The GT750 monoblock barrel is a very stiff design which is unlikely to twist or warp in action. Unlike individual cylinders, the three cylinders and heads are not free to move around and in theory should be much more reliable.

The head gasket has 75mm bores which is considerably larger than the 70mm cylinder bore. Was that because it was originally designed for a 75mm bore or was it just large manufacturing tolerances?  If anyone has access to the original designers, please ask them.  The rest of us will just have to work out what, if anything, we want to do about it.

With so many bolts and sleeve nuts, holding the head to the barrel, there isn't a lot of movement, so the head is somewhat close to going more or less back in the same place every time.  Right?

Not necessarily. We recently took a GT750 head and placed it on top of a barrel and turned it upside down to see how well the head and barrel aligned. the answer was that they were not close.

So the head and barrel were both drilled on the four outer 8mm bolt holes.  Thin wall 10mm dowel pins were inserted to ensure that both parts stayed in alignment.

The dowels were out of a Honda CB160 and used to fit between the barrel and head on all four outer studs.

Next, the barrel and head were fitted together and upturned and the mismatch was still there.

As can be seen quite clearly above, at one side the head is overhanging by 2-3mm. All three cylinders were similar, though not identical.

It could be that matched pairs form a running bike might be better matched, but this is a problem. Computer simulations using Bimotion cylinder head design program showed that on an average stock motor, it might be alright most of the time. But if the block was machined, bringing the pistons closer to the head, Maximum Squish Velocity (MSV) could range from 50m/s to 700m/s.  Since the limits for MSV are 30-50m.s depending on fuel, it can be deduced that this is not a good situation.

On big bore motors that situation is exascerbated by the larger overhang.

The simple solution is to machine out the overhang with a Dremmel or die grinder. A better solution is to machine all the combustion chambers to perfectly match the barrel they are fitted to.

Head gasket

Let's go back and re-visit that head gasket. With a 75mm bore measured at the steel sealing ring, there will be a 2.5mm gap all around the bore, right?  Well, not exactly. 

This used gasket is clearly oversized, as are new ones, and the holes are misplaced.  Relative to the center cylinder, the two outer holes are displaced outwards. The head gasket holes are not concentric with the combustion chambers.

That leaves a smaller gap on the inner side and a larger gap on the outside. Is that a problem?  On a stock motor it's probably OK most of the time, but if conditions are marginal because timing slips or jetting is off because of dirt or deposits in the jets, things can go pear shaped very fast.

Squish Band Heads

A few years ago, an enterprising guy who makes his living modifying two stroke motors, designed a squish band type insert head for the GT750 and offered them for sale. Unfortunately some of those motors torched head gaskets and the pundits were soon crying foul.

We have only examined 1 set of those inserts and found their shape and general design to be in accord with generally accepted design principals. The only design element that we would have preferred to be different is the tall ring around the outer shell presumably there to stiffen the inserts or to conduct head away from that outer edge. We would prefer to have teh upper surface ribbed and that wall removed, but that's not the main event.

One could argue that they would be better with O rings but that would have required a replacement head gasket or O ringing the outer edge of the  head.  Not a trivial task.

The set we examined and measured had an uncorrected Compression ratio of 13:1.  We have no way of knowing if they were race inserts supplied in error or if that head was sold to someone for racing and then onsold it.

So if there was nothing much wrong with the head inserts, why do they torch head gaskets? based on a sample size of one, we believe that the head gaskets fail because they are so large and that the gas in that ring either dteonates or becomes so hot that the gasket fails.

One could also argue that a flat top design, similar to say a TZ250 V twin, might be better.  Or maybe one of those funky looking toroidal shaped chambers would do the trick. The fact is, that those inserts are pretty much the same sort of shape as almost every aftermarket chamber insert on the market.  The real problem is the stock head gasket with oversized holes and heads that do not align with the bores.

If a different head design is to be tried, it must be aligned to the corresponding bores on a head that is doweled so that it goes back in exactly the same place every time. or people with stacks of spare parts, check the head and barrel before you port it and machine the surfaces to be sure that they work together as a pair, or machine the head to match.