What makes a good rally engine?
Discussion
There are no mini specialists in my country.
I contacted Omega Piston for information, but they could not help. I was told that "depends on application"
.
OK. Although they suggested the clearance between 0.05-0.1mm.
The engine was originaly built for Mini Se7en track racing, to be reved up to 8k. I convert it to rally use, and will set the rev limit to 7200.
Soo I decided to reduce the piston-bore clearance to 0.07mm.
I am still not sure about the honing patern. Call me a foul, but after my last engine, which developed that nasty smoke for the sole reason of a bad honing, I am overly keen on this.
The honing of the original race block was very shallow - 25-30 degrees at max, with a very fine surface. Shall I order my machinist to duplicate this, or shall I go for a "normal" 45 degrees pattern?
I contacted Omega Piston for information, but they could not help. I was told that "depends on application"
.OK. Although they suggested the clearance between 0.05-0.1mm.
The engine was originaly built for Mini Se7en track racing, to be reved up to 8k. I convert it to rally use, and will set the rev limit to 7200.
Soo I decided to reduce the piston-bore clearance to 0.07mm.
I am still not sure about the honing patern. Call me a foul, but after my last engine, which developed that nasty smoke for the sole reason of a bad honing, I am overly keen on this.
The honing of the original race block was very shallow - 25-30 degrees at max, with a very fine surface. Shall I order my machinist to duplicate this, or shall I go for a "normal" 45 degrees pattern?
Ok young Jedi, because you brought me tobacco I'll do this one more time for you. I've run all the calculations for piston expansion at your puny 62/64mm bore size. If the pistons are 4302 alloy you should be ok at 0.04mm clearance so I'd go 0.05mm to be safe. If they are 2618 I'd bump that up to 0.06mm.
Crank clearances should be factory spec as with any engine. I always stay on the low side of the spec tolerance. 0.03/0.04mm is plenty. What you already have sounds ok.
Ring gaps could be as low as 0.25mm. O.3mm should be fine. Bigger ring gaps just don't seem to affect bhp too much though so don't worry about it.
Piston/head clearance should be 1mm but your 0.8mm is ok but don't reduce it.
Valve stem/guide clearance should be stock i.e. 0.025mm inlet, 0.03/0.035mm exhaust. You can go even tighter on bronze guides.
Honing pattern should always be 45 degrees.
Crank clearances should be factory spec as with any engine. I always stay on the low side of the spec tolerance. 0.03/0.04mm is plenty. What you already have sounds ok.
Ring gaps could be as low as 0.25mm. O.3mm should be fine. Bigger ring gaps just don't seem to affect bhp too much though so don't worry about it.
Piston/head clearance should be 1mm but your 0.8mm is ok but don't reduce it.
Valve stem/guide clearance should be stock i.e. 0.025mm inlet, 0.03/0.035mm exhaust. You can go even tighter on bronze guides.
Honing pattern should always be 45 degrees.
I spoke to the managing director of Omega for you. The alloy is 2618 and he agreed exactly my estimate of 0.06mm clearance for that bore size. He also agreed my estimate of 0.25/0.30mm ring gap. The piston is still made he says which contradicts your understanding. Maybe a language difficulty.
That should be you all sorted now.
That should be you all sorted now.
Hi, sorry for jumping onto your thread but i was searching for advice on this subject and this came top in the results, i have a 1300 ford crossflow, bored out+40 a kent bcf3 cam and vernier, i did a dummy build and have about 1mm clearance, a few people have told me it is too close and i just went to an engine builder and he said just run it, it will be ok, i'm still not sure what to do either way.
Andemk1 said:
Hi, sorry for jumping onto your thread but i was searching for advice on this subject and this came top in the results, i have a 1300 ford crossflow, bored out+40 a kent bcf3 cam and vernier, i did a dummy build and have about 1mm clearance, a few people have told me it is too close and i just went to an engine builder and he said just run it, it will be ok, i'm still not sure what to do either way.
1mm piston - head clearance? Yes it's fine.yes piston to valve, since i posted this i have put it all back together and checked the valve clearances, it says in the specs for this cam to set .016" inlet .016" exh, so i did that, thinking it would increase the clearance as the valve clearances were really tight, but i did another dummy build and if anything it was worse, how could that have happened, surely loosening the rocker off would mean the valve doesn't get push as far????
Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff