Heads on 350

spub · Post by **spub** » 23 Nov 2009 04:45

Two questions:

Are the heads on the two jugs of a 350 interchangeable?

Were there changes over the years such that a late 70s head would not fit a mid 70s engine?

Thanks for your help.

hendre · Post by **hendre** » 23 Nov 2009 07:35

Yes they are interchangable and I would think any 350 or 400 head would fit regardless of year of production. AndrÃ©

EVguru · Post by **EVguru** » 23 Nov 2009 10:01

The whole point of the Lambertini design is that the components of the two cylinders are interchangable. The exceptions are different length pushrods (about 1mm) and the oil control rings (at least on OEM pistons for early engines).

All of the heads share the same bolt pattern. If you bore the 350 engine to 375 you can take advantage of 500 heads (501 heads on a 400). You could probably run an engine just fine with one 1973 head and one 1990 square fin head.

I'd probably have a look at the port sizes, shapes, and parting lines, just to make sure there aren't likely to be significant flow differences.

spub · Post by **spub** » 23 Nov 2009 17:52

Thanks for your replies. Both heads have same part number, and I suspected they would be interchangeable, but it does not hurt to double check. I am weighing options on fixing a stripped exhaust port thread. I cannot believe this stripped, since the threads run over 1 inch. I suspect this port was cross threaded at some time, since the front is fine and the rear was turning like butter. I can get a used replacement head, but this would mean lapping in the valves, and I cannot be sure of the condition of the guides, or how true the gasket surface is. If I rethread my stripped head the foregoing can be avoided. It appears costs of either approach would be about the same, assuming no issues with the new head.

EVguru · Post by **EVguru** » 23 Nov 2009 18:27

Stripped threads are not uncommon. Sometimes it's down to corrosion, but I think in many cases it's cross-threading, or trying to pull a poorly fitting/fitted exhaust into the head.

I've done several port repairs by boring out the port, lightly pressing in an insert and then welding.

You should note that the thread is not standard. The diameter and pitch (42 by 1.5mm as I recall) are, but it's a close tollerance fit and if you use the book figures you'll end up with and exhaust nut that rattles in the thread and is more prone to stripping.

I made a jig for my mill, but NLM do it on their lathe (they don't have a mill). I think they then machine the thread on the lather after the insert has been welded.

mgill · Post by **mgill** » 23 Nov 2009 20:38

I had a thread chaser made up. If you think it could help, you can
borrow it for the cost of return shipping. I'm in Canada.
Mathew

spub · Post by **spub** » 24 Nov 2009 05:07

Matthew: Thanks for the offer, but this port is well and truly stripped...as in missing quite a few threads....but thanks....

EV quite a nice jig - I looked at the port yesterday, and the wall is not so thick that I would ever trust my skills with this job....needs way too steady a hand and way more skills than I have acquired to date, so its hiring someone or installing a new head for me....but a tip of my hat to ya....

EVguru · Post by **EVguru** » 24 Nov 2009 09:14

Taking the parallel section of port beyond the thread as your reference, about 1/8" needs to be bored out. I made up a length of insert, cutting lengths off it as needed and boring to suit.

If you don't feel confident about doing the work, fair enough, but you only learn by trying! I found the hard part was making the weld, but then I was almost a complete novice with TIG. Why not put a help request on http://bbs.homeshopmachinist.net there may well be someone nearby who can help you out. If you go to a commercial shop it will cost far too much as a one off.

spub · Post by **spub** » 24 Nov 2009 18:02

EV: You are right, you only learn by doing, but this is not a task I want to repeat too often, so unless I am a mechanic or machinist (which I am not) whatever skills I pick up will have eroded next time I need them. Unfortunately, it appears those most familiar with this fix reside in the UK, and shipping of the head will cost as much again as the fix itself, if not more. There are more than a few machine shops in So California, but it appears that unless it is made clear this is not a simple matter of replacing a standard 42X1.5 thread and it will take some experimenting, things can go south...hence my considering a replacement head.