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MG Midget and Sprite Technical - Valve clearances with aftermarket cam

Hi all,

Going to do my valve clearances (never done them in the year iv owned it), I have a figure of 10 thou to adjust to.

But then I remembered that my cam apparently isn't standard. I don't think it is given its idle. My question is, will this change the valve clearance figures I should be adjusting to? In my mind, it shouldn't make a difference as either cam lobe should be the same diameter when the valves are shut, otherwise the valves wouldn't be shut? But when the valves are fully open there is obviously a difference between the cams..

Anybody confirm either way. And if the figures will be affected, how do I know what figures to use!!??

Cheers all!
Karl Bielby

Hi Karl,

I don't have a definitive answer, but I use a Piper cam in my 1500 and set clearances to their spec and their clearance spec differs depending on the cam profile.

http://www.pipercams.co.uk/pipercams/www/product_nav.php?type=C&cat=CM&man=26&engine=6

(Not sure on their clearances for the mild road, they look huge! I run the fast road).

Malc.
Malcolm Le Chevalier

Hi Malc,

I just found that as you posted it! Guess my issue is, I don't know what cam is fitted.. I'm not pulling the head off to find out!
Karl Bielby

Use 0.015” if in doubt... ‘A noisy tappet is a happy tappet’...

Mark
Mark T Boldry

Haha it's a good point.

I think initially, I will measure to see what they currently are. Then make a decision from there.
Karl Bielby

Karl

im assuming yours is a 1500... id measure and see what your valve lash is currently

Id go with to loose over to tight other wise you will be building a new head in a few hundred miles

make sure you know what type of rockers you have... standard or roller tipped

if there roller... gap gauge from the side NOT the front... fronts will give a false measurement do to how the roller tips work
Prop the cow killer

I can only assume they will be stock rockers, obviously all will be revealed when I pop the rocker cover (might buy an ally one?? Quieter and mine is a right mess)


Yeh that's what I'm thinking. Check what they all are currently. Take it for a good old rag and get it nice and hot, then measure again. If iv got about .005 I think it should be about right?
Karl Bielby

Malc will have to vouge for me but on a 1500 i think they are

.010
Prop the cow killer

I believe that is the cold value on a standard cam though?
Karl Bielby

Start at 16 thou and nip them up from there a tad until reasonably quiet. Echo Mark T that loose is far better than tight - especially on exhaust valves.

Non-standard cams do specify a clearance in order to match the profile and it is (usually) greater than book.
Oggers

Are you implying that I am some sort of 1500 guru Prop? I am not sure about that!

I can confirm (thanks to my handbook) that 10 thou is the standard clearance for 1500s. Unfortunately that doesn't really help us.

Cheers,
Malc.
Malcolm Le Chevalier

My page at http://www.mgparts.co.nz/advice/technical-notes/valve-clearances/ explains the opening ramps and how clearances relate to them.

Ramps vary considerably beteen profiles, for example the one I am using at present has a ramp extending to around 20 thou valve clearance, whereas the fast road cam I use has the ramp finishing at 11-12 thou valve clearance.

If you don't know your profile, ideally you'd measure the opening ramp and then select a figure just before the end of the ramp for day-to-day running, or just after for optimim performance for a short term event.
Just how much before the end of the ramp depends on how often you plan to check and adjust the clearances!

To measure the ramp, use a dial indicator to measure lift (I slide no 1 inlet rocker to one side and put the probe in the end of the pushrod), start with the rule-of-nine valve fully open and rotate the engine gradualy forwards. Once the gauge starts reading, record left every 5 or 10 degrees.

The pointers for ignition timing are accurate enough for this, you may need to step off some further marks on the pulley to get a greater range (with pulley TDC mark at the most advanced mark on the cover - usually 15 or 20 deg - and then paint a mark on the pulley at TDC pointer. Repeat if necessary.)

The ramp will be identified as the period in which the opening rate is constant, usually between 1.5 and 2.5 thou per 5 degrees of crank motion. The ramp usually lasts for around 20 deg. After the end of the ramp, the lift per 5 deg increases very rapidly up to a maximum of around 15 thou per 5 deg (the amount of the maximum is governed by follower diameter). This acceleration phase is ont relevant to this measurement other than confirming you are past the end of the ramp!
Paul Walbran

To add to pauls comment

as you rotate the crankshaft... turn in one direction only... dont give a small nudge in the reverse direction as that will ulset the readings because of the shift in the timming chain always keep the forward tention

to really get a visual prespective of what your doing and not just a pure mathmatical input out put... get some graf paper and plot all your data point...if for no other reason then its just a fun exercise to see what your cam looks like on paper by converting math into a visual perspective

malc...

id say your about as close to being a resident expert on the 1500 over most anyone one else on here... i trust your comments... you earned your learn and have certianly faced and over come alot of 1500 challanges


btw malc... i feel for you on the career front ...and hope the best for you ... any chance you can shift to teaching H.S./ collage or pick up a mechanical engineering degree to go with your chemical degree.

With shell oil laying off 6 to 9 % of its work force and new projections over the weekend of oil dropping to less then $20 per barrel in 5he not so distant future .... its looking like a long time before big oil rebounds if it ever does

the usa congress really screwed the crap out of world oil industry... wow did they ever....at the time i thought it would have had the opposite effect by selling our oil on the open world market and not keeping it for ourselfs as had been our policy going back to the mid 1970s . With congress repealing those restrictions that just created an even more glut supply of oil for the planet to absorb... IM DEFINATLY NOT COMPLAINING..cause i love cheap gasoline its already down from a 1/3 from what it was 6 weeks ago

but it sure is hell for anyone that makes a living from black gold and looks like it will be for decades to come
Prop the cow killer

Well I had a look today. Engine was warm, not hot really, 20 min drive prior. Pretty much all valves were .010 I had to adjust a couple (which I'm sure I could (not)hear after! Sounded much quieter). So I think il leave them at that, might still get a alloy rocker cover though!
Karl Bielby

Ha ha, thanks Prop! Credit where it's due, most of what I have learnt I have learnt from the guys and girls of the BBS.

Malc.
Malcolm Le Chevalier

Malc
Same here... i could not have learned what i (think) i know if were not for those that chimed in when i had a question

we are in good company
Prop the cow killer

Congratsz karl

Something i always do... add an extra .001 to what ever the spec is supposed to be... just a little insurance protection from being to tight

prop
Prop the cow killer

There's a hell of a lot of information in these forums! They are incredibly valuable to us all.

Cheers for all the help chaps!
Karl Bielby

This thread was discussed between 24/07/2016 and 25/07/2016

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