15cc Seal Help

[Vixen]

I have been re-doing the drawings of the 15cc Seal engine from fractius to decimal inches in 2D AutoCAD.

When I draw the end view of the crankshaft, conrod and piston, I find the piston protrudes 1/32" (31 thou.) above the top of the cylinder block and into the combustion chamber space. The Westbury drawings clearly show the top of the piston as being level with the top of the cylinder block. Question... Is there an error in the published conrod length or height of the cylinder block? or should the piston protrude?

There have been many Seal engines built which run successfully. so I am sure this question must have bee asked before. What did you do?

I have also checked the drawings and dimensions for the Seal Major. Here the top of the piston IS level with the top of the cylinder block, unlike the Seal minor. This makes me suspect an error in the Seal dimensions.

What did you do?

Mike

[Jasonb]

Adding things up confirms what you say, thick gasket maybe?

[Vixen]

Hello Jason,

Pleased you were able to add up those dimensions and get the same answer.

The question is not whether to add a thick gasket. The question is " should the piston crown be flush with the top of the cylinder block or should it stand proud?"

It is a simple matter to reduce the conrod length by the offending 1/32"  if that is what is required.

What have other Seal engine builders done?

Mike

[steamer]

As a question, is there clearance in the head for the piston with the 1/32 protrusion?

As just a point of general design philosophy, the Wallaby is flush as well.

Dave

[Vixen]

As a question, is there clearance in the head for the piston with the 1/32 protrusion?

As just a point of general design philosophy, the Wallaby is flush as well.

Dave

Hello Dave,

I'm not sure, the castings are not due for delivery for another two weeks. The CAD drawing says there should be 3 thou.  (talk about banging your head)

I could machine the cylinder head casting to give whatever clearance I wish, in the combustion chamber.

With the piston protruding, the top piston ring is also very close to popping out of the cylinder.

I'm pretty sure the published length of the conrod is too long by 1/32" (31 thou).

Many other have walked this path before me, It would be interesting to hear what they found

Mike

[steamer]

As a question, is there clearance in the head for the piston with the 1/32 protrusion?

As just a point of general design philosophy, the Wallaby is flush as well.

Dave

Hello Dave,

I'm not sure, the castings are not due for delivery for another two weeks. The CAD drawing says there should be 3 thou.  (talk about banging your head)

I could machine the cylinder head casting to give whatever clearance I wish, in the combustion chamber.

With the piston protruding, the top piston ring is also very close to popping out of the cylinder.

I'm pretty sure the published length of the conrod is too long by 1/32" (31 thou).

Many other have walked this path before me, It would be interesting to hear what they found

Mike

.003".......honestly....there's probably that much stack tolerance of the average build.....probably more!....but I get what you're on about.

Dave

[Vixen]

And it makes a large difference to the Compression Ratio.

Mike

[nj111]

Mike, I've not seen the drawings for this engine, so I may be going off at a tangent , but I recall a similar scenario with the Whittle V8 whereby the pistons protruded a similar amount - however that engine has a 31 thou thick spigot plate under the heads - which brings the pistons back flush.

[Jo]

I find the piston protrudes 1/32" (31 thou.) above the top of the cylinder block and into the combustion chamber space. The Westbury drawings clearly show the top of the piston as being level with the top of the cylinder block. Question... Is there an error in the published conrod length or height of the cylinder block? or should the piston protrude?

The drawing is wrong:

The piston sticks out of the top of the crankcase. This is not a problem as there is a 4.76mm deep recess under the head for it to do that. The ring (if using aluminium pistons with rings) will still be 0.4mm down the bore of the liner.

There is no Head gasket on either the Seal or the Seal Major

Jo

[Vixen]

The drawing is wrong:

The piston sticks out of the top of the crankcase. This is not a problem as there is a 4.76mm deep recess under the head for it to do that. The ring (if using aluminium pistons with rings) will still be 0.4mm down the bore of the liner.

There is no Head gasket on either the Seal or the Seal Major

Jo

Jo, If you look carefully at the cylinder head drawing, you will see the 4.76mm (0.187") occurs only in that part of the combustion chamber over the side valves. Whereas above the protruding piston crown there is only 0.003" clearance.

Also the top ring 0.4mm (0.01") below the end of the cylinder is unnecessarily close.

I still believe the conrod length is wrong and am trying to verify that as a fact.

Mike

[Jasonb]

Mike, where are you getting your 0.003" from?  I can see a 1/32" recess in the head over the cylinder and 3/16" max in the valve area.

The piston ring groove position and width is not shown so not sure where Jo gets her 0.010"

[Vixen]

I see we have been evicted from Jo's "Seal and Seal Major" build thread. I guess that's my fault for drifting off her build log story. Sorry.

Jason, I have taken all the measurements from my CAD drawings, which are exact copies of the original Westbury plans and dimensions, converted to decimal inches

I have also taken into account the fact that the cylinder centre line is displaced with respect to the crankshaft centre line. It does not make that much difference, just the 0.003" clearance in the head recess, I referred to.

Mike

[gadabout]

I have just recently completed the Seal 15cc and there are a number of drawing errors.. yes the pistons protrude 1/32 above the liners, I machined the cavities in the head to allow for this, mine has good compression and starts easily. If I remember correctly there is also a issue with the tappet lengths.
Mark

[Vixen]

Thanks Mark,

Your engine looks great and matches the style of the motor launch perfectly.

Thanks for the feedback about the protruding pistons and how you compensated by deepening the cylinder head pockets.

Thanks also for pointing out the potential problem with the tappet lengths, I haven't got to them yet.

Mike

[gadabout]

Mike, The cavities are cast into the head so if my casting was anything to go by they will need machining to compensate the piston pop up just to clean them up.
When I went to assemble mine I found that there wasn't enough space to fit the tappets in. I think they were about1/16th too long and when I added up all the dimensions found they were too long as shown in the drawings, I could have compensated by running the thread up the valve stems more I suppose, so the adjusting nuts went further but that would have compressed the springs more and left little compressed room for them .
My main block casting from Kirk was just over .250" too long and to get it down to drawn size I had to machine off all the boss features on the ends, I milled the features back in later
Mark