Rebuild 18R cylinder head *with pics*

[timbosaurus]

I'll be taking the head off my 18RC this afternoon expecting to find a burnt out exhuast valve in cylinder number 4. I remember rebuilding a mates mazda and having to take the head in for a rebuild after it had a burnt out valve, and i think I remember it costing (for a standard regrind, lap and replacement of the single valve) around $180.

This just leads me to a few questions.

The compression of the other 3 cylinders is pretty even and around 120psi. Am i right in assuming that (with one atmosphere at sea level being around 15psi), that my effective compression ratio, after a bit leaks past the old rings, is 8:1? (120psi:15psi) I know this isn't the formal definition of compression ratio, but is it a good approximation? I would guess standard CR for an 18RC is close to 9:1. Is that true?

I'm just curious about shaving the head while it is off. If it doesn't cost me much to do($50 maybe?), would it be worth having a bit taken off just to raise the compression a little. How much is usually taken off? I dont know how close the valves/plugs go to the piston normally, has anyone had an RC head shaved before? Or ever gone too far and had a major collision? Will i have problems of too high CR before i have the problem of valves hitting pistons?

The engine is no powerhouse... moves along ok just running a 32/36 and extractors/exhaust, but i'm still in the experimental stage of my life and like trying things just for the sake of it.

Would i notice any difference in power at all? Or will it just mean i'll have to run more expensive fuel and retard the timing?

I'm not trying to build a quick motor... the conversion is coming, but if anyone has done a bit of work to an old RC top-end to make it go ok i'd be interested to hear! I'm not touching the bottom end... i need it back on the road in a week.

Cheers guys,
Timbo

[Norbie]

You can't determine static compression ratio from dynamic compression readings; there are way too many variables affecting dynamic compression, valve timing being one of the big ones.

My advice, don't waste any money on an 18R if your conversion is on the way. Even if your total cost is only $200 you still lose; you can buy a whole car with running 18R for less than that!

[timbosaurus]

Thanks Norbie. I didn't think the compression test i did would give me an accurate static CR, but the numbers just seemed to work out right

you're right, i should just look at getting another head cheaply of someone rather than getting this one rebuild, but the rest of the engine is so reliable and in good condition (new fuel pump, points/plugs/leads, bottom end done not long ago, GC exhaust and a nice carbie) that i would think it would be a shame to put on an unknown head that might only last 1000ks. with a new head, the engine would be like new again

Having said that, any cast head that looks good on inspection should last a while shouldn't it?

I was planning on having the conversion underway by now, but a move interstate put a 6 month postponement (is that a word?) on that. So i want it to keep the 18R running nice and reliably for that time. I'm not terribly concerned about the money (within reason) as i still like learning what works/is a good idea, and what doesn't. But the $150 i could save on using a second hand head would be handy... ok norbie... i think you've convinced me. Now... has anyone got a good secondhand 18RC head in canberra that dont want?

Timbo

[schnitzel]

If you go a second hand head remember there are two types, but in your case with what you've described I would be putting anew valve in getting a full valve grind and skim your head, probably about $200 plus Vrs kit and a few hours work. Unless og course you can do the conversion now. You have just got to accept it is dead money as the 18 rc is worth Zero, I couldn't even give one away last year with less than 20k on it.

Regards

Greg

[timbosaurus]

Thanks Greg!

I have the VRS kit, so yeah, thinking around the $200 mark for a rebuild. I know any money on the old RC is dead money, i dont look at it any other way. But knowing that the engine will last as long as it needs to will be good piece of mind. And i'm not that strapped for cash.

Well, the head came off last night, and there IS a busted no. 4 exhaust valve. BUT... It hasn't burnt out like others i have seen. All the ones i've seen have burnt through between the valve and the valve seat and required both the head and valve to be repaired/replaced. However mine seems to have burnt a hole through the middle of the valve!





My only guess is that a leaking inlet manifold around no. 4 might have meant that cylinder was running lean. Anyone have any other suggestions?

Another thing i noticed was the top of the piston. It has been badly pitted. It's possible it has been like this for a long time, or maybe just happened recently.





