Flat topped pistons vs dished

[Eleven]

It is my understanding that all Alpine Series came with dished pistons. Mine are flat. I have a very late SV (October 67 build date). Nothing on my engine (matching numbers) looks modified but I also do not know if the engine was ever rebuilt, so I am wondering if Sunbeam ran out of Series Alpine pistons and tossed a set of parts bin types given the Series was ending production in a month or so.. Having written the above, the next generation of Alpine used the 1725 engine so that probably doesn't make sense.
1-Am I correct that all Series Alpines were dished?
2-If so, has anyone else found this in a stock engine?

I am asking because my Alpine has always been a triangulated bugger to start and has never idled well no matter what I do. I also have to use Supreme gas which makes using alcohol free impossible here.

 

[puff4]

I can’t imagine that flat tops were added to production SV Alpines - it would have increased the compression ratio considerably.

 

[jumpinjan]

Your pistons were probably replaced by ones from Victoria British. I bought many flat top pistons from them years ago.
You didn't mention if they are oversized or STD? It will be stamped on top.
Jan

 

[puff4]

My calculation is that going from stock dished to flat tops would change the compression ratio of a stock motor from 9.209 to 10.512.

At that compression you're certainly going to have to run premium petrol, and even then you're possibly risking detonation in #4 cylinder, which tends to overheat and preignight on SV cars because the coolant isn't being removed from the rear of the head (through the intake manifold heating pipe), as it was on earlier cars. That 'not idling well' could be a symptom of such preignition.

 

[Eleven]

Well, these are great replies. When I had the head off, I did an inspection of the cylinders and did not see any stamps indicating over sized pistons. However, I was not looking for them so may have missed it. At the time, I did not realize at the time that the flat tops were not stock.
Thanks for the info on the compression ratio; I know it is higher but not how much. Didn't want to hear that but its what it is. I don't want to do a Vizard mod because I don't trust myself to not butcher the head.
As to idling, once I found out the flat tops were not stock, I was thinking along the same lines and also for the hard starting.
I blew up a great engine in my racing Spitfire because I was too stupid to realize it was detonating so am leery of it now; especially with this old engine.
Boy, not anxious to do an engine rebuild. The oil pressure is good, doesn't burn much oil and have never overheated it. Not to mention the expense.
Well, more stuff to think about. Thank you both for your replies. As usual, I found them very useful and I very much appreciate your thoughts and help.

 

[puff4]

Well, hate to say this, but the Vizard mod is likely the best cure. It should easily bring your engine into spec.

 

[Eleven]

I know you are right. I wouldn't change pistons without rebuilding everything. It is against the laws of mankind to open up an engine and find nice bearings, end thrust in spec and everything just wonderful!
I have a bad head to practice on, any idea where to get a template and instructions? I have a cutting tool, just need to do it I guess.

 

[Eleven]

Oh, I promise not to call your Renown a Mayflower if you have source for a template....

 

[puff4]

I have the template - I made one in metal. I can mail it to you if you promise to return it. PM me your mailing address.

 

[Eleven]

Thank you! Keep it for now, I won't get to this for a while. I am changing out of wire wheels so am elbow deep in that. Next project is to sort my tranny shifting issues. I'll PM you when I get closer to doing this and we can figure it all out.
Take care and thank you again!

 

[puff4]

Okeydokey. Glad to help.

 

[puff4]

Oh, and as for how to use it, see my post here:

http://forum.sunbeamalpine.org/index.php?threads/vizzard-head-gasket.28463/#post-200846

 

[jumpinjan]

...stock dished to flat tops would change the compression ratio of a stock motor from 9.209 to 10.512.

Yeah no kidding. I ran Jan"s pistons (12.5:1) back in 1970 in my 1600 engine. Yes, I used Sunoco 260 gas.
Jan

 

[Jay Laifman]

If you are running flat top pistons, maybe other parts of your engine are modified. For example, how certain are you that you have a stock cam? Perhaps you have a cam that is particularly bad on idle. However, I would suggest that hard idle is more likely leaking carbs or issues with the distributor.

 

[Eleven]

Well, those are good questions; as a matter of fact I have wondered about the cam too. The distributor is a new Pertronix and the carb is a weber. I checked for leaks and there are none. It does seem to like somewhat higher rpms so have wondered if it is a Stage one or something. I have no information on the cars history but It would not surprise me if the cam was. Car had worn out Stroms and still has the stock exhaust manifold. If this is in fact the case whoever did this would not be the first to waste money on performance parts and not do the whole system.