Checking and setting TDC

[blaze_away]

Setting/Checking TDC 4.6 V8

I want to check and set a TDC mark on my front pulley.
To do that I want to use the tried and tested method of screwing a "piston stopping" device into No1 spark plug hole then bring piston up to the stop in both clockwise and anti-clockwise dorections making marks on pulley then the exact middle is definitively the exact TDC.

I have tried drilling an old spark plug out but its a mess and therefore I'm for a neat alternative,
Is a 14mm 1.25 pitch threaded rod correct ?

[ChimpOnGas]

There are many ways to find TDC, but mostly these can be divided into two distinct indication methods:

1. Air/pressure

2. Physical (my preference)

For the air/pressure indication you can use a compression tester assuming the gauge shows +/- pressure, just remove the Schrader valve and you're good to go.

For the better physical indication use a DTI which precisely shows when the piston is coming up, going down, and that exact dead moment where it's neither going up or down indicating TDC. To make using the DTI much easier buy something like Laser Tools Part No. 2833 which is nothing more complicated than a sleeved rod you screw in the plug hole.



You also want your DTI to be held firm so use a magnetic DTI stand.



Obviously the block is aluminium but you should be able to get a strong enough fix for the magnetic stand base off the near side exhaust manifold or their flanges. If you have none of the above kit you should be able to buy a cheap DTI with magnetic stand for £15.00 off eBay and the TDC tool for £18.00, lets say roughly £35.00 for everything you need.

Using a DTI is definitely by far the best method, but going back to the less precise air/pressure indication this kit is just £16.00 all in.

https://www.ebay.co.uk/itm/AST5005-TDC-FINDER-KIT-...

For that sort of money its really not worth messing about trying to make your own TDC indicator out of an old spark plug.

[phazed]

I tossed this together a few years ago.

Drilled out the plug, tack welded a nut to the plug and screwed in a long bolt with a ground down end.

From memory it was only about 15 minutes work. Probably got it on the shelf if you need it.

[ed_crouch]

My preferred method is to stop the piston about 30 degs BTDC with a piston stop. Mark the pulley at the point at which it stops, then CAREFULLY BY HAND turn the engine backwards until once more the piston touches the stop. Mark the pulley again. Bisect those two marks and you have exact TDC.

Miraculously the marks on ours were bang on... First. One. Ever.

One point of caution with piston stops. Make damn sure nothing can fall off it in the bore. Second note, put whatever system in place you feel necessary to make bloody sure you never, ever, EVER (!!!!!) try to start the car, crank it over, or otherwise cause the engine to rotate other than by hand with the stop in. You'll wreck the engine if you forget and leave it in!!!!

The DTI method works well with DOHC engines where the spark plug goes in from the top, i.e. parallel axis with the piston. The RV8 has the plug hole axis over at quite and angle and you may just end up bending the rod rather than moving it...

[ed_crouch]

P.S. I also made one - turned it up out of a bit of steel bar.

Happy to make another for a few beer tokens if needed. In truth you absolutely can use a cannibalised spark plug.

[ChimpOnGas]

ed_crouch said:

The DTI method works well with DOHC engines where the spark plug goes in from the top, i.e. parallel axis with the piston. The RV8 has the plug hole axis over at quite and angle and you may just end up bending the rod rather than moving it...

Fair point but typically you'd have the engine more or less on TDC anyway, so the amount of piston movement is so small bending a rod type TDC indicator with is round profiled end becomes pretty much a non issue, I've used this type of tool on countless old school push rod engines with no issues at all.

The profiled end is key as it'll skid over the piston crown, some TDC tools have a nice ball end for this reason.



This one is reach adjustable which is a nice feature too.

Physically and directly connecting the piston crown with a DTI is definitely the most accurate way to find TDC, but TBH a length of welding rod acting on your finger is the way it was often done back in the day, you just need to have the feel for finding TDC.

[phazed]

Hey Dave, that is just a shiny version of mine!

[Steve_D]

These are mine for different engines.

Can't agree using a dial test indicator is a valid option.
When the piston is coming up to TDC the dial will be moving nicely. As it gets close the piston is hardly raising at all and for quite a number of crank degrees there will be no discernable piston lift/fall. You can use a DTI in a similar fashion as a deadstop but because the DTI is meeting the piston top at an angle you cannot be sure the DTI is not 'skidding' across the top of the piston giving a false reading.

Steve

[geeman237]

Can't you buy a piston stop tool like this? I bought one from for Griffith.
https://www.amazon.com/Proform-66792-Dead-Center-L...

[ed_crouch]

The only thing I'd say is that for accuracy, I'd recommend having a stop whose tip can be locked into position. The adjustable ones as linked above are fine, but if you want it dead accurate, pop a lock nut on there to completely lock the adjusted position of the tip.

[blaze_away]

Thanks folks have now managed to neatly remove the ceramic insulator from an old plug and have cut a thread inside it with an M8 tap set. Just need a long set screw and a nut to complete the job.

[Steve_D]

blaze_away said:

Thanks folks have now managed to neatly remove the ceramic insulator from an old plug and have cut a thread inside it with an M8 tap set. Just need a long set screw and a nut to complete the job.

If you have a long bolt you can screw it in from below until the thread binds on the plain shank to lock it in position.
Then using my picture below to establish the length required cut the bolt head off and round it with a file.
The RV8 one is on the right in that picture.

Steve

Edited by Steve_D on Wednesday 11th September 07:53

[blaze_away]

Nice tip Steve Thanks

[blaze_away]

Update for completeness.....
Tried and failed to make one I was happy with so ended up buying one. Tried to get one of the adjustable ones but none in uk that I could find.

Bought this and cut it down a few mil
https://www.pedparts.co.uk/product/652/piston-stop...
Under about a fiver arrived next day job done and confirmed my bottom pulley marks are bang on.

[Classic Chim]

Get in

Are you getting any joy with improved mpg or about the same?

[blaze_away]

Classic Chim said:

Get in

Are you getting any joy with improved mpg or about the same?

Not yet tested it but initial indications are plug colour is good rovergauge values show short term trim switching around zero with long term trims at +15 and + 30 so looking hopeful. Just need a long run to check it out for sure.

Having a 123 distributor with programmable mechanical and vacuum advance map is marvellous.

[Classic Chim]

blaze_away said:

Not yet tested it but initial indications are plug colour is good rovergauge values show short term trim switching around zero with long term trims at +15 and + 30 so looking hopeful. Just need a long run to check it out for sure.

Having a 123 distributor with programmable mechanical and vacuum advance map is marvellous.

I’ve been really hoping you’d say that as I did encourage you ,,, easy with someone else’s money

Lovely looking thing I thought. Quality. Well worth it

[Garp635]

Hi have u still got this v8 timming plug cheers