Cam timing using degree wheel and dial indicator

[foster461]

I'm helping a club member re-assemble a TR3A engine, standard cam. Pistons are installed but the head is off. Cam is in the block. New timing gear and chain but no alignment marks on the timing gear.

 

As I understand it the normal way to time the camshaft from scratch is to:

 

Install the timing gear on the crank and attach the degree wheel and pointer.

 

Zero the degree wheel with #1 piston at true TDC using a piston stop.

 

Put a lifter and a dummy pushrod in #1 inlet hole and use a dial indicator to measure the lift of the dummy pushrod

 

Using the cam spec, rotate the crank until the pointer for the degree wheel reads YY degrees (eg 15 degrees BTDC).

 

Using the cam spec, rotate the cam so that the dummy pushrod starts to move and stop rotating when the dial indicator reads XX (for example .05")

 

Without moving the cam, put the chain around the timing gears and slide the small gear onto the cam.

 

Rotate the crank one revolution and double check.

 

Does that sound correct and for a standard TR3A cam what are the values for XX and YY ?

 

Stan

[iain]

Hi Stan

 

i spent yesterday afternoon doing just this.

 

This is my methodology slightly different to what you describe but allows you to find TDC on the Crank and max lift on theCam which by pure lift is a bit vague as the cam lobe like the piston crown actually doesn't move over quite a few degrees of rotation.

 

As you say put the degree wheel on the crank.

 

Find true top dead centre, so with the dial gauge on the piston crown , piston just before TDC, zero the dial gauge. Note the crankshaft degrees of your pointer...may be zero if you zero it or maybe another reading but its "X" degrees BTDC. Now rotate cranks clockwise slowly until you go past TDC and bring the dial gauge back to ZERO. Now you have a "Y" degrees ATDC. Difference between the two is "Z" degrees. Z/2 equals True TDC. Now move the crank to true TDC and if necessary zero and double check.

 

Next with the chain and chain wheel on, use a dummy pushrod in Inlet one. Perform the same procedure as above to find max lift on that cam lobe. That is take a reading just before and just after max lift off the dial gauge (using the same datum on the dial gauge) and 1/2 it giving you peak lift. Set the cam to this position. Now very carefully remove the cam bolts without losing the position.

 

Rotate the crank to the desired install figure.....for example mine is 103 degrees ATDC.

 

Now you have the crank and cam in the correct positon, all you have to do is get the cam wheel bolts to line up without moving anything! Remember the cam chain wheel offers a lot of adjustment with options for 1/2 and 1/4 tooth adjustment by using the opposite bolt holes and flipping it over!

 

Trust this makes sense

 

Iain

[RogerH]

Hi Stan,

the method in the TR4 WSM (probably the same for TR3 etc) is to get the crank at TDC on #1 and then get #7 & 8 cam lobes in an equal position

 

I will not go through the process as it is a bit long winded.

If you do this method be aware that the difference between the position of 7 & 8 when correctly set is very very small - just a few degrees on cam rotation.

I was looking for a large movement and it took me ages to get nowhere.

Doing it correctly took just a few minutes.

 

Hi Iain,

does ths mean you have a new cam installed. Did you find a reason for the noise.?

 

Roger

[iain]

Hi Roger

 

the cam checked out OK, the followers were fooked! Funnily the one with the chunk about to part company with the follower had no evidence of sticking.........

 

Apparently the followers ( phosphate hardened ones from the company that begins with C) need to be cleaned in paraffin, insert with a film of oil, checked for rotation, possibly fettled with 1200 w and Dry to ensure good rotation and not sticking, cleaned again with paraffin, given an even coating of cam lube and then inserted. You know where. I checked with County for their instructions.....less than useful they said do what your supplier says! Well I don't see any suppliers of said parts giving the advice I was given verbally, so perhaps this is the problem with premature cam follower failures in the past?

 

Then perform the cam and followers run-in as per the cam manufacturers blurb...(previously done)

 

So Cams back in, follower's are in, all degree'd up just got to reassemble the lot and keep my fingers crossed.

 

 

Iain

[john.r.davies]

Iain,

 

In the abscence of cam sprocket markings, there are two ways about this, you outline one, Roger the other.

The first requires a magic number of degrees, that the cam grinder if available should be able to give you. If not, you are stuffed!

