Compression

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James k
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Compression

Post by James k »

Hi,

I felt as if my car was low on power today so, immediately suspecting the head gasket, I did a compression test. Hot engine, throttle wide open.

1: 140
2: 140
3: 110
4. 95

After setting the valve clearances and noticing that those on 3 and 4 were too tight, the readings were;

1.140
2.140
3. 120
4. 120

So still lower on 3 and 4 but is it enough to suspect the head gasket?

The overall figures are quite disappointing too as they were 170 when I put the engine in, in 2015. I tried again with some oil in the bores and got:

1. 170
2. 170
3. 140
4. 140

Presumably this means the rings/bores are worn which, again, is quite disappointing.

I rebuilt the engine, as I said, in 2015 but I did not rebore it and only honed the cylinders and fitted new rings. I also did not have unleaded valve seats fitted and just replaced the valve guides and ground in the valves by hand.

Is it possible, or perhaps even expected given the level of the rebuild that the bores have worn in the time I've had the engine in?

With regards to today's compression figures, does it sound like the head gasket is going or something else? Is it going to be a case of having to take the head off.

Thanks,
James
oliver90owner
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Re: Compression

Post by oliver90owner »

If the valves were set far too tight - you don’t give details - I would initially suspect valve trouble, rather than head gasket (although head and block surfaces would be checked when the head is removed).

I’m suspecting, without sufficient information, that the head has original valve seats and has been run on unleaded fuel?

A leak-down compressed air test might provide more info, before stripping the engine (further than manifold removal).
James k
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Re: Compression

Post by James k »

Hi, thanks for the response. The valves were set too tight on 3 and 4. I can't say exactly how tight but it was too tight for any of the feeler gauges to fit.

You are correct that the head has the original valve seats and has been run on unleaded. Perhaps it's worth lapping them in although I remember reading that if the bores are worn, lapping the valves can exacerbate the problem.

Thanks,
James
James k
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Re: Compression

Post by James k »

I should add that the car is a daily driver and does get driven on A roads and motorways at high revs for extended periods. I imagine this probably makes it a good candidate for valve seat recession.
oliver90owner
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Re: Compression

Post by oliver90owner »

As I posted previously, applying, say, 10Bar (~150psi) to the combustion chamber is likely to indicate if the (exhaust) valves and seats are leaking - as long as you are not deaf! It is a simple check, no more complex than a compression test - if one has (or can make) a suitable connector and a compressor (even a decent battery-driven tyre inflator will work).

It may appear that you were (fully) aware of the need for suitable valve seat arrangements (while using unleaded fuel), but have chosen to ignore the likely outcome of not adhering to those requirements.

Again, complete information is best, for a decent distance guess for a diagnosis - you have again mentioned a dimension with no clarification. My sets of feeler gauges mostly (but not all) range as low as 0.015mm (which is effectively zero clearance in your case). That alone should provide the answer to your woes, if you are sure the valve clearances were set correctly at previous service/maintenance intervals.

Please tell us how lapping-in poorly sealing valves can affect other parts, such as cylinder bores - unless it is a high-mileage engine already requiring remedial work, anyway.
James k
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Re: Compression

Post by James k »

Hi,

I will try and track down an air compressor for a leak down test. If not, I might just pull the head at the weekend and test them with petrol down the exhaust ports.

I was indeed aware of the likely consequence of not fitting hardened valve seats. When I built the engine, I was on a tight budget and needed to replace an engine that was blowing copious amounts of blue smoke out the back and knocking at the bottom. It was a budget rebuild intended to last a few years, the plan being that I'd be in a better financial position by that time and be able to do the job 'properly. It's done seven so far as a daily driver so I'm not too upset at the prospect of rebuilding another one.

With regards to lapping in valves, I was told once that you shouldn't increase compression on an engine with worn bores as it will push past the rings more. I'm happy to accept that this is nonsense, though.

