Hi,

I just replaced the bearings as well as the piston and liner in my TT .61 pro engine. I was wondering if I will have to break in the engine again, as everything else has already been broken in. Any advice would be appreciated. Thanks!

I would take it easy (rich) with the plane for a few flights. It should loosen up fairly quickly.

Hi Jim,

Thanks for the reply. The reason I'm asking is because the compression feels pretty normal, much like it was before. There is no noticeable notchiness, and it flips over smartly and easily. When I dissassembled the engine, I cleaned off almost all of the rust with a soft scotchbrite pad, and everything is sparkling clean. i also changed the o-ring seal for the carb, and I hope my previous problems with it go away....[]. I'll be running it this weekend to see how it goes.

I'm surprised it was economic to perform such a major overhall. Most TT spares are priced so high that it's usually about the same price to simply buy a whole new engine.

Well, here in Thailand, a new tt .61 pro engine will set you back about US$ 125. The piston, liner, and both bearings cost US$55, so it was much more cost effective than buying a new engine. We have a dedicated TT dealer here that keeps quite a lot of spare parts in stock.

Cyborg,


I suggest you redo the break-in using [link=http://www.rcuniverse.com/forum/m_1850473/anchors_1850473/mpage_1/key_break%252Din%252Ctapered%252Dbore/anchor/tm.htm#1850473]this thread[/link] for guidance.

Doing the break-in in flight may be perfectly OK for Jim (w8ye), who is an experienced expert on engines, but for someone who is a bit less experienced, you are risking a lean condition, which has not been foreseen.

It is much safer to run the first two-three tanks with extra Castor oil, on the ground, where you can always adjust the needle, should anything prove wrong.


I know TT engines are pretty loose to begin with, but would the risk of a lean run (which will put your $55 Dollars to waste) be comparable to ~40 minutes of ground running with a smaller prop?

Thanks so much for the advice, I'll redo the break in this way. I have a question on the cylinder head. Like the hasty fool I am, I changed the glow plug because the engine would not idle without the battery attached. I managed to strip out the glow plug threads in my haste to change it. Question now is, I went to get a replacement head, but it was from the TT gp .61. It wouldn't fit! it was just under a mm wider, but everything else was the same size. I tried it with a piston liner for the gp .61, and it still wouldn't fit. What is going on here? i am assuming that both engines are using the same sleeve and piston, even though the parts numbers are different, as the gp uses the same body as the pro. I guess the major differences would be different carburetors and bushings instead of ball bearings. This has me stumped. Anyway, the dealer had to backorder the head, and it should arrive by the beginning of next month. Any comments on the different sized head?

The GP61 is actually a .65 (I believe) and has a different bore diameter -- therefore you won't be able to use its head on your 61Pro.

Cyborg,


XJet is perfectly right!

The TT.61GP is actually a .65 cid engine. This is probably to make up for the lesser specifications, with added cubic displacement.


You will need a genuine TT Pro cylinder head, or to go to a machinist (or a DIY shop), to make a Heli-Coil re-thread for the glow-plug.

It must be 1/4"-32 thread (UNEF).

Thanks for the advice...which leads to the next question. Since the displacement is higher, can we bore out the pro .61 engine casing and refit the larger piston, sleeve and head? Would there be any benefit from doing this? it sounds like a good way to get a little more horsepower from the engine...

Cyborg,


With the low prices you are paying for parts (much lower than Americans pay for the same), it would seem you should stay with the stock items. Boring the case would require a good machine shop and work costs money.

It could be a good experience, if you can prepare to live with a failure that would require making extensive replacements.

Hi,

I'm not too sure on this, but I would imagine that the Pro .61 may have more aggressive porting than the GP .61, simply because it is a sportier engine. If this is the case, then modifying the Pro .61 to take the Gp .61 piston/liner may just leave you with a slightly odd-ball engine with very few bnefits - maybe even less power (but possibly more lower down in the rev range???). Just a thought.

Regards
Warren

It won't work. The GP 61(65) is a completely different engine, having a different case, plus different porting & timing from the 61Pro. It is also a better real-world engine than the 61 Pro -- being ~10% lighter & making more power & torque across a broader RPM range than the Pro (which only has an advantage with smaller props at very high revs). A better bet could be to put the 61 Pro carb on a 61 GP.

Britbrat,


Dave Gierke, in the MAN 05/03 issue engine shootout found, the OS.65LA makes more power and behaves better than the OS.61 FX...

But this was not the case with Thunder Tiger. The Pro is a stronger and better engine then the larger GP.

Dar, I've read the article, but I've also personally lived with both of those engines. I have found that the GP generally outpulls the Pro with most decent-sized props. The Pro works a little better in high-rev applications, where it actually will slightly outperform the GP, but in most real-world situations, it is the other way around.

I'm not criticizing the TT 61 Pro -- IMO it is better than the 61 FX -- it's just that I think the 61 GP is better that either one of them.

The one thing that I dislike about the GP is the air-bleed carb -- pretty low tech & not as easy to set-up for smooth transitions -- but then again, the engine is a simple plain bearing machine --- one that really works well.