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MG MGA - Cam Timing
|I fitted new crank & cam sprockets and timing chain, all exactly aligned as per the manual (new sprockets came already punched with the timing dots). I then found exact TDC by using the piston stop method. With the engine on the bench I even made tdc punch marks on the flywheel. No.1 inlet valve set at 0.060" and turned the engine over until the valve just started to open. At this position the pointer mark was at 5 degrees after the TDC mark on the degree wheel. When the engine is turned anti-clockwise to line up the pointer with the 0 mark on the degree wheel (TDC), the inlet valve slackens slightly. So, not happy with the timing being 5 degrees off after having replaced the sprockets and chain. Can anyone tell me how this is corrected. Would the use of an offset key solve this problem? I've never used these keys, what do they do and how??|
|Frank, do you have a stock grind MGA cam installed?|
Yes as far as I can tell the camshaft is stock. I had bought it new from Brown & Gammons UK and fitted it in the car myself.
I've had mine to bits a few times and rebuilt it with reground crank and new camshaft, chain and sprockets. You line up the dots on the sprockets as per manual and that's it, then put the rest back. My TDC & +5 & +10 deg marks are on the bottom of the timing chain cover...
I wouldn't trust myself with the 'piston stop method'. Are you trying to be super accurate for some reason?
|Frank, -- Using the piston stop to find TDC is inherently accurate. However, I don't like the method you use to determine the cam timing (although I know it is popular). Problem is that it is exceedingly difficult to tell exactly where the cam lift starts, because it is such a gradual transition from the base circle to the beginning of the ramp up. In other words, not particularly accurate.|
For this reason, cam manufacturers will commonly specify the cam timing for points where valve lift is 0.050-inch up from the base circle. This point of the cam lobe is on a steady ramp up with more rapid transition and may be measured easily accurate with 1/2 degree rotation of the crankshaft. These measurements are usually done with a long travel dial indicator, measuring the ramp up and ramp down points on both sides of intake and exhaust cam lobes (4 separate can timing points for comparison and verification). This method of measuring and setting cam timing is explained in detail, on the Crane Cams web site. I also have details with photos on my web site.
That said, five degrees at crankshaft is 2.5 degrees at camshaft, which is a relatively large misalignment. Accurate or not, you are probably right that it is a problem. If you are confident in the measurement and ready to change cam timing, it is easy to do with an offset cam key, and no special timing sprocket required. Start with just one cam key with 1/32-inch offset, which will give exactly 3-degrees rotation (change of timing) to the cam (1.5-degrees at the crankshaft). You can buy these offset keys. You can also make a 1/32" offset cam key by milling opposite sides of a thicker crankshaft key. Details of that are also on my website.
Another related detail is that the cam sprocket has 40 teeth (and the crank sprocket has 20 teeth). If you skip the timing chain over one tooth (on either sprocket) you change cam timing by 9-degrees (18 degrees at the crank). Using one standard cam key, and one 3d offset cam key that can go either way, you can set cam timing -3d, 0d, +3d. Skip the timing chain over one tooth to advance cam timing +9d, and then the keys can be used to achieve +6d, +9d, +12d. In this manner you can walk can timing all the way around the clock in 3-degree increments. For finer adjustments you can likewise buy offset cam keys with 1d, 2d, 3d, 4d offset, With these four offset keys you can adjust cam timing in 1-degree increments anywhere you like.
In your case, the cam timing is currently retarded 5d at the crank (2.5d at cam). You can install a 1/32-inch offset cam key to advance cam timing 3d. This will show as 6d advance at the crankshaft, ending up with net 1d advance at the crank. This is a good place to set it for long term operation. With some running time and a little wear of the timing chain and sprockets, cam timing will retard slightly. Starting with +1d, wear eventually gets it to 0d, and later to -1d, all the while staying within +/- 1d of the intended set point.
|Frank Checking the cam timing the way you are doing it works OK as you are already well up the cam face at .060" Just make sure you check the .060" accurately as even a slightly worn rocker face can cause problems. If you have one use a dial indicator to allow for any wear. To tell when the valve is right on lift point spin the push rod and you will feel it just tighten up.|
|I will do another very thorough check today, checking TDC, the .060" gap at no.1 inlet valve and ensure that degree wheel is installed properly and that I am getting accurate readings. If I still find the cam to be retarded, which I probably will, I will follow Barney's advice and adjust the cam timing using offset keys as per the last paragraph in his post.Can someone please tell me where I can purchase a couple of these keys|
|A question for Barney. Please allow me to clarify a statement you made in the third paragraph of your post above.|
I quote "...Start with just one cam key with 1/32 inch offset, which will give exactly 3 degrees rotation (change of timing)to the cam (1.5 degrees at the crankshaft).
Am I right in saying that, using an offset key to rotate the cam by 3 degrees, the crankshaft will advance double that figure, to 6 degrees. Please correct me if I'm wrong.
I think I may be able to see where you went wrong.
Once you have found the exact TDC with the piston stop and got your mark aligned with 0 on the cam card, you should remove the piston stop assembly. From then on rotate only clockwise. If you go anticlockwise you will get all the slack in the timing chain altering your careful measurements.
Going clockwise and CCW when getting the initial zero position on the crank doesn't introduce any significant error because the timing chain is not interacting with your measurement.
Hope this helps
|Yes Frank, you got it. I posted a misprint. Rotating the cam sprocket 3 degrees does indeed rotate the crank 6 degrees. I got it right in the final paragraph.|
Make your own offset key with a milling machine:
Or buy them off the shelf:
MiniSpares - £6.00 exc vat
MiniMania - $16.96
Pegasus - $16.99
Moss Motors - $26.35-$30.95
mossmotors.com - part number 327-015
Thanks, reading the last paragraph of your post did make me wonder if there was a misprint in the 3rd paragraph. That is why I asked. Thank you for the info, and thanks also for the links re the offset keys.
|For Pete Tipping.|
Pete, I don't think I'm being super accurate, I am a perfectionist by nature, and what I do I do it as near perfect as possible. To me there's only one way to do something, and that's the right way. You have to remember that all our engines today have had many parts replaced with modern reproduction parts. Mine is certainly no exception. Repro parts are seldom produced as per the exact factory specs, and therefore changes in the factory original settings are bound to take place. At best, these can be corrected by using modern engineering technology.
|A common method to determine TDC on old motorcycles is to put on a degree wheel and rotate the crank to TDC using the piston as a guide. Note the degrees and then rotate the crank in opposite direction noting the degrees. Then split the difference and use it as TDC. I recognize these old bikes are gear driven but the theory should be the same.|
This thread was discussed between 06/12/2010 and 09/12/2010
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