Pistons hitting head

[triumph885]

I am building a super rocket engine out of parts sourced from auto jumbles etc. using DA10 cases,big journal crank,think flange barrels,JP pistons and wassel billet conrods. When assembled the pistons are proud of the barrels by 1.75mm and obviosly hit the head. What have I done wrong?

[bsa-bill]

three possibilities spring to mine triumph885

Wrong con rods
Wrong pistons

And  last and certainly least likely is the barrel has been skimmed, very unlikely this as it is usually the head that you would skim and 1.75mm is a massive amount to take off

My bet would be the conrods (didn't know Wassel made them)

Others will have more knowledge than I do but it would help if you gave the case number in full, conrod and piston info  and the travel of the pistons from Bottom Dead centre to top dead centre.

[Dynamo Regulators Mike]

My punt is its the pistons. The JP pistons for my A10, nominally 8:1 I think were too tall at the top edge by a good mm, and required a chamfer to fit. Oh, and the oil rings were faulty as well, which is another point to watch for. Portions of the oil rings would have taken 10k miles + to bed in, as witnessed by regular black and shiny portions on the surface to the bore. Replaced with a pair of GPM oil rings. Worth the fettling, it runs fine now with low oil consumption.

[A10Boy]

I dont have any confidence in most wassup stuff. But they wont have manufactured the rods themselves, they will have been made by a rod manufacturer somehwere. Can you find a brand or are there any casting numbers.

have you got some STD Bsa pistons to compare yours with?

[triumph885]

The rods are made by R+R ,but it sounds like it could be the pistons ref; manormike. Its almost certain the pattern parts are the problem!

[Beezageezauk]

triumph885,

What is the measurement between the centre of the gudgeon (wrist) pins and the start of the piston crowns?  I can measure mine tomorrow (Wed) for a comparison.

Beezageezauk.

[triumph885]

Borrowed a std piston which measured 1.7mm lower than the JPs from the pin to the edge of the crown. I returned the pistons to the engineers who rebored the barrel and supplied the pistons, after a long debate they machined the pistons for free but said there was nothing wrong with them as they had sold loads with no problem. They did however mention the rings have been an issue in the past as JP use several different suppliers. Anyway I now a nice set of pistons that fit [just need rings] thanks to everyone for their input, just goes to show you cant trust anything even new.   Just for reference the pistons were plus 30 not a common size maybe a dodgy batch!

[Beezageezauk]

Glad you got it sorted in the end.

I wonder how many times I've heard "I've sold loads of them with no problem" before??

Maybe standard retailers jargon for...I ain't gonna admit to anything!! 

But most of us realise that many of the new pattern parts need to be worked on to make them fit and be servicable.  Some parts more than others.  At least, I suppose, these parts are available and without them we wouldn't be able to keep our bikes on the road.

Beezageezauk.

[nigeldtr]

I was shocked to find the rings that came with the pistons for my A10 were way too small and hand about 0.030? gap when tested in the bore! Also, 1 bore I had to hone out 0.003? as it was too tight for the piston (tight spot half down the bore). The replacement set of rings were a bit tight in the bores so I had to file a little off. All-in-all, a lot of fitting needed to be done - so much for having a re-bore and just popping it back together! The advice I got here on the forum was a correct "check it before you fit it" sad but it really saves a load of time. I think I would have missed the piston crown being too thick - that?s a dirty trick!

I once bought new big end nuts and bolts for a commando rebuild and interestingly, the slits in the locking part at the top of the nuts were cut too deep, consequence was that after about 200 miles, the locking ring detached its self from the top of the nut, the nut then form the bolt, the bolt from the big end and  bang - totally knackered engine. I was told to sue for compensation HaHa!

PS Do the valves now have sufficient clearance when fully open?

Nigel

[muskrat]

G'day Nigel,
                I assume you mean clearance to the piston at TDC. You will need to bring the piston up to TDC and rig up some form of lever to push down on the valve that is just opening (inlet) or closing (exhaust) to measure the clearance. You will need at least 1mm.
  When playing with different cams and pistons I used a worm of plasticine over the piston then assembled the head and rockers. After turning the motor over a few times I would then remove the head and measure the thickness of the plasticine.
 If your clearance is insufficient the valve pockets in the piston will need to be deepened or a thicker head gasket (or 2) used.
Cheers