Not enough slack on timing chains

[265bhp]

Hi all

The next resto project I am undertaking is an XK120 but as the engine is so similar to the E I thought I would pose a question here please

I'm re-building the engine and I have got to the stage where I'm trying to put the chains around the sprockets that bolt to the ends of the camshafts

My problem is that I don't seem able to generate enough slack in the chains to do this and I am completely puzzled as to why this should be

Rotating the eccentric idle shaft isn't generating enough slack as I thought that was the issue

I thought it was maybe was the head gasket that being new and thicker than the old steel version was causing a problem, but I'm not convinced

I've taken the head off multiple times so could get a better look behind the timing chain cover...but I'm really stumped as to what I may have done wrong

Just wondered if you all had any thoughts?

Best
Jonathan

[mgcjag]

Hi Jonathan. ....what parts are new.....and whats original....eg a new chain could be short?..its possible to refit the adjuster wrong so its maxed out befor you start.....Im re rerbing an xk150......well worth you getting on JagLovers....they have an XK section with some very knowledgeable guys on 120/140/150.... have a look here it may help.....Steve https://forums.jag-lovers.com/t/xk-help ... unk/304665

[265bhp]

Hi Steve

It could indeed be a short chain, although all the new parts have come from Twyfords as a full rebuilt set…as they organised all the machine work for the engine so although I must check them on Monday, I have a reasonable confidence that they give me the right bits.

(Chains are new, as are all toothed sprockets)

Your suggestion of the tensioner being set wrong to begin with is a bit on my mind so I’ll have a little look at that tomorrow..and will check out Jaglovers too

I just hope I don’t have to take the timing cover off because that’s sump off and it’s just such a backward step to have to make

Oh well…All first world problems and tomorrow is another day. ….

Best
Jonathan

[abowie]

Hi all


Just wondered if you all had any thoughts?

It can only be that the chain is too short. C2256 is the same chain for all the XKs, Mk series and 6 cylinder E types.

All the other bits (assuming they are correct for the engine and correctly installed) are in fixed positions relative to each other.

PS. I'm assuming that you're fitting the sprockets into the chain with them disconnected from the camshafts. There will not be enough chain to get the chain on if you already have the sprockets installed.

[RICHOT]

Hi Jonathan,

What XK120 have you got?? - Mine is a 1950 OTS (the only model they made then).

Rebuilt it around 2007, can't remember having any issue with the timing chains.

As others have suggested, I suspect the new chains may be a tad too short. Have you asked Twyfords for advice?

Good luck,

Richard

[malcolm]

[/quote]

PS. I'm assuming that you're fitting the sprockets into the chain with them disconnected from the camshafts. There will not be enough chain to get the chain on if you already have the sprockets installed.

[/quote]

Sounds from the original post wording that this could be the issue?

[nefematic]

Just wondered if you all had any thoughts?

As Andrew asked, we first need to know if you are attempting putting the chain on the fitted sprockets - this will not work since chains are too short for this. The chain is lifted on the loose sprockets, which together with the fitted chain are then fixed to the camshafts. It‘s a tight fit.

[christopher storey]

The chains are not fitted over the sprockets : instead, you place the sprockets inside the chains and then fit the sprockets to the camshafts

[265bhp]

Hi Christopher…that is what I am doing…chains are over sprockets which are located in the sliding holes in the timing chain inner plate…as per Andrew’s picture

The best I can get is one sprocket fitted but I need less than 1mm to fit the other but the adjuster for the top idle sprocket is on its maximum position

I have counted the same number of links (50) for the upper timing chain and it matches my old one and I have undone the fine adjustment wheel and it’s spring plunger and re assembled..but I am not seeing any difference

It will doubtless have a simple answer but it is eluding me

I have spent another couple of hours investigating today and am getting no where..so I will now stop for the day and call Twyfords in the morning to ask about the chains they supplied


Ho hum….

[mgcjag]

Hi Jonathan...read the one from last post form the JL link i posted above.....it may well help you fit the sprockets.....Steve

[265bhp]

Hi Steve

Thanks for that ..it’s so strange having this problem as when I built the E Type I had no such issues…anyway….
I will call Twyfords in the morning and see what they say..but I am warming to the idea of the old style metal gasket..with either copper coat or well seal..maybe Tywords have a preference

Let’s see..!

[abowie]

Have you got the intermediate sprocket C25275 in backwards? The large sprocket goes at the front and is connected to the lower chain.

[265bhp]

Hi Andrew
It’s the right way round

Having talked to Twyfords this morning they are sure all their parts are the correct spec…so having thought about it all day I am going to take the sump off….timing cover off and strip all the sprockets….at least that way I can check old against new and have a better idea of what’s going on…something isn’t right…so better to go backwards for a couple of hours then I can rest easy

Thanks all for the suggestions….will report back

Best
Jonathan

[265bhp]

Hi guys.


It would appear that the simple answer to this question is in two words… “Head gasket…”

I decided to take the engine apart and having kept the original timing chains, and indeed, the ones from my E Type rebuild from a number of years ago, I compared the new chains with my old ones… Although there was a small difference in the length for the upper timing chain. It really was quite negligible so although I didn’t discover anything particularly startling, I did at least satisfy myself that I hadn’t built the engine incorrectly.

Speaking with Twyford they said they’ve never really come across this problem, so there wasn’t a magic solution… But what did seem to be the issue at hand was the fact that I was using a new style composite gasket with a thickness of approaching 2MM, where is the original steel ones are as we know…I suppose less than half of a millimetre

We also compare my new chain with another one they had in stock and all there was a fractional difference. It wasn’t major, but I took this new longer one to see if it would make a fundamental difference… It didn’t.

So with Wellseal applied to both sides of the metal gasket |(that I got from Stuart at Coventry Auto Components) I reassemble the engine and everything did indeed slot into place.

So perhaps the answer to this is that having had the head skimmed by the merest fraction (it was actually in remarkably good original condition, so didn’t need much skimming) the use of the composite gasket was just too much

Let’s see what happens when I start the engine up …

[abowie]

Interesting, and a good outcome.

Stock Payen gasket is 32 thou/ 0.8mm.

I've used Cometic gaskets up to 100 thou in the past, but that was to prevent overly high compression in a motor that had been skimmed a lot.

[MarekH]

It would appear that the simple answer to this question is in two words… “Head gasket…”

Full disclosure helps.

When people skim v12 HE heads, they worry about compression ratio but forget that the v12 has a single timing timing which goes up and down four times, so skim the base by "x" and the timing chain is lengthened by "4x". They then face the top of the cylinder blocks and the whole tappet carrier sits lower down and the chain "stretches" fourfold again.

That's the exact opposite to what you have done, but the same factors are in play but with different multiplication factors.

An old (worn) chain might have fitted without your being any wiser.

kind regards
Marek