GT750 Powered Race Car (D-Sports Racer)

[sav0r]

It's really important to get it right before you do my cylinder head....

Fingers crossed! Working on these GT750 heads is tricky because they all seem different. There's nothing to reference, especially so after they've been welded. But I think I have it worked out.

I grooved all the large (tube) bolt holes for snap rings. This means I can bolt the head to the fixture plate with zero protruding bolts or hardware. That makes facing the head (both sides) so much easier.







I am now ready to face the top side. Once the top side is faced, the head will be flipped over and bolted back down. I will reference it, square it up as best as possible, then actually machine the combustion chambers and spark plug holes. The plan is to thread mill the spark plug holes, but I will be taking them a size large and using a steel insert in the head. To finish, the head will have to be flipped back over, and the spark plug holes will be faced for the crush washer surface, at the same time I will set the depth of the spark plugs. With the plug holes being welded up I figured it would just be easiest to do it at the end.

[sav0r]

It needs a little deburring but it looks good!

[sav0r]

Oh, and my father sent me this.

[sav0r]

Faced off the gasket surface, then roughed out the combustion chambers, and bored the spark plug holes. That weld is some sticky stuff. There's some minor porosity in the welds. My father struggled with a broken TIG torch before I convinced him to switch over the MIG machine. He suspected we'd get some porosity, but we also figure it won't matter much. If the head design seems successful we can always fill the porosity and machine them again.



A video of surfacing. I just hand programmed this. I did it in two passes, the first at .02" depth, the second at .005" depth. I tossed the head on the surface plate after this and it's really good, better than I would have expected... I'll probably still lap it in a bit on the plate.

https://i.imgur.com/KDLpqqZ.mp4

Here's boring one of the spark plug holes. Pretty boring really...

https://i.imgur.com/5SOE7kg.mp4

There are only two operations on each combustion chamber left, but the run time will be around 3.5 hours to do those six operations. The first with small stepover using a ball end mill, this will still effectively be a roughing operation. This operation is needed to minimize tool load spikes for the final operation. The final operation will have an even smaller stepover, but will use a smaller ball end mill, and hopefully will produce a really clean and well finished surface. Fingers crossed, it's been a long road to get here. Let's not mess it up now!

[sav0r]

Well, i'm not super happy. I messed up the programming on the second to last operation, I should have left more stock for the final op. That means the final operation didn't get it cleaned up as well as it should have been. Nothing a little emry cloth and some elbow grease won't fix, but not exactly the results I was hoping for. Not a major F up, it should work fine, just not the finish I wanted. Tomorrow I set the heights on the top side of the spark plug holes, thread the holes, then ream them, and install the steel inserts. The hard work is all but over now, it's down to the details. I'm pretty tired currently, luckily grandma took my son for the night. Gave my wife and I a much needed rest after the crazy 12 days or so.

I talked to my dad today, he's pretty bummed out about all the Covid fallout. He doesn't think we will see any races this year if this keeps up. Not much we can do but keep our heads down, prep the car, and hope for the best. That said, now that my current projects are slimming down, my wife's RD125 is next. The engine is coming out in the next week or so.

The images make the finish appear pretty bad, but in reality the cusps are hardly detectable with a finger nail. However, they should have come out mirror like had I gotten it all right. The next time I'll have it worked out...

[sav0r]

This project is done for now. Spark plug sealing surfaces have been cleaned up, depth set. I milled the threads, but they will get steel inserts. I'm pretty happy overall with how this turned out. It's a great first try, on the second round it's only going to get better. Overall, this is the best head our GT750 engine has ever seen.

[sav0r]

My dad went through the motions of checking head volume, squish, etc.. Everything is looking good. The squish averages about .025" on all three cylinders. The motor will come back apart again and various items on the chassis will be checked over, but assembly for driving trim will begin soon. The new master cylinders are fully mounted with a new balance bar, the clutch cylinder has been revised, new clutch with Barnett discs and springs are in place. We should be able to run up through fifth gear without slipping the clutch from three on. And hopefully the car will be drivable in the pits too. I'm hoping to make a visits to my father's place this weekend, I'll try to grab some photos of the car.

