct tappet adjustment since) or maybe a shorter pushrod was used on the HC engines and Kevin could get these Oh yes, more variables.... Link to comment Share on other sites More sharing options...
johny Posted June 16 Report Share Posted June 16 In fact this from Chris Witor is interesting as its got the different head heights. The mk2 2000 LC had flat pistons and a head height of 3.4" while the mk2 Vitesse/GT6 (HC) also had flat pistons but a head height of 3.3". The latter also had pushrods 0.125" shorter than the first so it sounds like Kevin needs the standard 8" pushrods to get his tappets correct... head_applications_chart.pdf (chriswitor.com) Link to comment Share on other sites More sharing options...
Iain T Posted June 16 Report Share Posted June 16 Excellent advice Johny. That's why this forum is so good 😊 Link to comment Share on other sites More sharing options...
johny Posted June 16 Report Share Posted June 16 Well got to get some good out of all this time wasted on silly old cars (wifes opinion😁)... Link to comment Share on other sites More sharing options...
Iain T Posted June 16 Report Share Posted June 16 My wife is of the same opinion! Link to comment Share on other sites More sharing options...
johny Posted June 16 Report Share Posted June 16 Do sometimes wonder if shes right. Looks like 8" pushrods no longer available so that scuppers that idea... Link to comment Share on other sites More sharing options...
Iain T Posted June 16 Report Share Posted June 16 (edited) Adjustable length pushrods are available or perhaps try Canleys etc who have treasure troves of old stock and used parts? Measuring existing pushrods is first on the list to check. Edited June 16 by Iain T Link to comment Share on other sites More sharing options...
Kevinrpayne Posted June 16 Author Report Share Posted June 16 Chaps. I fear we are drifting ( interestingly) from the original question. I think we are all aligned that shimming the rockers is most probably a good thing to do in that it will allow for correct valve movement. Ive been able to adjust ( but only just) the tappet clearances, the question I had was “does anyone have a guess how much might have been taken off as part of the head skim ?” it feels like 40 thou doesn’t sound unreasonable to shim that amount and then that has the added benefit that it will give me an additional 2 mm adjustability on the tappets. Does anyone have a better value that that to shim by ? Thanks Kevin Link to comment Share on other sites More sharing options...
Iain T Posted June 16 Report Share Posted June 16 As Johny's post measure the thickness of your head. If it's 3.4" it's a standard LC head unskimmed. As to if it's possible to skim a LC down to a HC 3.3" I don't know. I assume the combustion chambers of a LC and HC are the same and it's just the extra 0.1" that reduces the compression? Iain Link to comment Share on other sites More sharing options...
johny Posted June 16 Report Share Posted June 16 I think youre wrong Kevin but the choice is yours. The document I posted shows that 0.1" need to be taken off an LC head to produce the HC version as they are a common casting 308351. Then it looks like your pushrods if original are 0.125" longer than they should be so with a bit of basic calculation you can calculate the pedestal packing required but your rockers are going to end up at a very funny angle... Link to comment Share on other sites More sharing options...
Kevinrpayne Posted June 16 Author Report Share Posted June 16 Ok great, sorry I missed that point, I’ll go for 0.1 and give it a go. I have the old push rods in the box of goodies, I’ll compare and report back if it makes any difference to the running thanks all kevin Link to comment Share on other sites More sharing options...
Kevinrpayne Posted June 16 Author Report Share Posted June 16 I should have added, I think that if they end up at a funny angle then I may have defeated the point of correcting the rocker angle so may go for less thanks Kevin Link to comment Share on other sites More sharing options...
JohnD Posted June 16 Report Share Posted June 16 5 hours ago, johny said: But surely you undo the tappet to compensate for the higher position of the pushrod (if theres sufficient adjustment of course) and the rocker hasnt moved position relative to the valve? As I see it adjusting the tappet is exactly the same as shortening the pushrod... Go your own way, johny! Link to comment Share on other sites More sharing options...
johny Posted June 16 Report Share Posted June 16 One thing I should correct is calling the rocker arm valve clearance adjuster the tappet which of it isnt as thats down at the bottom of the pushrod running on the cam, doh! Link to comment Share on other sites More sharing options...
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