My crossflow block's on a wonk

[Gridgway]

Sadly I have had cause to take the head off my newly built crossflow.  One of the things I have noticed (apart from some valve to piston contact) is that the block doesn't seem to be machined orthogonally to the bores.  One one side (the exhaust side, not that I think it matters which side) the piston crowns is dead level with the top of the block.  On the other side the piston crowns are about 0.33mm proud of the block.  By my schoolboy trig, that seems to be about 1/3 degree off orthogonal.  All the pistons in the bores as references are the same, so it's lateral wonk and not fore to aft wonk.

Whilst it's offensive to my sense of geometry, is that actually a problem?  It certainly means the squish will a bit different from side to side, if that matters.

Cheers

Graham

 

[jbcollier]

Hmm, about 12 thou.  Not great but not awful.  If it’s the pistons, that’s odd and easily fixable.  If it’s how the block is decked, that’s quite a bit and a lot more work to fix.  I’d start by talking with your machinist.

If you are building a hopped up crossflow, always clay your combustion chambers.  Doesn’t take long and then you know.

[Gridgway]

I'm pretty sure it's how the block is decked..  the trouble of that to fix it means a complete engine rebuild.

But I want it fixed.  I need someone to tell me not to worry about it!

[Gridgway]

And I have a couple of other problems with the block which means I want to start again.  Not sure the engine builder will agree with me though!

[Ainsley]

Well, if it was mine, I would get the engine builder to explain it. 

Then probably get a different engine expert to rebuild it.

[Roger King]

Are you certain of this? A cold engine will have a large (.030"ish) piston to bore clearance which means they can rock considerably and the piston deck height can be "adjusted" simply by pushing them from side to side.

If it's for real, I would most certainly be having a discussion with the builder. It may run, but it ain't right. However, the mere fact that you have piston/valve contact suggests some serious competence issues. You may now have slightly bent valves, and even if not, I would insist on them being replaced in case one of them decides to part company with its head.

[Gridgway]

Thanks Roger that's very helpful.  Would I be able to rock them by hand in the block? I hadn't thought about that as all 4 look the same.  But I certainly don't want to instigate a difficult conversation with the engine builder on a subject that actually isn't a problem!

The competence question is odd as he's a very well used and capable builder, but there are a few issues.  The compression ratio is too high and the first valve from the front has caught the edge of the valve cutout in the top of the piston which makes no sense how that happened to me.  Also two of the valve stem oil seals have come off and worked their way up the valve stems.  I wasn't expecting any issues at all.

[Roger King]

Yes, you should be able to rock them by hand. I don't know which pistons you have, and it's 20 years since I last built an engine, but many of the pistons available in my day (even forged ones) had valve pockets that would need opening out if you were using larger than standard valves. Generally, the forged pistons have deep enough valve pockets to cater for longer duration, higher lift cam profiles, but you should always check piston/valve clearance unless the engine is completely standard. On my build sheets I always noted that figure, not so much because I might need the information for future reference, but because it ensured I checked it.

Do you know the compression ratio? I would generally expect it to be in the range 10:1 to 11.6:1 depending on the engine spec and use. The lower figure would be for a mild road engine on 40DCOEs and the latter for a full race engine. There can be exceptions, such as supercharged and turbocharged engines.

[Gridgway]

Phew, Roger has saved the day again!  If you centre the piston the disparity goes away.  Glad I asked - you can see I've not built any engines!

Re the compression ratio, the Burton supplied Accralite pistons are approx 12.0 : 1 out of the box - which I presume is at a standard deck height.  I did some measuring and estimating and came to 13:1, but that's not at all an accurate figure.  But I think they will be 12.0:1 or so.

[Roger King]

Unless this is a race engine with an expected limited life, I'd rather see the CR nearer 11:1. However, without knowing more about the engine that can only be a guesstimate. Whatever, you must use the highest octane fuel you can get. Regarding the Accralites - unless they've changed since I last used a set they are really excellent and use one of the lowest friction and best sealing ring packs available, but I always had to enlarge the valve pocket diameters if I was going for something like 1.625" inlet valves and 1.375" exhaust valves. They were fine with standard valve sizes.

Edited December 28, 2023 by Roger King

[Gridgway]

It's definitely not a race engine, just peppy 1700 with a 245 cam.  It does have large valves, so I think we'll be machining to suit and also taking the compression ratio down to a more road reasonable level.  I was changing the head gasket for a thicker cometic one to reduce the CR when I found the piston damage,

It's only one piston that has hit a valve so I think we'll end up with a new piston and valve.  I think the set of 4 pistons that come together are in an engine set for balance purposes - well according to the Burton web site blurb!  But if the pistons are going to be machined, they can be weighed against each other then.