BSA B31 TDC and tappet position.

[briancairns]

Apologies to all , I have a silly quesion:
When engine is at TDC both tappets are free to rotate, and both valves shut ( on compression stroke).
If valves are shut should there be 0.003 thou tappet clearance ? are should there be no clearance at all?
The reason I am asking is I think I set timing wrongly and want to ensure that I find TDC on compression stroke. correctly.

regards
Brian

[rhyatt]

Some engines need 1 valve to be in a particular place before you set about adjusting the other.
This is due to that engines cam profile, so follow your makers instructions for your particular model., giving inlet and exhaust clearances.

however, my Royal Enfield stipulates that the engine should be at TDC (indicated by the ammeter just swinging off when the points open) and at that position , the pushrods must just spin but with nil clearance)

Just watch the inlet valve being opened and then closing , continue rotating your engine as you are then on the compression stroke , employ the usual bits of wire or screwdrivers as you approach the top and tdc

[limeyrob]

Plug out, kebab stick (they are narrow enough to fit vertically (or near enough).  Tappet cover off. Bike on centre stand in 4th so you can turn the engine one handed.  Keep trying to rotate the tappets, when both rotate feel for the piston.  Get it as high as you can - TDC (go back and forwards several times to be sure)  then check the tappets again to be sure.  If its not on compression stoke you won't be able to spin the pushrods with you finger.
Mark a line on the stick and another 7/16" above it, turn the wheel backwards, to the upper line. Its actually better to go back 1/2" and come up, this gets rid of the backlash in the timing gears.

[scifi]

Hi Brian, don't look at the cams to get your TDC position, look at the piston.  Insert something like a pencil into the cylinder then slowly rotate the engine (to avoid breaking the pencil.)  TDC is when the pencil is pushed out by the maximum amount.  If you have a side valve engine with an offset plug-hole then this might not work.

The idea of setting the tappet clearance whilst one valve is fully open, may be because it takes up any slack in the camshaft bearings, which could change the clearance by several thou.  Or it could be to ensure you are not on the cam quietening ramps.

[JulianS]

I suggest check the clearances the way BSA intended as detailed on service sheet 604. If you set them whilst the tappets are on the ramp of the cam you will get excess clearance.

[briancairns]

Thanks all,
Apologies for the slow reply ( computer problems).
I got bike sorted , I just wanted to ensure I was on compression stroke when finding TDC so that I may time engine, I was not setting valve clearances.I messed up and originally timed on exhaust stroke, even though I put thumb over plug hole etc. There was not much differnce in thumb pressure at the plug between compression and exhaust stroke.

Thanks all
Brian

[V500]

If that's the case then you have more problems...

[Janacek]

Just putting my B31 (1957) engine back together after valve trouble.
The Haynes Manual states that because the cams have quietening ramps the tappet clearance has to be set in a special way.Exhaust tappet with the inlet just closed,and inlet when the exhaust tappet is just touching the upper part of the pushrod.
I am finding this a bit confusing as tappet clearance is given as 3thou and rocker clearance as 10 and 12thou.
I am not sure weather to set the rockers,leave the feeler gauge in and set 3thou on the tappet,or close the rocker onto the valve and then set the tappet.
My gut feeling is to ignore Haynes and turn the engine to TDC on the firing stroke and set the rockers to 10 and 12thou and leave the feeler gauge in while setting the tappets to 3thou.
Any comments welcome.
Thanks, Terry.

[limeyrob]

I seem to recall the whole quieting ramp thing is a distraction, all it means is you have to be at TDC as both tappets are off the ramps.  This was because they were a new innovation then.  3 thou is correct (Service Sheet 701)

[briancairns]

If that's the case then you have more problems...

What sort of problems? ( I'm almost afraid to ask )

Regards
Brian

[V500]

Low compression, by the sounds of it.

[briancairns]

Low compression, by the sounds of it.

Ok, I tested compression, all good.........most likely my inexpereince caused the problem.
Thanks
Brian

[limeyrob]

Low compression won't stop a B31
Mine ran just fine with the top ring and half the top land missing, it had all gone down the exhaust and not even marked the valve!  What gave it away was a bit would come off and hit the plug and close the gap, it was driving me mad which was when I pulled the head to get to the bottom of it and found the mess.

[briancairns]

Low compression won't stop a B31
Mine ran just fine with the top ring and half the top land missing, it had all gone down the exhaust and not even marked the valve!  What gave it away was a bit would come off and hit the plug and close the gap, it was driving me mad which was when I pulled the head to get to the bottom of it and found the mess.

cheers, bike going very well at the moment