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MG Midget and Sprite Technical - Oil sucking
First 1275 engine build since the most recent discussion on breathers and oil sucking so before I forgot (again) I had a closer look.
The video posted years ago showed how oil builds up in the timing cover to a level that reaches the breather hole in the cover, so I had a good look at the escape route by which it drains from the timing gear back to the sump. In this engine, late 60s 1275, there are two holes running horizontally through the main cap, each leading to only a half hole on the rear of the cap, dropping vertically to the bottom of it. (see pic) The diameter of the half hole is less than the horizontal hole leading to it. That appears to be the greatest restriction to the drainage from the timing chamber. While oil could also discharge between crank and cap, the gap on this engine was well less than 1mm, so with cold oil and (ideally) minimal pressure head not much would exit by that route. Having small half holes makes sense from a structural viewpoint - the further they encroach the weaker the main cap. But would that be the case if the holes were made larger by extending to half-oval... wider while not increasing the encroachment? |
Paul Walbran |
OH NO--- Hi Paul Yep, I reckon you might be onto something The earlier caps for some of the smaller engines had the holes at an angle and the cap was shaped to expose a full hole With the crank laying flat(half stroke) the edge of the crank must go close to exposing the holes, maybe cut/grind the half round accross in the direction of crank travel into a taper, sort of teardropy shape so the crank helps pump the oil out as it spins--maybe If the corner where the horizontal hole meets the drains is at rightangles maybe round out/radius the corner there for better flow-? and that might get the hole a bit more exposed when the crank is laying flat also---maybe maybe found this pic of the early type on the left |
William Revit |
Oh no indeed. Lol.
Well why not eh? Given it's quiet at the moment. 😅 Have you got a sucker then Paul? Drilling/altering the front main cap as a 'potential' solution was considered, but unsure about weakening it, it wasn't attempted. Plus, given the amount of oil involved, it seemed unlikely to solve the problem anyway. Interesting, maybe significant though, that there are different hole arangements in the ealier caps Willy. Drilling an additional drain hole in the block was also dismissed. For those unfamiliar with the issue of oil sucking, take a day or two and have a read through this thread. 😭😅😅. http://www.mg-cars.info/mg-midget-sprite-technical-bbs/engine-breather-oil-sucking-yet-another-engine-2014091416212710294.htm |
anamnesis |
Nope, not this one - just contemplating the phenomenon during a full rebuild.
Looking at the drain holes, the horizontal ones through the cap are larger in diameter than the half holes leading away from them, so the half holes are definitely the point of greatest restriction. Point of my post was would modifying the cap make it weaker? In particular, if the half holes are made wider but no deeper (= don't reduce the front to rear thickness of the cap any further) would that weaken the cap? As to whether it would make a difference, my gut feeling is that it would - we know that most dont but some do, so the drainage must be marginal. In my head, when things are marginal a relatively small change tips the balance. An example here is that when oil suck occurs it lasts only a few minutes then clears. Presumably as the oil warms and gets a little thinner it starts draining away properly. So, if the sectional area of the half holes could be increased 50% by making them wider without narrowing the cap, it could help. Hence my question - would this weaken the cap? On a quick think, the stress in the cap will be greater at the narrowest point, and also greater at the midpoint and at the supports (where bolted). Given the proximity of the half holes to the centre the narrowness could be the predominant factor and if so the material either side of the half holes may not be contributing much to reducing stress concentration. Anyone have a nice stress analysis program? |
Paul Walbran |
"IF" I remember right---trying not to, the original sucking thread was more about finding the cause of the oil sucking more than modifying---There were a few different suggestions of mods to fix it but the aim was more -not to modify ,more to fix the cause.
