True TDC Question

Hello.

I watched the video on finding true TDC.

I'm unsure the exact definition of True TDC, to me it's always been the point of Maximum Lift.

The video was not clear on the reason why you need to find 50 thou down the bore on each direction in relation to point of maximum lift

however the video continues to show to then measure where 50 thou down the bore is achieved on one side of Maximum lift, and on the other side of maximum lift, and then calculate the middle-point of that.

Does the middle-point of 50 thou either way always generally equal or very close to the point of maximum lift?

This will be on my RB26 that runs a 2.8 nitto crank, I am degreeing a set of Kelford cams in so they perform the best.

Where in Australia can I buy a suitable gauge, base and the required extension?

You may be a little confused? They are basically the same thing - for me, TDC refers to the crankshaft pin being at it's highest point of it's rotation, and piston 'maximum lift' being when it is at it's highest point in the bore. For all intents and purposes they are the same thing, although the piston pin offset, if used, will throw it of an extremely smal amount - less than you could measure without very high precision tools!

Yes, by bringing the piston up the bore to a specific point from before, and after (do not measure down to as clearances, etc, will affect this), and spliting those rotation points you will get the point where the crankshaft and piston are at nominal TDC and highest point, respectively.

You do not need to use 0.050", specifically, I prefer a point further down as it's a little more accurate - just needs to be the same both sides and repeatable for accuracy - so make sure the DTI* and wheel pointers are stable - with alloy blocks and heads, you can make a platform for magnetic bases by bolting on a short piece of steel plate as convenient and required.

Some people prefer to use a physical stop that bolts to the block and that the piston is lifted up and against, to remove the hassle of getting the piston and DTI in just the right position. With engines where the head is already in place, you can buy, or make, a stop that is screwed into the spark plug hole that the piston is brought up against - I had several - one caveat with that method, though - you should move that cylinder's piston close to BDC so the stop can't accidentally be forced into a piston that is very close to TDC and can't be pushed down by the stop, damaging the piston. The first option may be suitable for you, just requiring a degree wheel - but as a DTI is used for checking camshaft timing, may as well get the whole package.

You should be able to buy a kit from any decent 'speed shop' or camshaft manufacturer. Your engine is metric, and so I'd suggest checking how the camshaft is times - some use imperial measurements, some use metric - make sure your DTI type matches.

*Dial Travel Indicator.

Just some info; piston bores to crank line should (and are normally) never inline as you don't want everything to be straight when at tdc, there is normally up to 0.1mm offset.

When a piston is at tdc there is a few degree dwell (either side of 'true' tdc) this is why you want to get a reference from either side to find the middle...Or tdc.

Dial test indicators or dti's are the best way of doing this, a plunge style with roughly 12mm of travel will be useful for an array of engine building tasks (cam timing and lift exercises)

Getting a finger style of dti is also useful for measuring floats of items like the crank and cams.

As Gord says getting a small piece of steel to use for a mag base is extremely useful in mounting your dti setup around the engine (put a 10mm hole in it so that it can be bolted up most places)

Hope this helps.

Ah, thank you, Dave, this old dog learned something there - I'm rather more 'old school', just aware of the piston pin offsetto reduce piston slap at TDC? It's why you normally have some indication of the 'front' for correct fitment.

It does seem to be a common practice, to reduce major thrust side loadings on the cylinder wall, and offer a friction reduction, and improve connecting rod geometry on the combustion stroke. For those interested, they're called Désaxé (or desaxe) engines. https://en.wikipedia.org/wiki/Desaxe.

Since a large offset can have quite a significant affect on the relative 'top' points, it may be an idea to check the exact proceedure for the engine in question.

Yes of course as you say piston pin off set helps the piston to travel uniformly down the bores reducing piston slap. Yes as you say pistons have an orientation more often than not and I have seen people out them in the wrong way to some interesting results *face palm*

I only know about the offset from my time ccm-ing inspecting blocks for wrc and dev work, I then questioned this with engine designers.

Yes as you have linked that's the big offsets but I'm taking about even your everyday 4pots for the same reasons as you've mentioned.

Using the dti or stop methods as mentioned will get a tdc from cylinder 1 and timing referenced from that point.