Is this a symptom of running lean and pinging too?

[timbosaurus]

You can also see in the close up valve pic that the valve seems to have sunken into the head around where the damage is.

Is it common, even with cast heads, for the valve to pull into the head like that? Or is it likely the the valve is just FUBAR'd and has distorted/worn so much? It would be nice if the valve seats didn't need any repair.


Cheers
Timbo

[Ben Wilson]

It looks like massive preignition to me, that valve is impressive...

[18rc turbo]

Are you sure that piston is pitted??? coz it looks like a major carbon build up to me.

It looks in the photo like there are still some original machining marks on the right side of the piston, and they seem to be under the 'pitting'.

Have you tried attacking the top of your piston (gently) with a razor blade or small screwdriver?? (being careful not to score the aluminium piston.)

As for that valve, thats awesome. I haven't seen another one burn through next to the stem. Did you actually have a leaky inlet manifold??? Because that is a plausible explanation.

I have a spare 18rc head, but its in VIC, and needs new stem seals.

[timbosaurus]

Thanks turboMan!

I took the head of in a hurry last night before i had to go and play touch football, so i didn't get to have a good look myself.

But you're right, it does look like the smooth piston under all the pitting, but the pitting does look metallic. I'll check it out tonight and post tmrw.

I didn't see any evidence of a leaky manifold, i'll have another look at that tonight too. That's just a theory i came up with looking at the photos this morning.

Thanks for the offer of the head... i have managed to find a spare from a kind canberrian, so i'll prolly just lap the valves on it, put the stem seals from the VRS onto it, and stick it back on. Save the cost of a rebuild at least.

[thechuckster]

that's carbon buildup on the piston - dodgy rings and a bad tune will do that

as one of those foolish to rebuild a 18R for performance purposes, can i suggest you don't? ;-)

cheers,
Charles.

[timbosaurus]

haha... thanks Chuck!

I certainly wont be rebuilding it for performance... i've heard your opinion on that before

But i do want the 18R to give me piece of mind for a while, so i'll use a second head i have, lap the valves myself and put it back on. Not costing me much at all.

The shaving of the head was just for curiosity. It would've been nice to know what sort of effect it has on general feel/drivability.

Chuck, you suggest it's carbon buildup do to dodgy rings/lack of tune which is quite possible and probable. The rings aren't too old though... maybe 18 months, but I remember when we did the rings, the pistons had a lot (probably an excessive amount) of clearance, and there is a bit of piston slap when it is cold. Nothing too bad for a non-performance engine though.

The dodgy tune is also quite possible, firstly, because it's a bitsa engine with a weber carb that has never been tuned properly, secondly because there may have been a leaky manifold to cause the high temps to damage the valve, and thirdly cos there's a bloody big hole in the valve!!!


The deposit is only evident in that one cylinder though, the others are ok. That is pointing towards a leaky no. 4 inlet manifold or it being caused by having no compression/exhaust valve.

Either way, nothing seems to fatal if put a working head back on (and of course replace all the gaskets), and I think it should last another few years after it's put back together.

Cheers
Timbo

[18rc turbo]

The shaving of the head was just for curiosity. It would've been nice to know what sort of effect it has on general feel/drivability.

Not really that much by itself. If you give the head a good port, extractors and all the good bits then it does make a difference, but as has been said many times on this forum, it isn't really value for money unless you can do everything yourself.

As you say, it should go for a few good years once you get everything back together.

Does the inlet gasket have any carbon build up around it??? coz the other option might be a cracked inlet manifold... (thats worst case though.)

[schnitzel]

I would still run a straight edge along the "new" head to check if it is true, and if not get it skimmed, is cheap and saves you having to do it all again.

That valve is certainly different, haven't seen omne like that before but would guess you have a manifold leak to cause that.

Greg

[mike1980ra40]

I would guess standard CR for an 18RC is close to 9:1. Is that true?

CR for a stock 18RC should be 8.5:1 18RG stock 9.5:1

[Screamn_Sleeka]

18rg 9.7
18rgr 9.2
18rgru 9.2
18rgu 8.7
18rgeu 8.3