I reckon that hole through the front of the block would be a foolproof mod that would fix it without having to mod the main cap----so started poking about to see if anyone had done it and found this pic of a front plate which appears to have the hole in it ready to go--just a hole in the block to line up with it and it'd be the go I reckon--- |
William Revit |
Also found this-description--for duplex chain--it also has the hole-----? https://www.somerfordmini.co.uk/plate-engine-front-mounting-for-duplex-chain-69-74 |
William Revit |
Yep. Definitely Willy. Rectify the cause, rather than find a way to work round the problem. But it failed. So a solution was found instead. But so much better if someone fixes the cause.
Interesting Willy. Haven't seen a front engine plate like that before. At least I don't think I have. Or are mine like that. I'll have to check, esp' as my rwa Midget was a '73. Yep point understood Paul. It's the main why attempting to enlarge those holes wasn't tried; esp' as, at the time, there wasn't an idea on how to test if it would actually work. But, as you say, -- "Presumably as the oil warms and gets a little thinner it starts draining away properly." I agree, so how about this for an idea? Rig up a front main cap at the bottom of a receptacle, so that it's the only drain. Pour in cold oil. Time how long it takes for the oil to drain through the existing original holes. Warm the oil a bit and time again. Warm a bit more, time again. Then enlarge holes, repeat above, and compare. |
anamnesis |
The puzzle to me in this and in the other mega thread was that this doesnt occur in all engines. I have never yet had experience of the full "oil sucking" scenario, and it seems many others haven't either. To those that have, its a serious issue that needs a solution. To the rest its a storm in a teacup, or a small tempest in a chaincase.I Clearly there are a lot of variables involved. For many these will cancel out but for the damned few they will conspire to accumulate to cause a real problem. |
GuyW |
"Interesting Willy. Haven't seen a front engine plate like that before. At least I don't think I have" That front plate is from an early engine (non-1275) as it doesn't have the reinforcement bracket. It seems to me that the large hole to the left of the camshaft is below the level of the hole in the timing cover, at least partially, so surely the oil should just drain through there. |
Dave O'Neill 2 |
The holes in the main bearing cap are angled downwards slightly. The drill bit is 6mm, so I'm guessing the holes are 1/4".
|
Dave O'Neill 2 |
Dave,
I don't think it is an overflow/drain hole. I reckon it's purpose is to route pressure/gasses to be vented to the chain cover breather. Hence there would be pressure acting to prevent oil draining back through there. At the same time, suction is coming from the breather via the pcv system. Sump pressure helps in pushing the oil, as pcv suction lifts it. And, it's height (top and bottom), relative to the entrance hole to the breather canister, means that by the time oil reaches that level, it's a little too late . See sketch. And I think the same must be true of the drain holes in the main cap. But all, at only those goldilocks conditions, of temperature and viscosity. Yep that's the mystery Guy. Why only some engines, and not most? And why of those some, it seems to start with a rebuild; perhaps it's first since being assembled in the factory? |
anamnesis |
Anam I wonder if the position of the overflow hole varies? Your drawing shows the bottom of the overflow hole above the bottom of the breather hole, whereas Willy's and mine (below) are lower down. |
Dave O'Neill 2 |
Maybe Dave, if so, maybe that explains things. Or maybe the heights in the drawing I posted are wrong. But I think even if it does vary in height, the pressure pushing from the crankcase into the chain cover, prevents oil flowing back into the crankcase via that hole. I just don't reckon it is an overflow hole. It's too high. There's no need for that much oil in the chain cover. |
anamnesis |
Why is level of oil in the chaincase not dependant in part on the flow in via the front camshaft bearing. This in turn related to camshaft end flost, oil pump delivery, crank bearing losses, sump surge and oil viscosity. If theres a problem to be solved, you can't just look at the oil return route. |
GuyW |
The holes through the maincap 'should' be more than enough to return the oil, it was decided last time that crankcase pressure was fighting against the oil returning through the holes and the fix then was to vent the crankcase from the dummy fuel pump hole and T that into the normal vent hose from the timing cover.
