Smoky TwinCam - A Definitive Answer?

[andyelan]

Hi Everyone

I've just carried out what I think is an interesting experiment.

My cars a '73 Plus 2 130/5 with 37K miles on the clock and all the time I've had it, it's smoked like a *******. I've had the cylinder head off and although there's a little movement in the valve guides the amount of smoke seemed out of all proportion to the wear.

Now I know Twin Cams have a reputation as smokers but this got me thinking. Neither the Jaguar XK engine (which has a similar head design) or the Ford Kent (where the block is from) seem to suffer in the same way, however, the one thing unique to the Lotus, is pistons. With this in mind I pull the pistons and sure enough there's huge ware between the top ring and groove, 15-18 thou or around 8 to 10 the limit. So, as I had a set of standard size NOS pistons, I simply honed the cylinder bores and fitted these and doing nothing else. The result is the smoke has now completly disapeared.

Of course I realise this was only a practical solution as I had some old pistons available. It would not make sense if new pistons had to be bought, not to rebore the eggine, also, it should be pointed out that it's the pistons themselves which have worn so new rings alone would have been a waste of time. An interesting outcome I feel nevertheless.

One final point, based on a previous thread I balanced the pistons to 1 gram before fitting (the old ones were 4 grams, ie book limit) and the improvement it's made to smoothness of the engine is amazing. Let no one say balancing is a waste of time

Regards
Andy

[rgh0]

Looking at the piston I suspect the smoke problem was due to the oil rings. The old style cast iron type you had were never all that good to begine with and the bore contact faces look badly worn. New pistons and new oils rings made the difference I believe.

Replacing the pistons was over due as a flogged out top ring land is the first step to piston failure.

cheers
Rohan

[andyelan]

Hi Rohan

Thanks very much for the comments

I too came to the conclusion the pistons were in a bad way but until I had some new one available for comparison it was difficult to judge just how bad they were. Of course non of this ring wear showed up on a compression test but then I don't suspose it would. With all the oil about, I guess the compression was actually higher than would otherwise have been the case. What I did find susprising though is that this is the second original Twin Cam I've pulled apart recently (both '73 Big Valves, the first with 76K miles and this with 36K miles) with both showing the same wear of the top ring grooves and scraper rings and from this I would estimate from this that the original Lotus pistons were only good for about 20K miles at best (I know Ford used anodised pistons in their later 1600 x-flow engine so maybe this is why).

I just wanted to write these findings down however as I though it might be of interest to others who have had problems with Twin Cams burning oil and smoking. Often, it seems, when anyone has a problem with excessive smoke and oil consumption the finger generally points at the cylinder head and valve guides. What I think I've proven here is that this is not always going to be the case and it seems very likley it could equally be the original Hepolite pistons which are actually at the root of the problem and possibly a general weakness in the Lotus Twin Cam engine.

Of course non of this would really help to anyone wanting to rebuild an engine proper since if one was having to buy a new set pistons and paying full price for them, then it would not really make sense not to have the engine re-bored at the same time. Then having gone that far, it would only be logical to overhaul the cylinder head too. I was fortunate in that I had some standard size old stock piston to hand, so I'd little to lose by just trying them (I was also intereste to see what the result would be). I have however known people who have just had their cylinder heads overhauled, with new valves, guides etc, in an attempt to cure smoke and oil consumption and then have found afterwards that their engines were no better. In these cases I would suggest it's actually worn pistons rather than valve guides which were at the root of the problem.

Finally, I noticed a few weeks ago there was a thread discussing a piston failure (Things in common - Piston Mystery). The top ring groove wear I've now seen makes me wonder if it might have been somthing like this which was at the root of this piston failure experienced by handi-andy

Regards
Andy

[handi_andi]

Andy

Your observations are very interesting especially with regard to the compression test as even with my badly damaged pistons not only was i getting even pressure in all cylinders but it was allot higher than expected. I just brought a new set of CPR forged pistons and the piston ring design is different than the original lotus ones. Will be interesting to see if my oil consumption changes too, although I never had smoking problems LOL

Cheers

Andy

[Foxie]

I recently also got caught with this. Before dismantling my smoking and leaking engine(~40K miles up), I did a compression test. The compressions were excellent, all 205+ psi. There was very little blow back from the crankcase vent.

On dismantling, I discovered massive wear on the inlet valve stems and guides. (Exhausts were not bad at all) The head was sent off for new guides to be fitted, with all new valves.

While the engine was out, I decided to have a look at the bearings. I discovered that swarf from the last crankshaft regrind had done damage, so a further regrind and new bearings

I did not bother to check the forged Omega pistons & rings

I put everything back together, hoping for a brand new engine.

The smoke was just as bad as ever

Well, might as well get the hoist out again