 

The other is termed "Equal Lift On Overlap" (ELOO) and relies on the fact that "Symmetrical" cam, like Triumph ones, have cylinders acting in pairs. While one is at TDC on the firing stroke, the other is at the end of one Four Stroke Cycle and the beginning of the next. As it does so, the exhaust valve is closing and the inlet opening, and they are open the same (small, as Roger says) amount.

 

ELOO can be summarised as setting the cam in this position on one cylinder while the corresponding cylinder is at TDC. For a TR3 that would be cylinder 4 for the valves and No.1 for TDC.

 

This has been discussed before here: http://www.tr-register.co.uk/forums/index.php?/topic/13546-uprated-camshaft-timing-settings/

 

Measuring the lift on the two valves is a bit of a faff, unless you can set up two dial gauges. Sounds expensive, but a gauge and stand can cost less than £20. With two, the rise and fall of the valves becomes obvious, and positioning the cam shaft at equal lift easy.

 

I should add that setting TDC is easier without a dial gauge! As the piston rises towards TDC, the movement per degree of crank turn gets less and less, until at TDC, normal tolerances allow it not to move at all, while the crank turnes several degrees. Where's TDC then?

Use a piston stop. This blocks piston movement somewhere in the bore - position not critical. With a degree wheel on the crank nose, note this position, then turn the crank the other way until the piston is stopped again. Note that position on the wheel.

Now, TDC lies exactly halfway between the two, in the arc that the stop prevents.

Because mid-stroke piston movement is maximal per crank degree, this is by far the more accurate method.

 

As Roger indicates, ELOO is recommended in the official Triumph workshop manuals.

John

Edited January 29, 2017 by john.r.davies

[Nigel Triumph]

I've used the method Iain describes.

 

The 'back and forth' procedure may sound long-winded but it gives a very accurate TDC setting and position of maximum lift on the cam lobe.

 

Trying to measure when the valve starts to open with a dial gauge isn't accurate. The cam lobes have quietening ramps before valve opening occurs and it's almost impossible with a dial gauge to tell where the ramp finishes and valve lift starts.

 

Triumph's 1/4 tooth adjustment through the offset bolt holes in the cam sprocket is very handy for getting accurate timing.

 

 

Nigel

[foster461]

I read the book Roger and did not like the #4 on the rock method because it sounded so vague. Iain's suggestion of checking crank position with #1 inlet fully open sounds easier to nail down and is what Roger Williams describes in his book. He suggests 105 degrees ATDC for the crank position after #1 inlet is fully open with the standard TR3 cam.

 

Measuring #1 on the rise sounds like it would be more reliable since the pushrod is already moving and we are not trying to measure the point where the lobe is in transition.

 

Stan

[Guest ntc]

Stan

With a std cam it is near enough with 7/8 on the rock.Number1 tdc.

[iain]

There is more than one way to skin a cat!

 

ELOO only works on standard (symmetrical) cams. Mine is not standard, hence the preference for the methodology I outlined.

 

Also using this method it is very clear that the TDC marks on the front pulley can be out by some margin....2.5 degrees on mine for the punched hole, hence the "comps department" had remarked the pulley in the correct place.

 

105 Degrees for a standard cam sounds right, as all the uprated Kent cams are installed at 103 degrees.

 

Iain

Edited January 29, 2017 by iain

[Guest ntc]

Without a vernier pulley you will not get it that close anyway,unless your lucky.

[peejay4A]

A word of caution, stating the blindingly obvious but in the heat of the moment easy to miss. Make sure you're looking at #1 inlet (which is the second pushrod from the front) and don't time it on the first which of course is exhaust.

[RogerH]

Hi Pete,

now who would use the first pushrod (and own up to it )

 

Easily done.

 

Roger

[john.r.davies]

There is more than one way to skin a cat!

 

ELOO only works on standard (symmetrical) cams. Mine is not standard, hence the preference for the methodology I outlined.

 

Also using this method it is very clear that the TDC marks on the front pulley can be out by some margin....2.5 degrees on mine for the punched hole, hence the "comps department" had remarked the pulley in the correct place.

 

105 Degrees for a standard cam sounds right, as all the uprated Kent cams are installed at 103 degrees.

 

Iain

Fair enough, Iain!

Degree method it is!

John