I just bought a 76 2.0 it actually runs very similar to my other 73 2.0. My mechanic pointed out the tin was butchered in order to install 1.7 heads in the 2.0 motor. Should I change the heads o just live with it until there is a problem?

Pictures?
1.7L heads have a different (4 bolt) intake runner bolt pattern and smaller valves than a 2.0L.
The only difference in the tins is the location of the spark plug holes. Either close up all unused holes in what you have or switch to the correct tins that are not that hard to find. Or wait for the engine to overheat and damage itself from airflow loss.
What intake system is it running? FI or carbs. Finding a set of 3.0 heads in good shape is not that easy or cheap.
Are you really questioning if you should fix it ?

The difference is performance oriented. As I understand it Porsche breathed on the 2.0 heads, but not on the 1.7s. I would say if you are satisfied with what you have, drive it. You might prowl the classifieds and find a pair of 2.0 heads for the future though. They have gone up in price at least as fast as the stock market.

It has two one barrel cheap carbs that actually run good. I have the tin coming and a friend has two 2.0 heads he will sell me. The tin I have is for a 2.0 but should I put the 1.7 tin in the engine compartment ? I'm satisfied so far with performance but I'm doing a compression test

With a two-barrel carburetor I doubt you'd feel any difference between the 1.7 and 2.0 heads. And if it runs well, as you say, then it's probably jetted right, so I'd say just drive it like it is.

Just as a heads-up there are two kinds of 2.0 heads, those with four intake studs from VW buses and the three-intake stud heads from Porsche. If the tins you're getting or your buddy's heads are for a bus engine then you won't see any gain at all, in fact I believe the bus 2.0's are lower compression than the 914/bus 1.7 heads, so you'll probably see a decrease in performance. The only reason to switch tins at this point would be if someone hacked new holes into your current tins for the relocated spark plugs from the 914 2.0 heads, and left openings that leak cooling air.

Those can't be just regular 1.7 heads since they will not fit on the bigger diameter 2.0 cylinders. So for sure, those heads were machined. The question is what else was done to them? Bigger valves? Chamber size? Ports size? And if they were recently rebuilt, you have a bigger chance of finding cracks in the 2.0 ones than the rebuilt 1.7.

If you assume a quick and cheap job on the 1.7, only opening them up for the 2.0 cylinders, then the compression should be very high compared with the original 2.0 so that may actually give you some benefit in part of the power courve. It's like the old trick of mounting cheap 1.8 heads on 2.0 to gain compression and improve performance.

Yes, the 2.0 914 heads have larger combustion chambers than earlier 1.7 heads and larger valves as well. With your 1 barrel carbs, the smaller valves are just fine.
By all means plug the extra holes in the tins where the 2.0 spark plugs went.

If your running carbs, you should be running slightly cooler due to the more fuel going through them. Your car MAY be more prone to overheating due to the air flow question. But if it runs drive it, but maybe not in the heat of the day in stop and go traffic.

Post some pics. Your mystery will be solved.

Great info thanks so much I'll post pics later

Here's some photosClick to view attachment

Other side pretty chewed upClick to view attachment

Speaking from experience, those little carbs run really smooth. Right sized and all...

Going with twin two-barrels may be popular, but most of the time people get them too big, and drive-ability suffers, despite maximum power going up.

That said, there is a noticeable difference when running them on 2.0 heads vs. 1.8 heads, which is probably what you have. (Only the casting number knows for sure. And even that is secondary to any mods that might have been done.)

There's about 5MPH more in 5th gear with the 2.0 heads...

Next question, do the manifolds have the 3-bolt pattern as well as the 4-bolt one? Mine are CB Performance, they do have a one-size-fits-all arrangement that needs washers to cover the slots.

Mike thanks so much. Not sure about manifold yet. Just trying to sort things out. I was pretty upset at first the heads were swapped out but after driving it pleasantly surprised. Will probably do a compression and leak down test and replace the tin to 1.8. I think I'll keep the carbs for now though I was tempted to go the Weber route like my restored 1972 VW bus

That head tin is sad....

Replace the tins. What is scary is what else on the engine was done to that Kuality of work.
When you pull the top tin see if all the other tins are there. Just helped a freind with his motor that gave up. They had no cooling flaps in it and no rear tin on the back of one head.
Fix it before it dies like his motor did.
Post more pics if you can of other parts of the engine. I bet this crowd can find more things wrong.

Will do