15 replies to this topic

[spaulwill]

I have had smoke coming out of the push rod holes when trying to start my new engine. I took the head off and noticed that there is a gap of 0.5 all round the piston so 1mm bigger than piston. The bore is measured at 71.1 mm and pistons and rings are new standard. Is this all correct or are my pistons too small??

[carbon]

Clearance at the top of the piston is to allow for expansion when running.

The critical clearance is measured at the bottom skirt of the piston, this should have been done by the engine builder.

[Stevie W]

Hi,

I am assuming your engine is a 1275cc as you say it's on standard pistons and rings?
The standard size bore for a 1275cc engine is 70.6mm so the bores seem a little on the large size.
Has the engine had a rebore and new pistons fitted? The first rebore is +0.020" which gives you 1293cc
This gives you a bore of 71.145mm, which appears to be what you have.

If its a newly machined block, the rings will need time to bed in and until this happens you should expect a bit of smoke and crankcase pressurisation.

Steve

[spaulwill]

Thanks Steve don't know the history of the block however the bloke I bought it off told me it was a straight 1275 however from you measurements looks like it could have had a rebore will using the standard pistons be a major problem or will I have to replace them for this much oversize? They are brand new rings and pistons. The original problem was I couldn't get the engine to fire despite having fuel and spark which lead me to think no compression especially when fuel smoke started coming out of push rod holes. But from what you say this could MBA normal. Excuse my lack of info I am new to this and am constantly learning thanks to you guys....

[spaulwill]

Oh forgot to say tha block has not been machined as far as I am aware.

[Stevie W]

Hi Paul,

It does sound as though your block has either had a +0.020" rebore or is worn ecessively making the standard sized pistons loose in the bores.
In order to get an engnie running without loss of compression (which you may have with loose fitting pistons) you must use pistons suitable to the bore size. A good engine machining shop will take your block and new pistons, or supply the pistons themselves, then bore the cylinders to suit.
Anything less than this is going to cause you problems!
What do your bores look like? Are there cross hatching lines in the bores or are they smooth with a defined lip at the top of the cylinder where the bores are worn.
The only smoke you should get on a newly built engine is maybe a little out the exhaust whilst the rings are bedding in and possibly a slight blue haze out of the oil filler cap hole until the rings seal properly.

Hope this helps!
Steve.

[spaulwill]

Thanks for this Steve looks like the block has been bored as the bores look clean with no lips. Think I'll take your advice and get it rebored correctly to may another +20 or would you recommend +40 to take it to + 60 or is this going to far are there any advantages to power going that big or is it better to keep it smaller. I am looking to do some local rallying the rest up is rally cam twin 1 1/4 su along with usual duplex lightened flywheel etc. thanks again

[Stevie W]

Hi Paul,

Just really need to get the bores measured accurately, so you know where you are. Try to keep rebore sizes to a minimum.
No point on going +0.060" if +0.040" will do the job. It certainly seems from what you've described as though your block has had a +0.020" rebore but has been fitted with standard size 1275 pistons!

Cheers, Steve

[tiger99]

It probably does not need to be bored again. Just ask your favourite engine reconditioner to measure it and supply a set of correct pistons and rings. Only if they find the bore to be non-standard in some way will it need to be bored again, if it is unworn, i.e. no lip. But they may advise honing, depending on the surface finish

[ANON]

what did you measure the bore with?

[icklemini]

whats it say on the top of the pistons?

[spaulwill]

I measured with a vernier gauge and it came in as 71.1mm they were all around this I know it's not exact but thats all I had. I have got a gap of about .5mm each side of the piston measured with a feeler gauge so pistons are around 1mm smaller than the bore. This has all come about as I couldn't get the engine to fire and there was smoke coming from the push rod hole when cranking. I took the head off and noticed what I thought was a larger gap than I would have thought but this cold be normal my knowledge does not go this far.

[spaulwill]

It says std on top of pistons they were 1275 std high comp from mini spares

[Stevie W]

Hmmm,

Still sounds like standard pistons in bores that have a +0.020" rebore, based on your measurements Paul.
It's probably best to strip it all out, get the bores measured accurately by a machine shop and go from there.
A measurement of 71.145mm on good condition, lip free bores with cross hatch patterning is a +0.020" rebore so you would need pistons to suit, then go from there!

Cheers, Steve

[spaulwill]

Brilliant thanks for all your help Steve need to put my pistons on eBay!!!!!