I've got a headache!

[Jentwistle3]

Hello Everyone! This is (these are) the silly question(s) of the week. After pulling the engine i found this:

The cam timing marks are aligned, and it seems that the crank is maybe five degrees advanced? I've got the engine all apart now, and I've found that the head has been shaved .040. Is that enough to account for this much advance?

Also, I've found the dreaded "L" connecting rods. In addition the DPO had reused the old rod bolts, so I suspect its a good thing that I've torn it down. Are these rods so awful that they need to be replaced or is a set of ARP rod bolts good enough for an engine who's idea of angry use will be speeding to Starbucks on Sunday morning?

The bores measure 3.267 across the board and still show cross hatching. The pistons look brand new and measure 3.245. Is that right? What is the correct piston to bore clearance?

Thanks,
John

[rgh0]

40 thou off the head plus some typical chain and crank sprocket wear will give you a 5 crankshaft degree shift from the standard marks which may not have been that accurate originally also.

The early rods are OK for a 6500 rpm on occasions road car but it is critical to have a rev limiter in place in case of a missed gear change and the best possible bolting given the small rod bolt size - so get those ARP bolts and torque them right if you stick with the early rods.

22 thou piston clearance is to much. You need to measure it at the wrist pin height at right angles to the pin as pistons are neither round or straight cylinders. It should be typically around 2 to 6 thou and the right number depends on the actual piston manufacturer specification

cheers
Rohan

[billwill]

I can't quite see how the timing marks can be aligned AND the crank timing be out by 5 degrees,.

Surely it can only be out by one tooth position approx 10 degrees on the cam sprockets ( presumably 20 degrees on the crank sprocket), or by an amount of wear on the sprocket teeth that would be so obvious that you would not want to reuse them and would only be a few degrees in any case?

When the chain stretches and the sprockerts wear you would expect to see the timing marks on the cam sprockets go out of line by that amount.

[john.p.clegg]

From Miles Wilkins Page 155 " Note that very few engines left Lotus with the camshaft-sprocket marks spot on.Do not worry if the marks appear to be slightly out,one up,one down...this is normal"

Trust the Pulley marking for TDC and time your camshafts to this correct TDC with a timing disc/vernier ( if you've got one)..

John

[rgh0]

Hi Bill
The rough numbers I did in my head when I replied as follows

The pitch circle diameter of the crank sprocket is about 50mm, thus circumference is about 150mm so 5 degree of crank movement turning moves the chain about 150 mm x 5/360= 2 mm.

If you take 40 thou off the head, then the cam sprockets are 40 thou / 1mm closer to the crank and the crank needs to turn 2.5 degrees to take up the chain slack. Similarly if you have 40 thou wear and stretch in the chain over that length turn another 2.5 degrees to take that up.

The crank sprocket wears much faster than the cam sprockets ( fewer teeth and more load) so 10 or 20 thou wear on the crank sprocket is not uncommon and hard to see from a quick visual check and that would add a bit more additional crank turn

You may also have a skimmed block and / or a line bored crank which changes the cam to crank distance also. plus the various sprocket and cam markings and keyways / dowels location will not be perfectly accurate either.

All told if you just bolt together an engine with current parts of unknown history and accuracy getting it all within 5 degrees is doing well which is why I always set cam timing from first principles using offset dowels or adjustable cam sprockets

cheers
Rohan

[englishmaninwales]

I don't have much experience on re-building the Lotus twin cams, but on other engines with unreliable TDC and cam sprocket marks I have used dial gauges and vernier cam sprockets- one analogue dial gauge to determine TDC and another pair of digital dial gauge for measuring and adjusting valve lift at TDC on each camshaft (being very mindful of close proximity of valve and piston when adjusting).
Is this a suitable method on this engine?
Malcolm

[john.p.clegg]

Yes,you've got it,maybe use a marked sitck down the spark plug hole to find approx 90 degrees before and after top dead with a timing disc (you can only use a timing disc with the engine out) to find top dead more accurately...then use the dial gauge to determine opening and closing of the cam followers to ascertain MOP then adjust as necessary...don't forget to paint mark everything for next time..

John