Author Topic: Cam Sprocket Timing Marks  (Read 213 times)

Offline Laverda120

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Cam Sprocket Timing Marks
« on: January 25, 2019, 11:45:55 AM »
I will be rebuilding the top end of the CB250RSA (again!) in the next couple of weeks.
I have a question regarding the cam sprocket timing marks that I think applies to this engine and every other SOHC engine. The Haynes manual states the usual of making sure the cam sprocket timing marks align with the top edge of the cylinder head (i.e. are parallel with the mating face) to ensure accurate valve timing. Once the marks are aligned and the timing chain is on the sprocket and the sprocket is bolted to the cam the chain tensioner can then be freed to allow the cam chain to be tensioned correctly and automatically. This all sounds great, however I have noticed both on the 250 and the 400/4 previously when the cam chain tensioner is freed and takes up the slack/play in the can chain it also moves the cam sprocket in the process thus knocking the timing marks slightly off line and no longer parallel with the head. I'm talking of maybe a 1-2deg movement. This is also when using a new cam chain.
Is the above something Honda takes into account with the design of the engine or should I allow for this when setting the valve timing by setting the two alignment marks slightly off to allow them to become parallel when the tensioner is released to take up the chain slack?
1976 Honda 400/4
1977 Honda 400/4
1981 Honda CB250RSA
1977 Rickman Honda CR750
1999 Honda VFR 800FX
1955 750 Dresda Triton
1978 Moto Morini 350 Sport
1982 Laverda 120 Jota
1924 Triumph 550SD
2012 Stanley Battery Drill

Offline Bryanj

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Re: Cam Sprocket Timing Marks
« Reply #1 on: January 25, 2019, 12:13:20 PM »
If you can move a whole tooth and get the marks exact then yes move it but what you usualy find is one tooth moves it farther away on the other side. Marks need to line up when chain is in running tension situation and it is worth rotating a couple of turns to check where cam marks are when crank is correct. Hope that all makes sense, as someone who did this for a living i find it difficult to explain easily what i did without thinking--------bit like me trying to understand a "techie" when my computer does not respond to the threat of a 4lb lump hammer!

Offline Laverda120

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Re: Cam Sprocket Timing Marks
« Reply #2 on: January 25, 2019, 12:30:22 PM »
Thanks Bryan. I'll give your suggestion a go. But yes, moving it one tooth takes it too far the other way. There is virtually no play, the marks either line up pre-tension or else they are a long way out by going another tooth on the sprocket.
When I refit the head and cam I'll turn the engine over a few times pre-tension and again post-tension to see the difference.
1976 Honda 400/4
1977 Honda 400/4
1981 Honda CB250RSA
1977 Rickman Honda CR750
1999 Honda VFR 800FX
1955 750 Dresda Triton
1978 Moto Morini 350 Sport
1982 Laverda 120 Jota
1924 Triumph 550SD
2012 Stanley Battery Drill