RV8 TDC

need4speed · Post by **need4speed** » Mon Jun 18, 2012 11:45 am

What's the trick to finding no1 TDC when the heads are torqued down and there's no dizzy?
I'm led to believe that I should not trust the mark on the crank pulley as they can be a good bit out. Will it be a case of poking something down plug hole?

sidecar · Post by **sidecar** » Mon Jun 18, 2012 12:59 pm

It depends on how accurate you want to be, if you are just after TDC roughly then poking something down the plug hole will do. If you want true TDC for setting up timing tape or marking the damper wheel then you really need a piston dead stop. You then turn the engine both ways round and TDC is between the two points where the piston hit the stop.

I have seen a serp engine where the pointer is cast into the front cover being quite accurate, the older adjustable pointers can be miles out.

need4speed · Post by **need4speed** » Mon Jun 18, 2012 3:08 pm

Its in order to set my cam timing.

sidecar · Post by **sidecar** » Mon Jun 18, 2012 5:07 pm

need4speed wrote:Its in order to set my cam timing.

If you have a non-adjustable cam chain and sprockets then I think that you will be able to just get the piston at TDC by poking something down the plug whole. You can then fit the gears with the marks pointing at each other, it will be quite clear if you are out at all. If you are using an adjustable vernier gear set and you are going to dial the cam in then you will need to be more accurate. AJMHO!

Jono FD3 · Post by **Jono FD3** » Mon Jun 18, 2012 5:10 pm

cam timing should be just the case of ligning up the two marks on the sprockets..... would think this is the only way with cam timing?!

Jono

need4speed · Post by **need4speed** » Mon Jun 18, 2012 6:56 pm

Yea I've got fully adjustable vernier timing gear....

sidecar · Post by **sidecar** » Mon Jun 18, 2012 8:19 pm

need4speed wrote:Yea I've got fully adjustable vernier timing gear....

I see, really then you are into the realms of using a degree disk if you want to get the cam timing spot on. In order to set the degree disk up you will need to find true TDC. The disk will actually help you with this anyway.

mgbv8 · Post by **mgbv8** » Mon Jun 18, 2012 8:26 pm

So you need true TDC then.

I've lost count of the TDC tools I've made and lent out over the years.

I make mine by cutting the earth strap off a spark plug. Then break away the porcelain and a good chunk of the centre electrode. I like to ba able to push an old bolt or bit of stud about 5mm into the hole so I can braze the stud in nice and straight.

To be sure of the stud length required bring no1 up to the top as best you can and then use a tool of some sort to get a rough length from where it hits the piston and the top of the plug hole. Make the stub on the end of the plug sobut 10mm longer than this. Hit the end of the stub with the ginder to form a smooth ish dome on the end. Then you are good to go.

Instructions are given above re forward, backward and mark in between the two

sidecar · Post by **sidecar** » Mon Jun 18, 2012 9:15 pm

There is a picture of adjustable one that I made here....

http://how-to-build-a-pilgrim-sumo.wiki ... by-members

martin_t · Post by **martin_t** » Thu Jun 28, 2012 11:48 am

mgbv8 wrote:So you need true TDC then.

I've lost count of the TDC tools I've made and lent out over the years.

I make mine by cutting the earth strap off a spark plug. Then break away the porcelain and a good chunk of the centre electrode. I like to ba able to push an old bolt or bit of stud about 5mm into the hole so I can braze the stud in nice and straight.

To be sure of the stud length required bring no1 up to the top as best you can and then use a tool of some sort to get a rough length from where it hits the piston and the top of the plug hole. Make the stub on the end of the plug sobut 10mm longer than this. Hit the end of the stub with the ginder to form a smooth ish dome on the end. Then you are good to go.

Instructions are given above re forward, backward and mark in between the two

Sorry but I still don't understand!!! how do you know it is at TDC when it hits the old plug etc (whatever you use as a TDC tool)? what if you have made it slightly too long? Won't the piston just nudge up against it and give you the impression its at its highest postion, even if its not? Never used one so excuse the lack of understanding!!

Martin

Quagmire · Post by **Quagmire** » Thu Jun 28, 2012 11:57 am

Doesnt really matter how long the stop is, although if its waay too long or short you will have problems.

The critical thing is that you rotate the engine by hand one way until it hits the stop, mark the position on the crank pulley as accurately as possible.

Now rotate the engine the other way until it hits the stop again.

TDC is now exactly halfway between the marks you made...

At least I think thats the theory.

martin_t · Post by **martin_t** » Thu Jun 28, 2012 12:50 pm

Quagmire wrote:Doesnt really matter how long the stop is, although if its waay too long or short you will have problems.

The critical thing is that you rotate the engine by hand one way until it hits the stop, mark the position on the crank pulley as accurately as possible.

Now rotate the engine the other way until it hits the stop again.

TDC is now exactly halfway between the marks you made...

At least I think thats the theory.

But if you aren't actually at TDC because your "tool" isn't long enough or short enough (pause for laughter!), won't you be marking the crank pulley in the wrong place, or am I misunderstanding...(which is quite possible!!).

Thanks

Martin

ChrisJC · Post by **ChrisJC** » Thu Jun 28, 2012 2:02 pm

If the tool is too short, then yes, it won't work as the engine will be free to rotate.

But if the tool is long enough to hit the piston, then it works fine.

Chris.

martin_t · Post by **martin_t** » Fri Jun 29, 2012 7:30 pm

ChrisJC wrote:If the tool is too short, then yes, it won't work as the engine will be free to rotate.

But if the tool is long enough to hit the piston, then it works fine.

Chris.

Sorry to bang on about this. But if the tool is to long and the piston hits it, then you are not at TDC. So you back it off a bit but how do you know the exact point of TDC? The piston either hits the tool or not, but that exact point has got to be impossible to calculate with this method?

Martin

Ian Anderson · Post by **Ian Anderson** » Fri Jun 29, 2012 7:50 pm

Martin please re read the explanation

Fit the spark plug thingy
Rotate the crank till the piston touches the thingy
Mark the pulley

Then turn the engine backwards until it touched the thingy again

Mark the pulley

Now midway between the two marks you have made is tdc

Ian