Piston Dwell At Tdc

[wardy18 33 1 Cars]

I thought it might be of interest to others in my situation for when timing up Lift at TDC on aftermarket Cams/pulleys

 

I had always measured TDC and marked it on my flywheel by sticking a dial gauge on a mount down the spark plug bore with a bit of stiff wire to reach the piston, however i always found this didn't give very accurate results as the wire has a tendency to bend!!

 

i have always been told to bear in mind the piston dwell at TDC and have always ignorantly disregarded this due to the inaccuracy of the wire down the plug hole effort, so i thought, whilst the head is off, to accurately check my TDC marking on the flywheel and i was shocked to find that piston dwell at TDC accounts for quite a lot!!

 

my 2mm wide mark on the flywheel has now become a 4mm wide mark which represents the TOTAL time at which the piston is at TDC and so represents the piston dwell

 

this doesn't sound a lot but when setting lift at TDC on your vernier pulley to within 0.1mm then making sure that you find TDC at the middle of the dwell and not just at the start where the piston first hits TDC is very important!!

 

sorry for my previous ignorance to all those that offered advice and i hope i have got this right and can be a help to others!!

 

opinions please and if this is in the wrong thread please move

[James_R 3]

That's why you locate TDC by measureing a fixed stroke making it then goign past TDC to it and diving by two

 

But good post to highlight it.

[Sandy 191]

The piston rocks a bit too, which doesn't help, when going in at an angle. On vertical plug engines, with a good positively mounted DTI, you can nail it, but otherwise, the positive stop method (a spark plug with a bit of bar welded on the tip), is foolproof.

[pug_ham 244 3 Cars]

i have always been told to bear in mind the piston dwell at TDC and have always ignorantly disregarded this due to the inaccuracy of the wire down the plug hole effort, so i thought, whilst the head is off, to accurately check my TDC marking on the flywheel and i was shocked to find that piston dwell at TDC accounts for quite a lot!!

 

my 2mm wide mark on the flywheel has now become a 4mm wide mark which represents the TOTAL time at which the piston is at TDC and so represents the piston dwell

How well did you find the marks you made matched up to the timing plate & flywheel marks (if you used a standard XU flywheel) as used in the finding TDC topic by petert?

 

Graham.

[sutol 0]

That's why you locate TDC by measureing a fixed stroke making it then goign past TDC to it and diving by two

 

But good post to highlight it.

 

 

That's the way to do it.

You need to stop the piston with something solid just before TDC, mark the pulley then rotate the engine the other way and it will stop again just before TDC and mark the pulley.

You should then have two marks, measure exactly half way between them and that is absolute TDC.

[wardy18 33 1 Cars]

How well did you find the marks you made matched up to the timing plate & flywheel marks (if you used a standard XU flywheel) as used in the finding TDC topic by petert?

 

Graham.

 

Before i fitted the Ally flywheel i matched up the new and old flywheel and traced across the TDC timing mark. I did find when measuring correctly that this mark seems to be only the very start of TDC

 

I have drawn a quick pic, hope it makes sense, you can see in the second pic where I've added to the timing mark onto the original one, you can see that the original timing mark (first pic) didn't sit in the middle of the timing plate mark