Porting tools

[GeorgeHurst]

Right on, that's good news. Off to the old tool shop I go to find a suitable drift in that case.
They have a pretty serious looking pillar drill (2hp I think) for £80, I'm thinking I might buy it if I manage to sell some parts to fund it, as I'm thinking of doing some lightening and balancing - questions on that to come in due course!

[bmcecosse]

[Alec]

Hello George,

I would always re-cut the seats when replacing valve guides, it's extremely laborious to just lap the valves in to correct a minor discrepancy in concentricity. (I'm fortunate, having a lathe so I can make my own cutters)

There is another option if you feel your guides are in good condition, drift them part way out and do the port work, or else mark them accurately and replace them as you wanted to slim down the port ends?

Alec

[GeorgeHurst]

Thanks Alec. part drifting them out sounds like a good idea, I'm going to get a drift and see how I get on. Though I'm not too fussed about spending ages lapping them in to correct any error... time I have - money I don't!

[Alec]

Hello George,

you also seem to have patience, which I don't have a great deal of, and I hate lapping in valves.

Do try and get a good fitting stepped drift as the guides are rather brittle and easily damaged.

Alec

[bmcecosse]

It must be a stepped drift........

[GeorgeHurst]

Aye aye... I had a go on taking one guide out with a heavy duty concrete chisel and it soon split! Got it out with a nail punch though. Going to try and get to the second hand tool shop this weekend to find a suitable drift.

Thanks very much for the advice
George

[MartinB]

Spending a lot of time lapping valves is not good for the valves, the seat shape will be wrecked. Seat should always be recut after fitting new guides, there are some very bad valve guides made which are not concentric. The non concentricity does not have to be much to make the valve/seat mismatch quite bad. The lapping process should be very quick and just the final finish to the seats.
Also, recutting the seats allows accurate seat widths to be obtained and matching the valve outside diameter (normal production ones tend to over hang a bit).
Whilst I havent developed an A series head on the flow bench, I have developed quite a few more modern heads on my flow bench and the seat shape/configuration of valve seat angles can have considerable effect on the flow characteristics of the port. I have found that it is not just a case of hogging the port out rather it is careful reshaping of critical areas like short side radius in the port, valve seat angles (3 or 4 angle seats) and valve back cut angle and chamber shrouding. All of these also depend upon the port angle as the seat and valve shape requirements can be very different.

The porting should also be matched to the cam characteristics to maximise the benefit of the head work. A cam that bangs the valve open very quickly to a high lift will need a port which flows well at the high valve lift, a milder cam which opens more slowly will benefit from a fatter flow curve and could easily sacrifice some high lift flow for better low lift flow for an overall power gain.

I would stick to following what the 'Bible' says, it should give you a decent head for road use.

Also, a lot of people don't believe in flow benches, but that is normally because they haven't got one!!

Sorry if my post has got a bit technical, I could got on for hours!!!

[GeorgeHurst]

Cheers Martin... no worries on getting technical, I can't seem to get enough of technical stuff at the moment, my head is becoming a bit of a sponge!

I'm definitely going to follow 'the bible' to the letter, rather than just hooning a massive chunk out of every port.

Take your point on seat cutting, but what I have decided to do is to 'colour in' the seats with a black chalk marker pen and then give the valves a very quick lap - just enough to take off the marker pen where the seat and valve make contact. I'm hoping that this will then highlight any irregularity in concentricity. I can then make up my mind as to how far out they are and if they need to be recut.

I'm going to get the guides from Minispares who I understand are a pretty reputable lot with there long track record and the fact that they manufacture alot of their own kit.

Thanks again
George

[les]

just putting my hand up for the-- 'always recut seats when fitting guides'-- vote, not doing this is making too many assumptions!

[bmcecosse]

Buy decent guides - concentric ones! Earlier the idea of knocking guides out 'part way' was mentioned (can't see the advantage myself) and then knocking them back in.......... Guides should always only be moved in the one direction - from top to bottom. Yes - of course to get that last 1% of extra power - there is more to reshaping a head than just opening the ports/throats/removing guide boss - but doing all that 'DIY' will give you most of the extra power available. If you want the very best - you need to pay the price to one of the well established head modifiers - they know what they are doing - they have the flow benches. And hopefully they have the results to justify the cost........ No point trying to re-invent the wheel!
Adrian Dodd comes well recommended in the land of the Mini..........
Best way to check valve seat is dark room - valve assembled to head - bright light up the port. Can you see any light around the seat??? Amazing how well that shows up any problems.

[GeorgeHurst]

Another useful trick, cheers.
That's why I'm going to buy from minispares as I'm hoping of all the guides I could get theirs will be the most likely to be concentric.