Iron 4G Head Question.

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John.reader
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Iron 4G Head Question.

Postby John.reader » Wed Jun 24, 2020 11:49 am

I am building up an Iron square four, and I'm not sure what to do about the cylinder head. Although the head dates from the 1940's its been in my possession for the last 40 years, and as the engine came to me in a dismantled state, its probably been out of use for many years before that so it would have seen very little running. Certainly the valve guides are hardly worn, and the joint between the cylinder head and the rocker box looks as though it hasn't been disturbed since the day the engine left Ariel's. However due to the amount of time that it has been in storage the head is a bit dirty and rusty, and my intention was to strip it right down, and after painting, to fit new gaskets under the rocker box and reassemble. I managed to get the seized rocker shafts out without damage and also the valves, but in order to remove the rocker box I need to remove the valve guides and these are stuck solid with a mixture of rust and carbonised oil. I think that if I do try to dismantle it further I run the severe risk of damaging the non obtainable valve guides and the even more unobtainable rocker box. I know that the gaskets under the valve guides are not leaking as I have filled the valve guide recesses with paraffin and after leaving it overnight there is no discernable leak. So the big question is do I leave well alone, or do I try to dismantle it further. What's worrying me is what I have read on these pages about differential expansion of the two different metals. If the movement is under the valve guides then if that space is full of carbonised oil then do I risk cracking the rocker box when the engine gets hot. Is the hole in the bottom of the rocker box larger than the external diameter of the valve guide to allow for the movement, although if the valve guide is pushed down hard on the rocker box its difficult to see how it can move anyway. If the hole in the rocker box is larger to allow for a sliding contact there then no wonder it leaks. So do I continue to try to strip it down, with the resultant risk of damage, or do I leave it as it is and reassemble it, grind in the valves and fit it to the engine. If I do have to remove the valve guides, how do I get them out without damage. Hitting them with a drift seems a bit drastic and liable to damage them, but any valve guide puller will bottom out on the bottom of the rocker box and risk cracking it. Any advice would be much appreciated. Many thanks, John Reader.

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john.bebb
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Re: Iron 4G Head Question.

Postby john.bebb » Wed Jun 24, 2020 1:16 pm

Hi John; - I have no experience of the iron head but much with my ally Mk1's. I cannot really advise; - it's up to you to as to whether to risk damage by fully stripping to inspect & service or to leave & risk motor damage due to historic carboned oil crud & rust. Personally since the motor is in bits, I'd strip it fully... I know the adage "If it ain't broke - leave it" ... but in your case you don't know if it's broke - only leave it when you KNOW it ain't broke!!!
As for pulling the guides; - I suggest a good few days of thermal cycling in the domestic oven - 2-3 hours at max then allow to cool to room temp...
liberally apply parafin / penetrating oil to soak overnight (maybes in the deep freeze!),(molecularly; - parafin is one of the most "seeking" liquids & probably better at penetrating than most proprietory penetrating oils)... repeat process daily till you become bored.
Irreplaceable guides? - rubbish! they're only phosphor bronze billet & though it's not an easy material to machine (it gets v. hot, & is v. grabby - use carbide cutter with zero or negative rake & slow cutting speed); - any decent lathe turner / engineering machine shop should be able to replace for the cost of all three testicles. Penta Precision, Portsmouth claim to be highly proficient with phosphor bronze.
When comes to drift; - ensure drift bar is reasonably decent fit - (if not - custom machine one), Iron has greater coefex than phosphor bronze so leave head in deep freeze overnight before the dreaded drift day only remove from freezer just prior to drift. Head should be firmly supported but not rigid... suggestion would be to use a base of 3/4" multi-ply wood on a blanket on a concrete paving slab (blanket reduces shock waves which may crack heads)... fit your suitable drift, grit your teeth, clench your buttocks & welt the drift with a club hammer...
Cheers, John

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Re: Iron 4G Head Question.

Postby malcolm.johnson » Wed Jun 24, 2020 3:31 pm

The two iron 4G`s that I have worked on had cast iron guides not bronze. The first one I replaced the guides in bronze and may do the same with the second. I would personally use a press and a custom made stepped spiggot if they are really tight, rather than hammer blows to the same. Easy for you to control force you use. Make a square wooden frame to clear the rocker box and support the head if you wish. The rocker box guide holes are a clearance to the guides. There are 4 cylinder head bolt holes within the rocker box, I would get 4 suitable nuts, bolts and washers to nip the box in place, not relying only on the two either side of the inlet port, you will need these on assembly anyway.

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paul.jameson
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Re: Iron 4G Head Question.

Postby paul.jameson » Thu Jun 25, 2020 7:16 pm

From the description you give, I would be tempted to leave well alone. The rocker box will not crack due to differential expansion because the holes around the valve guides are bigger than the external diameter of the guides. If you take the guides out, you will cause a degree of wear in the holes in the head. While it will take several guide removals and replacements to cause a problem with loose guides, the less this is done the better. If you do remove the guides, you will find that you need a variety of gaskets on reassembly, all of which need to be exactly the same thickness. I had difficulty finding a full set for my current 4G when I rebuilt it. I had to replace the guides as the old ones were very well worn. If you talk to Bruce Longman he will tell you that it is well nigh impossible to achieve a seal between the rocker box and the head. In his experience 500 miles is a good mileage before the leaks start. I have managed that and more with both my 4Gs now but slight leakage is becoming less slight, I fear. It is significant that the Mk I head is specifically designed to overcome this leakage issue.

You won't lose much by leaving well alone - perhaps a set of head gaskets if the worst comes to the worst. Diving in, without availability of replacement cast iron guides, is a big risk in my book. But Drags had replacement guides until recently so give them a ring and see what the prospect of availability is if you do decide to replace. I would not fit bronze guides due to their greater expansion than cast iron; which seems to me a recipe for trouble in one way or another.
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Now Archivist once more - but not Machine Registrar.

John.reader
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Re: Iron 4G Head Question.

Postby John.reader » Fri Jun 26, 2020 1:01 pm

Thank you all for your replies. I did try removing the guides by making a wooden frame to surround the head and then turning down a half inch bolt to fit in the valve guide and then trying it under a press at my local garage. Unfortunately this only resulted in shattering the end of the cast iron valve guide, not surprising really as its only pressing down on the thinnest point on the end of the guide. Needless to say I only tried the one before giving up. As its only lost about 1/4 inch off its length and as its an inlet valve so not subject to so much heat I suppose I could still use it as it is if I had to, although not ideal. I wish now that I had left it alone, but water under the bridge I suppose. I have been in contact with Drags, but they have run out, and will not be ordering any more guides until they have enough back orders to make it worth their while, if ever, so not much hope there. I might have a go at making one to replace the damaged one, although my ancient lathe isn't really up to it, otherwise I'm becoming more inclined to just follow Paul's suggestion and put it back as it is. Am I likely to encounter problems from running with one inlet valve in a shortened guide? Again any advice much appreciated. Regards, John.

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Re: Iron 4G Head Question.

Postby will_curry » Fri Jun 26, 2020 2:02 pm

Having had to do this under other circumstances - Yamaha SR125 many years ago - do make sure
that there isn't anything still attached to the guide that's going to fall off under the effect of the
moving inlet valve. A secondhand SR125 top end was available if not exactly cheap.

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Re: Iron 4G Head Question.

Postby malcolm.johnson » Sat Jun 27, 2020 2:40 pm

There are an awful lot of engines with cast iron heads successfully using bronze guides, out there Paul.


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