Thanks,
James
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Re: Compression

Post by Bowie69 »

Yep, that last bit is true in that it can happen, but nonsense as far as it being a problem!

If the bores are that worn, it is just someone making excuses for not repairing the engine properly...
oliver90owner
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Re: Compression

Post by oliver90owner »

It was a budget rebuild intended to last a few years, …. It's done seven so far as a daily driver

Seven years? I might well have enquired how often you might have checked the valve clearances during that time, but I won’t. :lol:
James k
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Re: Compression

Post by James k »

Is it lasting seven years good or bad, I can't tell what you mean. :lol:

The clearances have been set every six months, since putting the engine in.
James k
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Re: Compression

Post by James k »

I should add that I last checked them in February. I've never had them closing up significantly before but I have done a fair amount of motorway driving since February. There's no blue smoke out the back and the engine doesn't seem to burn oil.
simmitc
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Re: Compression

Post by simmitc »

It's not time that causes wear so much as mileage - 7 years at 1,000 miles per annum is vastly different to 7 years at 15,000 miles per annum!
James k
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Re: Compression

Post by James k »

Indeed. It's done about 25k miles in that time. Not a huge amount but I don't know what to expect from a budget rebuild.

I'm going to decoke the head and lap the valves this weekend, after checking how well they're sealing. I'll check the head for flatness too. I assume it's still the consensus to leave the carbon on the pistons alone.

I'm going to start disassembling the original engine at some point soon, with the aim of slowly rebuilding it with a rebore, crank regrind, unleaded conversion etc. The gearbox bearings are noisy too so I plan to rebuild that once I've got the engine done and the current one out.

Thanks,
James
James k
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Re: Compression

Post by James k »

Hi,

I've now finished lapping the valves. I decoked the head, cleaned up and lapped in the valves and replaced the top hat seals on the inlets, leaving the exhausts with no seals. There was no damage to the head gasket and both head and block surfaces are flat. The valve seats are pitted though and definitely need recutting, and unleaded exhaust seats fitting. I think I've improved the situation but it's not a long term solution.

The compressions are now reading 145 on all cylinders. Still not great but better than before and at least consistent. The engine is running more smoothly too.

I'll get to disassembling and inspecting the original engine soon.

Thanks,
James
oliver90owner
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Re: Compression

Post by oliver90owner »

James,

Sorted. Good. At least for the time-being.

A few points which may help with future ‘rebuilds’. Rebuilding is not the same as reconditioning.

Were the valve seats recut when the new guides were fitted? A requisite part of the process, over-looked at your peril.

The 170 psi original compression pressures were carried out when and how? I suspect cold and before the engine was run?

Fitting new rings to worn bores is not absolutely an ideal situation. Did you remove any ‘ridge’ in the cylinders (you should have done, if honed properly)? In fact, was there a ridge at the top of the bores? That can be a good indication of bore wear.

Bores should be checked for both ovality and wear. First requires an internal micrometer, the latter is easily carried out by checking ring gap in a worn and unworn parts of the bore. What were these values when you decided to re-ring it?

What were the side-clearances between rings and piston grooves? Again, over-looked by many. Often, new rings are fitted to out-of-spec pistons. The top ring groove is the usual culprit.

Your 25k miles is likely fortuitous if everything is still satisfactory.

The 145psi readings are perfectly acceptable.

Driving at high speeds, for long periods, is not good for heads, without hardened seats, if running on unleaded fuel without any ‘additives’. But you should have simply diagnosed the valve/seat problem yourself as the valve clearance reduction is a huge clue.

Not recutting the valve seats, (if that was the case) may well have been partly to blame for the problem arising.

Worn bores/pistons/rings are often suspected if oil consumption is high (and oily deposits on plugs), excessive crank case breathing, condensation in the valve cover, etc. Not usually noticed as a fall in power (unless pretty obvious signs are missed).

