500cc DomiRacer

Hello,

I am planning to build a 500cc Domi Racer engine to compete in classic races in Japan.
I plan to use parts from JS Jim for the pistons, connecting rods, valves, and cams.

After careful consideration, and taking into account the compatibility of the ignition system and various other parts, I concluded that using the crankcase from a Commando 750 would offer the most benefits.
I will use 500cc parts for the crankshaft, connecting rods, cylinder, and piston to create a 500cc engine.

So, I have a question for everyone: has anyone ever built a 500cc engine using a 750cc crankcase?
As shown in the photo, I did a test assembly, and it seems to be working fine.

In Japanese classic racing regulations, as long as the displacement meets the requirements, other factors aren’t usually an issue.

I cannot answer your question, but I’m very interested in your build! Please do a ‘build thread’ …

An excellent project but it would be better to use later 88ss/650 crankcases because the 88 barrels bolt straight on. The 750 cases require work to line up the barrel flange to crankcase stud holes. You can fit the early points in timing cover parts to the Domi crankcases with a little work (I did it on mine).

If unlimited funds are available go with late 750 Commando cases, alloy 750 barrel, a one piece 59mm stroke crankshaft, special lightweight cam followers, carbon fibre push rods, lightweight rockers and valves etc. Tune for up to 10,000 rpm. (That is probably just me dreaming!)

Whatever you do steer clear of 72 750 cases, ie any set of cases without the large sump plug. The cases you currently have do have the large sump plug, this has a filter so if you do have a blow-up large debris will not get to the oil pump.

It's my understanding that Domiracers used pivoting finger cam followers, which I toyed with for my 650ss. You're not considering going that way?
Cheers

Thank you, everyone.



I did a test assembly of the case and tried fitting the 500cc barrel, but as expected, the stud positions were off by a few millimeters.


When using the 500cc case, there are still issues to resolve regarding the use of the JS cam (Commando type) and the ignition system (planned to be Tri-Spark).

As shown in the photo, the cams differ between the Domy and Commando types, and it looks like I won’t be able to use the Domy cam sprocket if I use the Commando cam.

I’m also considering whether I can replace the timing side parts and the timing cover with Commando parts.


I’ll keep you updated.

Probably telling you what you already know, but the optimal timing for a Commando and a 88 are a bit different.

Also, a high lift Commando cam might be a bit aggressive for an 88. You may end up with quite a bit of flutter/bounce with those steep ramps designed around such a long stroke motor. Jim will have good advice though. A super light valve train will be a must.

Is it the cam tunnel/trough in the cases being too narrow for the high lift cam that you're running into?

@hybridracer, I'd be very interested to see your ignition set up on the domi cases.

t ingermanson said:

Probably telling you what you already know, but the optimal timing for a Commando and a 88 are a bit different.

Also, a high lift Commando cam might be a bit aggressive for an 88. You may end up with quite a bit of flutter/bounce with those steep ramps designed around such a long stroke motor. Jim will have good advice though. A super light valve train will be a must.

Is it the cam tunnel/trough in the cases being too narrow for the high lift cam that you're running into?

@hybridracer, I'd be very interested to see your ignition set up on the domi cases.

Click to expand...

I used a battery powered mk1 Boyer electronic ignition unit in the Commando timing cover, driven by a 4S cam. I can’t do photos because I stripped the 88 engine in 1984 to put the parts back on the 750 Combat race engine for a final few races.

In order to fit the 4S cam and Commando timing cover the following was done -
increase clearance in the cases and the bottom of the cam follower tunnels for the cam lobes,
fit a thrust washer to the cam (on the Domi cam this is an integral part of the cam),
fit Commando intermediate gear/sprocket, cam sprocket and nut,
Grind clearance for the chain on the tensioner mounting boss, the tensioner will not fit behind the chain so I reversed it and made a spacer and longer studs so it fits in front of the the chain.
There didn’t seem to be any oilway issues in the Commando timing cover so that bolted straight on.
I think that covers it.

I only used it for 3 race meetings because of a fuel starvation problem with the 32mm Amal concentrics. They had worked fine on the 750 but on the 88 the fuel stopped flowing above about 6000rpm. Prior to cutting out, the potential to thrash all but the fastest Commandos was clearly there but I had already started loosing interest and didn’t have the time or facilities to sort it out.

I heard back from Jim.

He mentioned that there’s a 500cc version of the JS cam, so it looks like I’ll be able to use the 500cc cam sprocket and intermediate gear.
Next, I plan to order the following kit and sprocket to install the Tri-spark.