You can actually buy a neat little alloy housing with a 1/2" hose fitting for doing this. If i had an engine apart i'd be drilling a decent sized hole (1"-? if there's room inside the block) through that front plate and the front of the block to create an extra vent between the crankcase and the chain cavity which would also work as a drain if it was positioned where that hole in the plate is. |
William Revit |
You pipped me to the post Willy. 😅.
"Why is level of oil in the chaincase not dependant in part on the flow in via the front camshaft bearing." It is Guy, and not just in part; rather, almost entirely. 😁. Imagine that. Every comment and question about oil sucking is being repeated, again. Improved drainage 'might' tip the balance Guy. The oil in the timing cover is pretty well entirely dependant on the feed from the end of the camshaft. That's exactly where the problem starts. At the goldilocks moment, in some engines, too much oil is squirted in, which completely overwhelms any existing ability for it to drain away fast enough, to prevent it being sucked up the breather, by the pcv system. Draining, which can only be by the oil flowing back into the sump, via any openings in the block (either cap holes or block face holes), is further opposed by the pressure caused by blowby, pushing against any excess oil attempting to drain. The problem *IS* too much oil for a short period. It's not insufficient drainage. But if drainage could be improved, it may just tip the balance back from sucking, for the short interval of time needed, for the goldilocks temperature and viscosity to pass. The Tesco philosophical approach. 😄 And as Willy says, a different solution works, by 'splitting' the pcv suction. |
anamnesis |
I wasnt suggesting that improved drainage was the wrong way to go. Rather, what was in my mind is a likely explanation of why some engines start to do this after a rebuild or refresh, when they hadn't done so previously. |
GuyW |
If just drilling a big hole through the block solves the issue why did BMC not do it, as it seems the drillings / grooves would have been much more expensive to form ? Is there a requirement to have a controlled flow out of the chain case to allow a limited sump quantity of oil to create the oil bath required for the timing chain lube ? R. |
richard b |
There's no indication BMC/BL acknowledged the problem, even if they knew about it Richard. In the late 1970's when oil sucking first happened to my engine, I went to a main BL dealer/service garage/parts centre. I explained the problem to the head mechanic. He'd never heard of it. I rang the number in Oxford where I was told the experts were in technical. They'd never heard of it either and told me it was impossible for oil to be sucked up the way I described it to them.
I still have my original Haynes WS manual in which I wrote the BL Oxford technical telephone number I was given. I'm sure you're right about the oil at the bottom of the timing chain cover. A small amount, carried round by the chain, and as it drains, kept topped up by the small amount entering via the end of the camshaft. It's just some engines, that under the right circumstances, are flooded for a short while as they warm up from cold. |
anamnesis |
Maybe it wasnt acknowledged as a problem back then because it wasnt seen as a problem. An acceptable trade off for some engines, under specific and short term combination of things to blow out a big blast of oil smoke for a few minutes. No great problem at the time and not seen as especially important. The trade off was a design that maintained an immersion sump for the camshaft chain to cool and lubricate a component which was well known for wear.
It was a time when drivers were expected and encouraged to warm up engines before setting of, and to drive gently for the first couple of miles. Those that didn't probably experienced the blue smoke cloud, if they happened to be looking in the rear view mirror. And they may have briefly had that 'oh sh*t, I've blown the engine' feeling and were just relieved when it cleared a few moments later. And, ringing a technical support in late 1970's would be no different to today. If its about a known fault that the manufacturer doesn't want to admit to you will get the "no sir, we've never come across that before" response. |
GuyW |
Yep all most likely Guy. Esp' --
"It was a time when drivers were expected and encouraged to warm up engines before setting of, and to drive gently for the first couple of miles." Add to that, it really is only a very small number who seem to have ever had this exact problem. Plus it only seems to happen after an engine rebuild. My engine was over 12 years old when I rebuilt it and first had the problem. So I really do wonder if BL did know, other than perhaps from the odd mention by blokes like me. Certainly not something that would warrant money and time to investigate. |
anamnesis |
This thread was discussed between 11/08/2023 and 14/08/2023
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