Tuning Twin Cams (C&CC Article)

Post by: **ardee_selby** » Mon Oct 31, 2011 7:06 pm

Post by: **rgh0** » Tue Nov 01, 2011 6:40 am

Thanks for the scans - an interesting set of thoughts from some of the early legends of twin cam development.

I love the casual way they talk of 85.75mm and 86.75mm bores ( even if the did blow one of them up). Getting this bore out of a Ford 1500 or 1600 casting is very very very dfficult and requires careful ultrasonic testing and careful offet boring to centralise the bore in the casting and a lot of luck or lots of blocks to test in terms of the bore casting roundness and casting outside diameter.

Just some facts about achieving a 85 mm bore:

1. Cylinder spacing is 96.5mm
2. The outside diameter of the bore casting is typically between 89 and 91 mm. It is very very rare to have a block with the casting diameter round and consistently over 91 mm on all bores. This is the same for all block casting numbers or "L" blocks, no block castings have thicker bore walls than others
3. The minimum possible gap between the bore castings is around 4 to 5 mm, you cant sand cast for a smaller gap as i undestand it. This is why around 92 mm bore casting outside diameter is the absolute maximum you ever see. Ford cast the blocks for the thickest possible bore wall without siamesing the bores. Cosworth brazed in a whole new solid siamesed cylinder pack into the Ford external casting to get their big bore engines to work.
4. A 91 mm casting OD which is the top end of the normal range and a perfectly centred bore of 85mm in that casting gives you 3mm wall thickness. Given that the casting will not be perfectly round - maybe +/- 1mm - you end up with a minimum wall thickness at the thin spot of 2mm which will probably crack !!!
5. 85.75 mm and 86.75 mm bores as described in the articles give you even thinner walls and very little chance of them not failing
6. So how does the new QED block achieve the claimed 90mm bores - I have no idea, but maybe they have siamesed the bores like Cosworth did ?
7. The normal blocks also have a 8mm oil feed cross bore between cylinders 2 and 3 if you lucky this in centred between the bore casting and you can live with a small thin spot of 1.5mm wall between the oil feed and the 85mm bore. if unlucky you break through here also.
8. If linering a block you need to bore it for maybe a minimum liner thickness after machining of 1mm - that means a 87mm OD liner for a 85mm bore. You will be need to be very careful and be very lucky to have a block with sufficient metal not to break through the casting wall when boring for this liner. Even if you dont breakthough when boring for the liners the liners are also likely to split as not enough support behind them from the very thin block bore casting remaining.

May be I am negative on big 85 mm bore engines after having a couple fail with split bores early in my engine building life copying the sorts of thoughts in these articles. But when I did the measuring and maths it became clear to me it was a mugs games trying to go above 83.5mm on a routine basis.

cheers
Rohan

Tuning Twin Cams (C&CC Article)

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