Author Topic: Top Dead Centre thoughts!  (Read 3894 times)

Offline LJ.

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Top Dead Centre thoughts!
« on: 04.12. 2006 09:36 »

Easy Peasy! No problem in finding or carrying out this simple task, but there is one or two things that creates thought.  ????

While setting up the timing on the A10, I found the TDC through the top of the cylinder head and measuring from a mark on the cylinder head to the top of the piston. Noticing the bottom of a piece of wire that I used as my marking gauge there was carbon. My thought is this... Should this carbon/coke be taken into calculations when setting timing? *dunno* as it can be quite deep, perhaps up to a millimeter or two, and is this why perhaps a more acurate timing is set by using a degrees disc? A lot more hassle in taking off the primary cover etc.

What do you reckon?

LJ.
Ride Safely Lads! LJ.
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1940 BSA M20 500cc Girder/Rigid- (SOLD)
1947 BSA M21 600cc Girder/Rigid-Green
1949 BSA A7   500cc Girder/Plunger Star Twin-(SOLD)
1953 BSA B33  500cc Teles/Plunger-Maroon
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1961 BSA A10  650cc Golden Flash-Red

Offline 1KCBC

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Re: Top Dead Centre thoughts!
« Reply #1 on: 04.12. 2006 09:55 »
For me just only remove the spark plug and use a pen put in a hold to
check top dead centre.Never give me a problem for timing setting.

Jaran.
1951 A10GF,(looking for  A10 swinging-arm)

Offline fido

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Re: Top Dead Centre thoughts!
« Reply #2 on: 04.12. 2006 12:51 »
You have to use some sort of datum for the degree disc so you would probably still end up with something down the plug hole. I would not worry about the carbon. There is more inaccuracy introduced by the fact that you can't get the wire, pencil etc parallel with the piston, as the plug hole is at an angle. The degree disc does eliminate that error because you are only finding TDC rather than 5/16" or whatever before tdc.

Offline a10gf

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Re: Top Dead Centre thoughts!
« Reply #3 on: 11.12. 2006 11:30 »
Yes, a degree disc is great, good feeling to get a readout and facts. But the pen works quite good as well (when one knows the pen well  ;) )

E.

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Offline dpaddock

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Re: Top Dead Centre thoughts!
« Reply #4 on: 15.12. 2006 23:48 »
OK, folks; here's how to get TDC regardless of carbon or anything else, using the primary chain as a linear degree wheel.

Gather the following materials:
  A plastic straw from a spray can nozzle; a fine-point black Sharpie pen; a 3-4 inch long 1/4-20   
UNC threaded rod or screw (1/4" Whit for you purists); a large trombone paper clip and a small spring paper clip.

Get your Beezer on its center stand so that the rear wheel is free of the ground. Select 4th gear. Remove the primary case cap. Remove the spark plugs. Check and adjust the primary chain tension.
 
Remove the chain case screw nearest the cap opening. Thread the 1/4" Whit fastener into the chain case screw hole vacated in the previous step. Fasten the trombone clip to it with the spring clip.

Grasp the straw and insert it vertically into the left (near side) spark plug hole. Rotate the rear wheel until the straw is fully upward (either direction is OK). Bend the trombone clip so that it points to the nearest chain pin center.

Now, rotate the engine three links forward. Mark the chain link pin against the pointer with paint or some other mark (the black Sharpie should do). Also, with your Sharpie, mark the straw against any convenient reference point on the head. (The head fin surface nearest the straw is suggested: remember this point.)

Next, rotate the engine backward until the straw mark aligns again with the reference point .
Mark the chain plate with your Sharpie.

Measure the distance between the chain marks. Divide the distance between the chain marks by two and rotate the engine forward by that much. Voila, TDC!

Mark the straw and set timing after marking your staw with your desired advance. You will have two marks on the straw after this procedure:  TDC and your advance mark, all referenced to the head fin surface. Save the straw!



David
'57 Spitfire


Offline a10gf

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Re: Top Dead Centre thoughts!
« Reply #5 on: 19.12. 2006 01:40 »
dp, thanks for this xcellent & informative post.

E.

