18R head machining, camshaft upgrade

**rtrona** · 03-02-2011, 08:52 PM

Hey guys,
Ill get in before everyone says get an 18rg. (Theyre hard enough to find in tasmania, and most of them are flogged out anyway).

Ive just brought a cam from another member off here, it has a fairly mean grind to it. I am wanting to up the compression to suit it. I rang an engine machinist who said i would need to adjust the cam timing after he decks the head. also he mentioned something about the standard chain tensioners would need modification? How true is this? Everyone i know with aftermarket cams havent had to do this.

All i was planning to do was get the head planed, ported, and upgrade the valve springs which i already have. Is there anything else that i should be doing? I will be running the 32/36 at this stage until i have enough spondoolio to purchase some Weber 45DCOE's.

Id just like some people's opinions, would an adjustable cam gear be needed? Are there other ways to raise compression on the 18r?

Cheers guys

**mattysshop** · 04-02-2011, 09:19 AM

i have floating around somewhere a pair of twin sidedraft manifold adaptor ally ofcourse, that i made up for my 18R to run the twin solex's off an 18RG... this was close to 10 years ago... basically for the same reason as your describing... i had a 2 RG's, one good (went into my old mans ra40), one parts motor with corroded water jackets..

my 28 with the 18R with a bit of compression (20 thou off the head) a 70/30 style cam, and the twin carbs made it on par with the 18RG in dads ra40.. valve bounce above 6800though.. haha..

you must use some sort of extractors, they won't fit with the cast manifold, honestly it's probly one of the best things to change straight away anyway..

PM me if your interested in the manifolds..

**Shifty** · 04-02-2011, 09:34 AM

Hi mate

If you want to stick with the 18R, you're on the right track with your mods.

1. Your friends are probably fine because they haven't increased compression. If you just machine the head for a fresh surface, and fit a small cam, there's no problem. If you fit a big cam, you need to increase compression and that's where the 'problem' comes in. It's nothing to do with the cam itself.

When you shave the head like this, you are changing the height of the motor, by taking meat off the bottom of the head. Accordingly the distance between the crank and camgear is reduced. To make up for this, you should get an adjustable cam gear, and may need tensioner mods to take up any slack you introduce into the chain (because now it's too long as it has to run over a smaller distance).

2. Shaving the head is the only cheap way to improve compression. Your other option is to get a new set of high compression pistons fitted. At the same time you can probably go to 92mm, I can't see any reason why you couldn't do this (the same as you would with an 18RG). Starts to get expensive that way, though!

3. You'll need to do carby/carbies & extractors at the same time. A single weber manifold would be great. You can run twins but then you have twice as many carbs to buy, overhaul, and tune.

**rolleraction** · 04-02-2011, 10:08 AM

+1 to Shifty's commentss ^^^^

**rtrona** · 04-02-2011, 11:10 AM

Hey guys thanks for the feedback. The engine itself is probably only 15thousand old from a rebuild. Already have extractors and exhaust sorted. Does anyone know anyone who does high compression pistons for the 18r?

Thanks again

**Shifty** · 04-02-2011, 11:37 AM

Prettymuch any good performance shop or engine builder should be able to get you some.

**mattysshop** · 04-02-2011, 04:22 PM

several of my 18R's have had vernier style gears.. actually i can't remember one that wasn't!

i never bothered with it though...

**prp120** · 05-02-2011, 07:22 PM

also remember that there are 3 different styles of combustion chamber designes in 18r heads. When i did my 18r-c up ( a long time ago). Mine is a 77 model(12/76) and had the shitty pollution head but an earlier engine has a great flow head with larger valves and less combustion area( higher comp)( actually resembled a 308 yella terra head design)which when i fitted this head ( ported and polished, big ass cam and 45 dcoe carb) upped my bhp from 129 to 152 cant remember torque figures. ( the 129bhp was from original head but all tricked up too). So hunt about before jumping in or grab the later model head for lower compression and fit a whistler

**Dazza** · 09-02-2011, 05:21 PM

The 18RC head had big casting burrs right where the ports meet the valve bowl area which made them flow poorly. you can open them up with a die grinder so they are as good as the earlier heads. If you run a DGAV weber you can gain a bit by opening up the inlet manifold so it matches the base of the carby and adaptor plate. Even with the standard carby the inlet manifold was smaller than the base of the carby.