Finding TDC on a rotary with trailing plug hole
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Rotary Enthusiast
Joined: Aug 2008
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From: Portsmouth UK
Finding TDC on a rotary with trailing plug hole
I have a 20B I have an uncertainty about the TDC with. i hope this can help solve it and be useful to all rotary guys out there.
With a bit of help from a few of the engine builders/tuners/highly skilled members on here I think we can come up with something worthy of a sticky unless I am missing a trick?!
The question about finding TDC on a rotary is one that gets asked quite a bit from what I can tell and most of the answers don’t help if you have your engine built and in the engine bay. Even so what I have read still seems to leave a reasonable margin for error? Such as lining up the keyway at 90degs.
So first question is how would you find TDC for definite when building an engine? Is there a mark defined somewhere internally for this or does it rely on lining up the rotor position by eye somewhere roughly?! I know how you do it on a piston engine and that gives a defined no argument position if done correctly with the correct tools.
Asides the above, that there may be an exact science for, let’s look at those of us with engines built. The only true points we have to visualise are the pulley and pointer at the front and the position of a rotor tip as it passes the plug holes.
I gather that there may be some variations on plug hole position between different series motors but with some input from other people I think this can all be covered.
So my suggestion is as follows:
Step #1
Take a protractor and stick it to the front pulley. Make a pointer to align at a given point on the protractor or use the point on the front cover.
Step #2
Rotate the engine until the rotor #1 has a tip centre of the small trailing plug hole in the housing. Note the position on the protractor.
Step #3
Rotate further until the same tip is centre of the larger leading hole. Note position on protractor. This step has a bit of a margin of error due to the size of the hole and may in fact not be needed. But I am suggesting this in case there is variation between different series of engine on the separation between the plug holes. A physical distance could also be measured between the holes as a double check.
Pause for thought..
If the rotor tip is always dead centre of the plug holes at Bottom Dead centre we could now work out the degrees to get the rotor tip to this point and then rotate the crank around accordingly to mark TDC.. I am just trying to get me head around the reduction ratio and firing.. I think a 60deg rotation would give us TDC though?
My issue here is I believe due to the above possibility that the plug holes positions vary slightly on different series engines BDC may not always fall middle of the plug holes. Someone may be able to confirm this though.
Step #4
This is where the rest of the community come in. calling on all you engine builders, tuners, DIYer’s with spare motors. If you have a part built engine 20B, 13B etc that has a known TDC mark on the pulley please can you do the following:
Step #4a
Align the TDC mark with the pointer on the cover.
Step #4b
Mount protractor wheel to front pulley and note position of protractor relative to engine point or separate make shift point.
Step #4c
Rotate the engine counter-clockwise until a rotor tip becomes visible at the centre of the small trailing plug hole. Note position on the protractor.
Step #4d
Post the degrees you have just marked on here with the engine type you have marked it from and the series/year of the engine block if known or applicable.
I am hopeful we can build a small table that shows how to mark TDC within a very small margin of error using this method. I think it could help a lot of people out. Me included!
If anyone see’s any flaw in my plan or a shorter route that I have not consider please jump in.
On a slight side note I have think I have most of the parts from a junk 13B FD engine in my shed. Could I mock this up using the FD trigger wheel to help myself or others with my 20B TDC issue?
i.e. I believe the rotor housings are the same for a 20B and a 13B? So I should be able to follow my above theory to find where TDC sits. And I have the bonus of a trigger wheel that should be marked at a known point as a reference? Don’t recall what the FD has on the cover for TDC alignment actually but I am sure I can find that out.
Thank you in advance guy’s I really hope we can put something useful together here.
With a bit of help from a few of the engine builders/tuners/highly skilled members on here I think we can come up with something worthy of a sticky unless I am missing a trick?!
The question about finding TDC on a rotary is one that gets asked quite a bit from what I can tell and most of the answers don’t help if you have your engine built and in the engine bay. Even so what I have read still seems to leave a reasonable margin for error? Such as lining up the keyway at 90degs.
So first question is how would you find TDC for definite when building an engine? Is there a mark defined somewhere internally for this or does it rely on lining up the rotor position by eye somewhere roughly?! I know how you do it on a piston engine and that gives a defined no argument position if done correctly with the correct tools.
Asides the above, that there may be an exact science for, let’s look at those of us with engines built. The only true points we have to visualise are the pulley and pointer at the front and the position of a rotor tip as it passes the plug holes.
I gather that there may be some variations on plug hole position between different series motors but with some input from other people I think this can all be covered.
So my suggestion is as follows:
Step #1
Take a protractor and stick it to the front pulley. Make a pointer to align at a given point on the protractor or use the point on the front cover.
Step #2
Rotate the engine until the rotor #1 has a tip centre of the small trailing plug hole in the housing. Note the position on the protractor.
Step #3
Rotate further until the same tip is centre of the larger leading hole. Note position on protractor. This step has a bit of a margin of error due to the size of the hole and may in fact not be needed. But I am suggesting this in case there is variation between different series of engine on the separation between the plug holes. A physical distance could also be measured between the holes as a double check.
Pause for thought..
If the rotor tip is always dead centre of the plug holes at Bottom Dead centre we could now work out the degrees to get the rotor tip to this point and then rotate the crank around accordingly to mark TDC.. I am just trying to get me head around the reduction ratio and firing.. I think a 60deg rotation would give us TDC though?
My issue here is I believe due to the above possibility that the plug holes positions vary slightly on different series engines BDC may not always fall middle of the plug holes. Someone may be able to confirm this though.
Step #4
This is where the rest of the community come in. calling on all you engine builders, tuners, DIYer’s with spare motors. If you have a part built engine 20B, 13B etc that has a known TDC mark on the pulley please can you do the following:
Step #4a
Align the TDC mark with the pointer on the cover.
Step #4b
Mount protractor wheel to front pulley and note position of protractor relative to engine point or separate make shift point.
Step #4c
Rotate the engine counter-clockwise until a rotor tip becomes visible at the centre of the small trailing plug hole. Note position on the protractor.
Step #4d
Post the degrees you have just marked on here with the engine type you have marked it from and the series/year of the engine block if known or applicable.
I am hopeful we can build a small table that shows how to mark TDC within a very small margin of error using this method. I think it could help a lot of people out. Me included!
If anyone see’s any flaw in my plan or a shorter route that I have not consider please jump in.
On a slight side note I have think I have most of the parts from a junk 13B FD engine in my shed. Could I mock this up using the FD trigger wheel to help myself or others with my 20B TDC issue?
i.e. I believe the rotor housings are the same for a 20B and a 13B? So I should be able to follow my above theory to find where TDC sits. And I have the bonus of a trigger wheel that should be marked at a known point as a reference? Don’t recall what the FD has on the cover for TDC alignment actually but I am sure I can find that out.
Thank you in advance guy’s I really hope we can put something useful together here.
Last edited by misterstyx69; Sep 23, 2016 at 01:14 PM. Reason: wrong word.fixed.