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Offline dpaddock

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Re: Top Dead Centre thoughts!
« Reply #6 on: 19.12. 2006 14:25 »
Erratum: you'll have THREE marks on the straw, including the first one you placed as the reference if you don't erase it as I did to avoid confusion.
Also, I crimped a spade lug on the straw to hold it more easily and to orient it in the hole.
dp
David
'57 Spitfire


Offline Pollock

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Re: Top Dead Centre thoughts!
« Reply #7 on: 19.12. 2006 22:24 »
You know..TDC is good and set it...but I am a firm believer in if it doesn't perform well then make a small modification. Off the subject but I had a 1991 VW fox that was easy to time with timing light...easily outlined in the manual ...line the marks up and torque down the dist settings. Ran good when timing was dead on... But, don't you know by just advancing it a bit and man  it ran so much better and actually got the MPG it was suppose to...It is like anything when was your bike built....Tues, weds, thurs, which I am told are good days...Friday? I have heard Thurs was a big party night in the UK especially for married men. Correct or Not?
Just food for thought.
Pollock

Offline snowbeard

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Re: Top Dead Centre thoughts!
« Reply #8 on: 21.11. 2007 18:16 »
sorry to dredge this up, but more for the collective I guess.  Thanks to dpaddock for that, I think that'll be how I do mine when the time comes!!

so, specific to the first question about carbon buildup on the pistons.  is there any chance of creating a hot spot for detonation by scratching the carbon/oily mess on the piston?  not even scratching the actual piston metal, just disturbing the buildup in only one spot? 

obviously it's not a big deal because everyone has set their timing this way since the dawn, but I just wanted to check.  there may just be a lot of really clean pistons out there!
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Offline fido

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Re: Top Dead Centre thoughts!
« Reply #9 on: 22.11. 2007 22:03 »
The detonation problem with carbon is because it has lower thermal conductivity than the metal of the piston and therefore does not lose heat into the oil, the cylinder etc quick enough. If you knock a chunk of this carbon off you are going to get a cool spot rather than a hot spot.

Online RichardL

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Re: Top Dead Centre thoughts!
« Reply #10 on: 07.03. 2009 21:03 »
So, it's a flurry of activity as ridable spring fast approaches.

Here's my take on a TDC and BTDC finder. The graticule was drawn in AutoCad for 1/32" increments, printed 1:1 and taped to the straws. I beleive there is no issue with my domed pistons and there certainly is not the hypotenusal issue related to devices that thread into the plug hole.

Richard L.
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Online RichardL

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Re: Top Dead Centre thoughts!
« Reply #11 on: 07.03. 2009 21:05 »
Oooh! I should have cleaned up that head before such a closeup, Mr. deMille.
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Offline A10Boy

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Re: Top Dead Centre thoughts!
« Reply #12 on: 08.03. 2009 13:18 »
LJ
I would have thought that if there was enough coke on the pistons to upset the timing then it would be time for a de-coke. The timing could be set accurately before putting the head back on.

Regards

Andy

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Online RichardL

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Re: Top Dead Centre thoughts!
« Reply #13 on: 08.03. 2009 16:26 »
Am I correct that 34 degrees applies to a specific cam and compression?

While on this subject, my books indicate 3/8" for Road Rocket and other higher cmpression models, and this is the value I have been using. However, to my recollection, I can't recall any reference material that relates this to cam grind. So, my question is, if anyone knows, with a 356 cam, 9:1 pistons and automatic advance, is 3/8" still applicable. If your answer includes "you've got the wrong cam to go with your pistions," or vice-versa, or, "why are you running godforesaken 9:1 pistons?", or the like, those corrections will need to wait until the next engine rebuild. Hopefully that will be by the inheritor 100 years from now. In such case you might recommend a 'tween setting that considers the conditions as they are. Thanks, to any and all, for any advice in this area.

Richard L.
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Online RichardL

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Re: Top Dead Centre thoughts!
« Reply #14 on: 08.03. 2009 17:01 »
Oh oh! Can a man my age use the term "my bad?" Before I modified it, my previous post made an incorrect statement about which pinion was which. Sorry, for my stupidity.
Richard L.
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