The Triumph Dolomite Club - Discussion Forum

Just had a chat with my engineering firm and they have hit a snag boring the 1850 block out. Their cutting tool is snagging on a piece of the casting on top of the main cap. I wondered if anyone had encountered this before and if there is a solution?

Options given to mare to put on a stop on the boring tool to reduce the depth, so it doesn't hit this bit of the casting or grinding this bit of casting away. Does anyone have any advice?

Why do the main caps need to be fitted when machining the bores?

xvivalve wrote: Why do the main caps need to be fitted when machining the bores?

I assume the casting he was referring to is at the top half of the main caps, which are part of the block casting. He is saying that is fouling his cutting tool.

That is the area that does break. Try to bore only the stroke needed and have as much flesh left around the main bearings.

Jeroen

soe8m wrote: ↑Thu Aug 25, 2022 6:30 pm That is the area that does break. Try to bore only the stroke needed and have as much flesh left around the main bearings.

Jeroen

So what you're saying is I should ask him to restrict to rebore to the length of the piston stroke? Does anyone have exact measurements I could pass on?

cleverusername wrote: ↑Thu Aug 25, 2022 6:46 pm

soe8m wrote: ↑Thu Aug 25, 2022 6:30 pm That is the area that does break. Try to bore only the stroke needed and have as much flesh left around the main bearings.

Jeroen

So what you're saying is I should ask him to restrict to rebore to the length of the piston stroke? Does anyone have exact measurements I could pass on?

Yes. Blocks suitable for competition are the ones that don't have bore marks in that area and unfortunately it's about 1 out of 20 that has. That area is the weak point of the engine and some machineshops bore even deeper at a rebore. I alway's round the edges with a dremel to have not that sharp bore edge.

Just the stroke and a bit is enough and will keep the engine stronger.

Jeroen

Here the rounded edges. Also shotpeened this area to have any scratches and edges removed what can become the start of a crack. When the bore marks are already in the threaded holes it's useless for serious power. This is such a rare one that hasn't and will be ok.

Jeroen







And here what happens when bored too deep. When the main bearing area where the bolt goes in has such a crack you can throw away the casting.

soe8m wrote: ↑Thu Aug 25, 2022 6:59 pm

cleverusername wrote: ↑Thu Aug 25, 2022 6:46 pm

soe8m wrote: ↑Thu Aug 25, 2022 6:30 pm That is the area that does break. Try to bore only the stroke needed and have as much flesh left around the main bearings.

Jeroen

So what you're saying is I should ask him to restrict to rebore to the length of the piston stroke? Does anyone have exact measurements I could pass on?

Yes. Blocks suitable for competition are the ones that don't have bore marks in that area and unfortunately it's about 1 out of 20 that has. That area is the weak point of the engine and some machineshops bore even deeper at a rebore. I alway's round the edges with a dremel to have not that sharp bore edge.

Just the stroke and a bit is enough and will keep the engine stronger.

Jeroen

Thanks, I will pass that on and see what he says tomorrow. I may well be asking for more advice because I am passing what the engineer told me down the phone. This is outside my area of expertise, so I might not even be describing the problem properly.

Ok update, spoke to engineer and he is going to put a stopper on the boring tool. The issue is, he normally run the tool all the way through and waits till it hits the void to stop. In this case the casting was hitting his tool and rapidly blunting it.

So hopefully, by putting a stopper just beyond the length of the stroke, the issue will be resolved.