Recommended engine bores

[marode]

Hi!

based on a blown head gasket, I am in the process of overhauling my standard twincam head (to QED 360 spec). After taking a look to the engines bottom end I am a bit confused about the existing bores. The block has been bored to 83,9mm (approx. measurement) and has +060'' pistons fitted (no liners). I have found an invoice for a set of pistons from Burton dated in 1990. The bores still seem to be ok, neither I can see or feel any heavy signs of wear at the piston ring return point, nor are there any marks visible in the bores. But: from what I have read, the block should not be bored over +040'' or 82,58mm (as stated in the manual as "grade 4"). Two weeks before taking the head off I had measured about 12.5 bar compression on each cylinder on a warm engine.
What would you do with the bottom end? Leave it the way it is? Fit liners and new pistons?

My car is supposed to be an 1970 +2S. No 50/1937. Most likely with its original engine.

[512BB]

Marode,

Your figures are slightly confusing. 82.55mm is the standard Lotus bore size before going oversize. At that size, a block will have STANDARD size pistons.

83.50mm bore size equates to 40 thou oversize, but you say that your bores are 83.9mm with 60 thou oversize pistons. In any event, your block must very soon shout ENOUGH boring, it will not take any more.

However, if you convert your 12.5 bar pressure, to English, and it equates to anything over 160psi, there is very likely nothing wrong with your pistons or rings, and you just need to concentrate on getting your head right, ie have you established why the head gasket blew?

Leslie

[marode]

Marode,

... have you established why the head gasket blew?

Leslie

Not yet. On the last run the car it was overheating to >120?C and I immidiately stopped the engine and took the car to the garage on a very last 5 miles run with three cool down stops to prevent overheating. The only thing I can tell is, that I have had extreme air pressure in the cooling system just after one or two minutes of engine running. Must have been exhaust gas. Taking a closer look to the head gasket, I can?t really tell were the air had taken the wrong way. I have to wait for my engine rebuilders comment after he has pressure testet the head. Hope there are no cracks.

If you think, with 160PSI of compression the rings and pistons should be ok, I think I leave it the way it is.
I will renew the crank oil seals and all the other bottom end gaskets, as the engine had suffered from massive oil leaks before. I was little concerned with the notice of oil coming out of the oil dip stick neck, as this may mean I had too much pressure in the crank case.

[512BB]

Marode,

I did not say that 12.5 bar WAS 160psi. I MEANT, FIND OUT what 12.5 bar is in psi. IF it is around 160 or above, then your rings will more than likely all be fine.

However, if you are taking the engine out completely anyway, to re gasket, then it would be a very simple step to go a bit further, and slip the pistons out as well, to check that all the rings, are in fact, okay.

If it was my car and engine, I would strip it down entirely, whilst you have the chance. Check all parts, ie crank etc., to see if they were within tolerance, fit all new bearings, seals, and water pump. Then rebuild. Your costs will be minimal, so long as all parts ARE within tolerance.

Doing it that way, you can have confidence in your engine for many years to come.

Good luck,

Leslie

[holywood3645]

12.5 Bar = 181.3 psi (12.37/1 CR) pretty high for a road engine!. Cylinder head cc may be the source of you problem. Maybe look at some thicker head gaskets, or ?? Pistons maybe even a New head..... Sorry

James

[512BB]

Marode,

I have just found out that 12.5 bar = 181.29 bar, so there is definately nothing wrong with your pistons and rings.

The only other thing that I can think of, that may have caused what you THOUGHT was the head gasket blown, is POSSIBLY, that you have some porocity in a bore wall, due to it being taken out to far more than Lotus recommended, at 93.9mm you stated, and that has then pressurised the cooling system. But would you then get 181 psi in a compression test? I do not know? Did you get 181 psi on all cylinders Marode?

Your head needs to be checked for FLATNESS though, without a doubt.

Hopefully Rohan, Brian or Nick [AVRO] will comment soon, to give their opinion.

Leslie

[512BB]

James,

180 psi is fine for road engines. All the engines that I have built for my cars over the years, have all measured around 200 psi for a few thousand miles after a rebuild, then they drop down a little, after that. And that is with standard thickness heads, not skimmed.

I do not believe in skimming heads to make them look all shiney, like some shops, if there is no need, and they are flat. Keep the thickness in the head for as long as possible.

Leslie

[holywood3645]

512BB... Well we only have 91 Octane gas here in the US.

12.3 is a little too high for our flavor.

James

[elj221c]

512BB... Well we only have 91 Octane gas here in the US.

12.3 is a little too high for our flavor.

James

James,

I think that you are thinking compression ratio, not measured compression on turning the engine over with a compression gauge in the plug hole. (warm engine with the carbs wide open, of course!)

Even we don't do that kind of compression ratio here.

The confusion between psi and bar as well......

[AHM]

Marode,

My suggestion would be that you do some calculations to work out what the compression ratio is for your engine. CR is likely to be on the high side as it increases with bore size - assuming that the combustion chamber size remains the same.

Once you have calculated the CR, you will know if 12.5 bar is high or low, and hence an indication of the condition of your engine.

If the condition of the pistons and bores are ok, you can adjust the CR with different thicknesses of head gasket.

[AHM]

I just scribbled some calculations:
CR will be about 10.6:1
consensus is 160 psi is ok for a standard engine so plus 10% for increased CR and 180 psi looks similarly ok

[marode]

Roy, you are right, I was talking about compression on turning the engine over, measured with a compression gauge in the plug hole. (warm engine, carbs wide open). I remember I had measured approx. 10.5 bar (152psi) on a cold engine a few month before. All cylinders were within 12.1 and 12.5. (175 / 181 psi) on a hot engine. I am sorry, I can't tell figures about the head thickness at the moment, cause I gave it away. I will soon get more info about the need of skimming or not and final head thickness.

Cheers, Matthias

[Chancer]

My Elan has been off the road for close to 30 years half of that in my ownership, last year I had to find another storage facility for it so took the opportunity to free the engine, get it started, flush the oil and cooling systems and better prepare it for the next bout of storage, fresh oil and coolant etc.

Anyway I was dead pleased to have 220psi on all 4 cylinders when hot, listening to the figures on here I wonder how it is going to cope with modern fuels???

AFAIK its a standard stromberg motor although probably rebuilt a couple of times.

[rgh0]

I would have a close look at the bores for any cracks as well as the cylinder head. There should have been an obvious point of failure visible on the head gasket if it was a blown gasket. Cylinder head cracks are rare but a bore crack in a block thats taken to 83.9 mm is very possible.

If both block and head are OK and the rest of the engine does not need rebuilding

i.e. The compression pressures suggest it is basically OK and I presume oil pressure is OK around 30 to 40 psi when hot and it does not blow oil smoke or rattle badly from the cam and valve gear or main bearing areas it probably does not need rebuilding.

Just check the head and block for flat matching surfaces and reassemble with a new head gasket and see how it behaves if you dont want to engage in a full rebuild at this time

regards
Rohan

[marode]

Hi folks!

I have disassembled my engine into every single bit and had taken the stuff to a rebuilders shop for some measurements and tests.

The bore measures 84.03mm, Piston 83.93mm (0.060''- type from Burton). So, with 0.10mm I am at the end of the scale for the piston to bore clearance now (I have found figures of 0.076 to max. 0.091 in the manual). The block was pretty cold, because I had left it in the trunk over night, so measurements won?t become any better with room temperature.
Also when fitted a single piston ring taken from the piston inside the bore, the gap at the ring was visibly about 1mm, which according to the engine builders opinion, is way too much.

The crank (six bolt) measures 0.75mm taken from original spec at the ends and where the rods are mounted it measures 48.44mm, so also about 0.75mm taken from original spec.
I have 0.030'' bearings fitted everywhere, which would have to be renewed. Sadly I cannot find any 0.040'' from ACL and I was warned to use the quite cheap 0.040 ones from e.g. Burton for two reasons, because of bad quality of the crank bearings and also the crank itself may become very weak when grinded more than 0.75mm to take 0.040'' bearings.
The crank should be grinded, there are quite a few marks visible. At the surface, where the rear oil seal is running, there is a big tic, apparently from clamping without the use of a sleeve. This could be repairable by using a sort of bushing, which would be available according to the engine rebuilder. Last but not least, the crank was not nitrified after the last grinding, but was apparently nitrified once. The test fluid showed nitrified ends of the shafts, but where the rear oil seal was running, it was gone, also at the main and con-rod surfaces. The good point is, the crank circulates well.

Pistons have a few scratches, not too bad.

In real life, the exhaust gas was not very blue, but oil consumption was a lot, I cannot say if all the oil was burned or just dripping off. The engine literally was covered in oil, all gaskets were very old and hardened, one reason why I had started the rebuilt. There was one thing that might be a sign for too much pressure in the crank case: I have seen oil coming thru the dipstick tube, when the engine was running.

Discussing all that with QED has left some irritation about the correct bore sizes for the pistons. QED told me, the correct size for a 0.060 piston (the onces they are selling) should be 84.074mm (which would lead to some 0.04mm left to be removed now). But my calculation would lead to a bore to piston gap of 0.144 then. It may differ, when using the QED 0.060 pistons. I had thought 0.060 is 0.060 in any case before. hmm. Other question is, will my 618F "L" block take a 84.074 bore?

I guess 84.074 calculates from standard bore size: 82.55mm + 0.060'' (1.524mm) = 84.074.

What is the recommended bore to piston clearance with 0.060 Pistons?

What would you do, having in mind, that a new block is about 1.500 or 3.300 GBP and a new EN19 crank is 875,- or EN40 is 1.200,- ?

regards
Matthias