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Overheating leading to partial tight turnover

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Hi eveyone,

I am new to the forum and looking for some advice on my latest engine build. I have some theories, on the issue, but would like to draw on people's experience here. This engine is in a gravel stage rally car, for info. The engine was re-build because of excessive oil consumption, basically using about 1 litre of oil per 30-40km competitive driving. Turned out to be the valve seals and guides


JDM B18c Type R (96 spec)

Built to 100% factory spec, with regards to bearing clearance, ring gaps, compression


Running .25 over spoon pistons, which are match weighted OEM pistons

New oil pump

Only thing that is not factory spec is a block guard(which in hindsight, wish I had not fit). The block was bored and honed after the brace was fitted.

Break In:

The engine has been broken in, roughly following guidance from HP academy on the street - basically got it running and got straight out onto the street and did some 5-10 sec pulls and then into overrun, gradually building the throttle and rpm up to about 5000rpm. I got about 50km on the engine.

Next I entered some local rally sprints with 4x4km stages where I gradually built up the intensity of driving over the next 50km, building to some WOT pulls through 2nd, 3rd, 4th

At this point the engine is feeling strong and not showing any major signs of issue, however on one pull through the gears, I remember noting that it felt like the engine was struggling when at the top end of 4th, but put that down to super strong wind blowing down the straight that day.

First Stage Rally:

Basically the engine is pulling strong, but quickly starts to overheat a little (don't have accurate temp gauge unfortunately, but OEM gauge is just over half way, which is not typical from previous running before the re-build).

We turn the heater on and back off to only using about 5000-5500rpm, which keeps the temps from going any higher. So we carry on, with the temps stablised.

At the first service stop after about 35km competitive, we turn the engine off and try starting again and the starter will not turn the engine over. The engine turns with a spanner on the crank and feels a bit tight, but only really on compression.

Once cooled a little (10-15mins), it starts no problem and is running great again. We go out on the next stages and basically the same happens as you would expect.

First thought, was whether I had messed up the compression and we are running super lean and heating the piston crowns up and causing excessive heat, but check that on the dyno ....

On the Dyno:

Ran the car on the dyno last week and did some AFR checks at various rpm at WOT. The AFRs are certainly not lean (from my knowledge, but please correct me if I am wrong), running at about 12.5-11.9 at 7000-8500rpm @ WOT.

The engine sounds great and is pulling strong.

Theories : (in order of likelihood from my opinion, but please do share your thoughts!!!)

1 - I am thinking perhaps one of the ring end gaps is too tight and as the engine is getting a hot, the gap is closing and then introducing more heat into the bore(s) which is then creating a self perpetuating cycle as more heat is produced the ring gets tighter which equals more heat.

2 - The thermal expansion on the block guard is higher than the rest of the engine and is distorting the bores, which in making the factory spec gaps in-adequate.

3 - the block guard is causing water flow restriction which is hurting the cooling and if I can keep the temps down into the normal range the ring gap my be marginal, but not a problem.


- Try and increase the cooling capacity. Currently running a half width twin core rad and a 16 row oil cooler, both with shrouds and ducting, which has work previously. So could move to a custom full width, twin core rad and move my oil cool in front of it. Also consider wrapping the headers, which are right behind the rad in a full width configuration.

- Pull the head and sump off, then pop the pistons out and take 0.1-0.2mm of the top ring gap and 0.05-0.1 off the 2nd ring and 0.20mm off the oil rings. Probably on the larger side, to ensure I don't have to do this twice. Taking care to note the ring positions and putting back in the same place. Put it back together and see what happens. Perhaps carefully put some more holes in the block guard too - block has a drain plug in, so flush out the metal.

fyi OEM ring specs are

Top - 0.20-0.35mm, with service limit of 0.60mm

2nd - 0.40-0.55mm, with service limit of 0.70mm

Oil - 0.20-0.50mm, with service limit of 0.70mm

Any help and guidance is much appreciated and feel free to ask questions if I have missed any information which will help in the discussion.

NOTE - another good investment that will be happening is some good gauges and a wide band for AFRs - spent so much on this, if getting some good gauges saves a destroyed engine it is worth it, since the B18c is getting hard to find and expensive.



Hello, my thoughts are the ring gaps are too tight I would be setting them to 0.45 top 0.5nd and leave the oil ring as it is but before just grinding them remove the pistons and inspect for any damage. the ring ends will show wear on them often looks shiny if they have been touching, I wouldn't go any more than the measurements I have given. the overheating could be compounded by the rings also but I don't have any experience with the block etc so i cant comment if they are a issue

Regards Ross

The OEM gap spec's have a rather wide range, if aparently a bit on the tight side - exactly what did you use?

It is certainly possible the rings are just butting, but I would expect a total engine failure if that was the story - you can buy cheap boroscopes to have a look for any evidence of scoring of the bore which would indicate such a problem.

It may be perfectly normal for the engine to be a little harder to turn by hand, especially on compression, when it's warm as there 'should' be less leakage.

That the starter doesn't turn the engine over - no movement whatsoever? - isn't that uncommon if there are other issues, such as a smaller/weak battery, light gauge wiring (including on the earth side), worn/burned commutator, etc - or heat soak - if the exhaust runs close to it, a heat shield should help. Next time I'd suggest checking the voltage between the starter terminal and it's body, and that across the batter terminals when cranking is attempted - it should be within 0.2V if possible, as most starters will require around 9V to even operate and any excessive drop can really hurt cranking performance - if either power or ground return is over ~0.1V, then look for the reason. I'd also suggest removing the starter and checking it - use some fine W&D and brakeclean to remove the oxide and crap off the commutator (it just needs a light clean) and check the brushes still have plenty of material left on them, make sure the bushings and solinoid pivot points are well, but not over, lubricated - I usually use a little moly' grease and engine oil - that bit of maintenance can make a big difference.

The cheap bore scope options sounds like a good idea, before I pull the engine apart. Hopefully I can take a look this weekend.

When setting the gaps, I went for the smallest gap, top 0.20, 2nd 0.40, oil 0.20mm

Another question :)

If the ring gaps are just closing, would you see bore scoring? The pressure on the bore would have a fairly even distribution?

I was very careful to de-burr the rings after setting the gap. Learnt that the hard way previously :)



Bit of an update...

So, I have pulled my engine apart and checked all of the ring gaps. All of the top rings were in the 0.4-0.45 range except one, which was 0.35-0.4mm range. So the tighter one got increased to 0.4-0.45mm range. The 2nd rings were all ok.

Lesson here for me it so make and save notes on the build to refer to.

But, I don't think the ring gaps are my issue??

I will have the car back out testing soon, so I will be able to report back if I still have cooling issues and seizing issues.

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