Compression tests should include resetting valve clearances before carrying out the test when a problem is suspected.

Regarding flatness, of head and deck, do remember to check lengthways, crossways and on the diagonals. The usual initial check is by using a light behind the straight edge, only checking with feeler gauges if required.

Good luck with your other engine rebuild or recondition.
James k
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Re: Compression

Post by James k »

Hi,

Thank you for your detailed reply. It is good to know that 145psi compressions are acceptable. The original compression readings of 170 were done with the engine hot after its first run.

From memory, I did the following to the engine. It was a while ago though so I may have forgotten some bits.

-Engine fully disassembled and cleaned, including removing oil gallery plugs, cleaning out and fitting replacements.
-Bores honed and new piston rings fitted.
-New main and big-end bearings and thrust washers fitted.
-New valve guides fitted with mini guides and top-hat seals on inlets.
-New rocker shaft.
-New cam followers.
-Head and block skimmed.
-New oil and water pumps fitted.
-Duplex timing chain fitted.
-Spin-on oil filter conversion.
-All parts inspected for wear and any washers etc replaced as appropriate.
-New thermostat and housing.
-Carb disassembled, cleaned and reassembled with new jet and throttle spindle.

I assume this qualifies as 'reconditioning' rather than rebuilding as the block was not rebored and the crank not reground.

I should point out that the engine I reconditioned was not the smoking, oil burner that I referred to earlier. It was another engine, already in reasonable shape.

There was a minute ridge at the top of one cylinder, from what I remember, which was honed out. None of the bores had any ridges after honing.

Valve seats were not recut and instead ground in by hand. I was advised at the time that this was acceptable if a uniform grey ring could be achieved, which it was.

I do not still have the measurements for the various clearances but I do remember measuring the ring clearance and it being acceptable, along with the bores and crank, which I had someone else check.

Regarding additives, I do try to use Castrol Valvemaster but am admittedly not particularly well disciplined with it. If it runs out, I'll just run on unleaded while I get round to ordering more.

I feel I may be underselling the engine slightly. It doesn't smoke or burn oil or make any bad noises and it runs basically fine, pulling up most hills in top gear and not having a problem with doing 70mph on the motorway. The bores are not scored and the honing marks are still visible. Aside from this head issue, and leaking a bit of oil, it hasn't given me any problems. It was my first time reconditioning an engine and, like I said, was done to budget constraints so I'm reasonably happy with the service it's given. But... I do know that I could do better and I do feel that it doesn't quite have the power it did when I put it in.

The next engine I intend to qualify as a 'rebuild' with no corners cut. Assuming the current engine holds out for a while, which I daresay it will, I don't really have any time constraints so can do the job extremely carefully.

Out of interest, how many miles would you expect to get out of a 1098 rebuilt to 'as new' condition?

Thanks again for taking the time to reply in such detail.

James
oliver90owner
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Re: Compression

Post by oliver90owner »

Your next engine might be regarded as a recondition... Rebuilding, in my book, is just putting back together - maybe with some new parts - but not to the exacting standards of the official workshop manuals.

Reconditioned would, hopefully, mean fitting the appropriate grade of piston, depending on the bore diameter, or the bores honed to accept the pistons supplied - as I said, back to factory specification.

Crankshaft would have to be within factory specs regarding diameter, ovality, straightness, too.

A lot of ‘reconditioned’ engines are likely not as per factory specification - often with inferior warranty.

I have generally only rebuilt engines in my distant past. But I don’t claim to be a reconditioner - more a repairer. A fairly large proportion of the engines I have rebuilt have long become obsolete, with spares (to factory specs) simply no longer available.

Our current vehicles have covered about 170k and 135k miles with no known engine repairs (beyond normal service items) and no mechanical problems at the present time. Our previous vehicles had covered 160k (clutch failure finally scrapped it) and 195k (turbo failure). I don’t think these older designs are likely to last much over 100k? And likely less?
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