[sav0r]

The car is looking really good. The compression ratio came in right to 7:1, stock being 6.7:1. Perhaps a spot higher than we really wanted, but it may help the systemic over-rev we weren't pleased with. My father has picked over a number of the small details on the car, nothing major to report, but this is how racing machines work. If this lock down process continues (fingers crossed), I may start machining some of the more elaborate parts for the car. Currently it uses cast iron front upraights from a Triumph, that was common for the period. We can however update those. I'd also like to get some of the rear drive train on the lathe and lighten it up, maybe replace the cast steel seal carriers with billet aluminum bits, just generally shed some weight from the car. Almost all of the major weight that is easy to get rid of is unsprung weight, for a sub 1000lbs car that's a very important area of improvement. Especially so for rough tracks like the street circuits we generally compete on. Here are some photos. Keep in mind I'm posting 12mp photos, the ports look rough but in reality they are smooth as can be. I spent the rest of the time working on RD125 parts, I'll get photos of up for that project tomorrow.

[sav0r]

The head is officially ready to go on. My father has been picking through a few other issues around the shop (valve job on his Mini Cooper S and stand alone ECU on his hot rod Mini), so the DSR has taken a bit of a back seat. He says he ironed out some timing irregularities he think existed, definitely small but perhaps troublesome. The new chain tensioner bracket that i mentioned machining on a few posts back is in, and it seems to be really nice and stout. Other than that, not much has changed.

[sav0r]

It's all back together. My father modified the exhaust a bit, going to a larger ID perforated tube to the silencers. He also has the PI System (now owned by Cosworth) hooked up and is trying to calibrate the three EGT's. Keep in mind this data logger is something like 25 years old, we are running the software on a HP Windows 95 laptop from a 3.5" floppy disc haha. So while the data logger is certainly modern technology in relation to the car, we've done our best to run vintage data logging too.

It runs - https://i.imgur.com/YwRdZw2.mp4

[tony27]

Nice work on the waterpump gear & new combustion chambers, doing that sort of stuff is very satisfying
It's funny just how sticky weld material can become compared to the original base material, there must be a slightly different make up between the 2 materials. I have a head for my CZ400 that was welded & machined which has the same porosity issues as your head, never seemed to affect how it ran although when it turned up I was pretty unhappy after having seen the work done by the company on other heads
Did you end up correcting the slight misalignment between the cylinder bores & combustion chambers I've seen mentioned before? I assume because the whole combustion chamber was fully welded & you measured the top of the block to make your sub-plate that you would have been able to get the positioning exactly right

[sav0r]

I think if my father had started with his MIG machine the porosity issues wouldn't exist. There was some issue with his TIG torch and he kept getting bad welds. The torch ultimately was replaced, but that was after he MIG'd the head basically problem free. Virtually all of the areas with porosity were touched by that broken TIG torch. Old dirty aluminum has proven tough to weld in general though. What also surprised me is how quickly the head naturally anodized. It was like 20 minutes at most after machining that it lost its shine. Whatever alloy these things are made of it doesn't react like any modern variants.

I got the head misalignment mostly corrected but it's not as good as i was hoping. Next time around I'll fix the head to the table, then bolt the cylinders to the head, then I'll use the touch probe to find the center of each cylinder. I'll set each cylinders center as its own work offset. The cylinder will get removed, then the program to machine the combustion chambers is run individually for each cylinder based on the real life spacing of the cylinders. This will assure that the combustion chamber is perfectly placed for each cylinder based of the cylinders themselves, also meaing the cylinder head is custom matched to each cylinder set and might not fit to other cylinders as precisely. What I tried to do was model the cylinders and reflect that onto the head model. Even after creating a dxf by touch probing the cylinder I never was able to get an accurate reflection back onto the head because there's simply nothing worth referencing on the head itself. I hope that description makes sense. Hind sight as they say is 20:20, and in the case of this head work I learned a ton. The carburetion has changed right off the bat, so something seems to have worked haha.

[sav0r]

Today they announced the cancellation of the Pittsburgh Vintage Grand Prix. It always takes place in mid to late July. I suppose it's for the best but it certainly takes the edge off the last 9 months worth of preperation.

[sav0r]

I was approached by a gentleman in Australia about my water pump gear. So I made him one and it came out pretty much perfect. The finish is exceptional.

[sav0r]

That carb thread got me thinking about Lectrons so I got a hold of my buddy that offered us a set. It took him a day or two, but here's what he sent me.







They are 2mm larger than our VM's that we ran last year on our Water Buffalo, and they came off a GT750 from one of the cars my father raced against. Period for the vintage of car, and likely a sweet upgrade. He's going to send them to us. We will have to work out throttle cables, but I think we are going to give them a try.